PRECAMBRIAN
GEOLOGYOF THE PICURIS RANGE,
NORTH-CENTRAL, NEW MEXICO
BY
PAUL WINSTON BAUER
B.S.,
M.S.,
University of Massachusetts, Amherst, 1978
University of New Mexico, Albuquerque,
1983
DISSERTATION
Submitted in Partial Fulfillmentof the
Requirements €or the Degree of
Doctor of Philosophy in Geology
The New
Mexico
Institute of Mining Technology
and
Socorro, New Hexico
December, 1987
ACKNOWLEDGMENTS
I thank
my committee members Dr. JamesRobertsm,
M.
Dr. JonathanF. Callender, Dr. Jeffrey A. Grambling,
.:D
Christopher K. Mawer, andDr. Kent C. Condie for all aspects
of their assistance and guidance.
I am particularly
grateful
to
Michael
L. Williams
for
his
encouragement,
interest, and friendship during the past four years. Other
individuals
whose
contributions
were
sincerely
appreciated
are Dr. Rod Holcombe, Dr. Ron Vernon, Dr. Tim Bell,
Dr. an'3
Sam
Bowring. For assistance in the field
I thank Roge-
Smith, Dave Plummer, Jim Walker, and Rick Lozinsky.
I gratefully
by
the
(Frank
New
acknowledge
Mexico
Rottlowski,
Bureau
the
of
Director),
financial
Mines
and
National
support
pmvided
Mineral
Science
Resources
Foundation
Grant EAR 8018506 (to J.F. Callender), the Geological
Society
Xi,
and
of
America,
the
Finally,
Resnick,
for
the
University
special
her
New
of
thanks
support
Mexico
New
to
Geological
Society,
Sigma
Mexico.
my
wife
throughout work.
this
and
best
friend
Joan
ABSTRACT
The
Picuris
extensive
Range
exposures
in
of
north-central
Early
New
Proterozoic
Mexico
contains
metamorphosed
supracrustal and plutonic rocks. The supracrustal roc'cs can
be
divided
into
Group,
exposed
schist
at
range;
and
three
in
Pilar,
the
lithostratigraphic
groups:
southern
the
the
part
restricted
to the
Ortega
Group,
of
northern
exposed
part
in
the
the
range;
of
Vadito
the
felsic
th?-
central
part
the range. Four distinct granitic plutons, which rang?- in
age
from1680 Ma to1450 Ma,
Picuris
crop
out
in southernmxt
the
Range.
The
Vadito
Group aisstratiqraphically
complex,
heterogeneous sequence of volcanic, volcaniclastic,
an83
clastic sedimentary rocks. In the southwestern Picuris
Range,
where
consists
of
rocks
three
are
best
exposed,
lithostratigraphic
the
units
Vadito
Group
(from tonorth
south: the Marquenas Quartzite, the Vadito schist, and the
Vadito
amphibolite)
that
may
be
separated
by
bedding-
parallel ductile faults and/or unconfomities. Granitic
plutons
which
amphibolite)
The
intrude
nowhere
felsic
Vadito
intrude
schist
at
schist
the
Pilar
(and
Marquenas
perhaps
Vadito
Quartzite.
consists
of homogeneous,
feldspathic, quartz-muscovite quartz-eye schist that
probably represents metamorphosed, altered, phenocrystic
felsic volcanic rocks. No plutons intrude the felsic
of
iii
or may not be equivalent to some
schist. These rocks may
part
of
The
the
Ortega
continuous,
(Ortega
Vadito
Group.
Group aistransgressive, stratigraphically
metamorphosed
Quartzite),
sequence
of basal
interlayered
quartz
quartzites
areyite
and
pelitic
schists (Rinconada Formation), black phyllites (Pilar
Phyllite), and laminated phyllites and schists (Piedra
Lumbre
Formation)
accumulated
in a shallow
that
marine
setting. Nowhere are Ortega Group rocks cut by pluton:.
Although
it
is
lithostratigraphic
the
felsic
unknown
groups
schist
at
how
are
these
related,
Pilar
occupy
three
the
Vadito
similar
Grou?
and
structural
positions below the Ortega Group. The south-dipping
boundary
between
Ortega
a near-bedding-parallel,
Group
and
ductile
overlying
reverse
Vadito
fault
that
Grolp
is
co?tains
recrystallized quartz-rich mylonites. The south-dipping
boundary
Pilar
between
Ortega
and
isa bedding-parallel,
underlying
ductile
felsic
shear
zone
schist
at
that
contains pristine quartz mylonites. Kinematic indicators
suggest
that
Ortega
Supracrustal
deformational
moved
rocks
history
southward
have
that
over
the
felsic
undergone
a progressive
resulted
in
the
formation
three major generations of penetrative structures. All
three
supracrustal
rock
schist.
groups
contain
a bedding-parallel
schistosity (S1) that is the earliest tectonite fabric
recognized. A south-dipping extension lineation(L1)
of
iv
commonly accompaniesSI.
The Ortega Quartzite contains
other Dl structures indicative of localized, beddingparallel simple shear strain. These shear structures formed
throughout
much
Crustal
folds in the
of
the
shortening
Ortega
ductile
deformation
during
D2 generated
Group
(Hondo
history.
major
syncline
tight
and
Copper
Hill
anticline) and moderate folding in the Vadito Group. The
synclineis the
Hondo
dominant
structure
in
the
mountain
range. Most F2 folds are tight to isoclinal, plunge
shallowly westward, and are overturned
to the north. Nearbedding-parallel, steeply south-dipping faults and local
large-scale
imbrication
of
Ortega
Quartzite
accompaniei
D2.
D3 is characterized by an intense, pervasive, easttrending
schistosityor spaced
the
rocks.
S2*, which transectsF2 folds in the Ortega Gr83up,
late
in
the
cleavage
in
folding
most
Ortega
that
formed
dominant
the
Group)
probably
is
cleavage( S 2 * in
schistose
history.
Dl, D2, and D3 structures developed conditiox
with
of
approximately coaxial principal strain axes. Througholt the
deformation
for
history
strain
heterogeneous
on
all
The
preferredstratigraphic/kinematic/tectonic model
the
Proterozoic
evolution
Picuris Range calls
for:
only
was
part
of
the
rocks
exposed
in
the
1) Vadito Group rocks (or pe-haps
Vadito
Group)
setting
sometime
prior
to
rifting
of
early
crust
this
of
scales.
formed
17002)Ma;
at
inoran
arc
back-arc
around
resulted
in
1700
Ma,
voluminous
felsic
V
volcanism (felsic schist at Pilar) and plutonism, followed
by: 3) crustal stabilization and subsequent accumulation of
Ortega
4)
Group
rocks
extended
were
sediments
on
some
type
buried 12-15
to
km depth
orogenic
event,
due
of
and
perhaps
continental
subjected
to
margin;
to
an
convergence
an3
collision from the south. During
Dl, shear is concentrated
between
the
overriding,
mechanically
stiff
Ortega
Quartzite
and incompetent felsic schists.As shortening progresses,
north-directed
shear
moves
Vadito
Group
rocks
northwarl
over
the Ortega Group. As shortening continues, strain evolves
from
shear-dominated
nucleation
to
andgrowth of
more
major
homogeneous
folds
in
bulk
the
shortening
Ortega
by
and
Vadito groups. As folding evolves,a well-developed,
slightly
Late,
highly
oblique
foliation
south-directed
tectonized
develops
shearing
Ortega
occurs
over
earlier
along
Quartzite-felsic
the
structures.
already
schist
boundary.
This model of Proterozoic shearing, folding, and
faulting
histories
and
Rio
histories
belts.
in
the
Picuris
recently
Mora
proposed
area),
described
Range
and
is
in
a number
for
is
nearby
also
of
similar
to
uplifts
similar
to
Phanerozoic
Proterozoic
(Tusas
geologic
orogenic
Range
TABLE
OF
CONTENTS
.........................................
ABSTRACT ..............................................
TABLE OF CONTENTS ......................................
LIST OF FIGURES .......................................
LIST OF TABLES
........................................
CHAPTER 1 . INTRODUCTION ................................
Purpose and Scope
..................................
Wethods ............................................
ACKNOWLEDGMENTS
ii
iii
vi
xiv
xix
1
1
4
Access. Physiography. Physical Features.
................................... 6
Previous Work......................................
7
CHAPTER 2 . GENERAL GEOMGY .............................
13
Regional Geologic Setting
........................... 13
Local Geologic Setting
............................. 17
General.......................................
17
Phanerozoic
Rocks
.............................. 17
General..................................
17
Paleozoic
Rocks
........................... 19
Del Padre sandstone
..................19
Cenozoic
Deposits
......................... 20
Picuris
Tuff
......................... 20
Tesuque
Formation
.................... 20
Ancha
Formation
...................... 22
alluvium .............................
22
and
Map
Area
vii
.............................. 23
General ...................................
23
Basic Precambrian Stratigraphy and Structure
........23
CHAPTER 3 . PRECAMBRIAN STRATIGRAPHY....................
26
General Statement..................................
26
Vadito Group.......................................
30
General field relations and previous .....
work30
Precambrian
Rocks
..................... 31
General ...................................
31
Schist....................................
32
Amphibolite ...............................
32
Marquenas Quartzite Formation
.............33
Southwestern
Picuris
Other
quartzite
Range
and
..........34
conglomerate
............................ 34
Calc-silicates............................
35
Southeastern Picuris Range
..................... 35
General ..................................
35
Quartzites and conglomerates
..............36
Amphibolite and biotite schist
............37
Felsic
schist
............................. 37
Porphyroblastic schist below Ortega
.......39
Andalusite
schist
......................... 39
Felsic Schist at ..............................
Pilar
40
General........................................
40
Felsic
schists
Northwestern
General
Picuris
Range
(Pilar
......
cliffs)41
...................................
41
viii
Quartz-muscovite
Northeastern
Picuris
...................41
schist
.....................
Range
43
................................... 43
Quartz-muscovite
schist
...................43
Eastern blockof the Picuris Range
.............44
General ...................................
44
U.S. Hill .................................
44
Comales
compground
........................ 45
General
Depositional
schist
environments
at
Stratigraphic
Pilar
of
and
the
the
felsic
......Group
46
Vadito
Positionof Marquenas
................................
Group.......................................
Quartzite
Ortega
General
..............................
Statement
....................
General...................................
Lower
quartzite
...........................
Upper
quartzite
...........................
Rinconada Formation...........................
General...................................
R1/R2
member
..............................
R3
member
.................................
R4 member .................................
R5
member
.................................
R6
member
.................................
Pilar Phyllite Formation
......................
Ortega
Quartzite
Formation
47
49
49
51
51
54
54
55
55
56
57
58
58
59
60
ix
....................... 61
Sedimentology of the Ortega Group
..............63
Plutonic Rocks.....................................
65
General ........................................
65
The Granite
of Alamo Canyon
.................... 65
Piedra
The
Lumbre
Formation
Granite
of Picuris
....................
Peak
67
.......................... 67
Puntiagudo Granite Porphyry
................... 68
Rana Quartz Monzonite
......................... 68
Penasco Quartz Monzonite
...................... 69
Pegmatites ....................................
70
Cerro
Alto
Metadacite
..................71
CHAPTER 4 . GEOCHRONOLOGY ...............................
76
Introduction........................................
76
Previous
Work
....................................... 78
Discussion..........................................
80
CHAPTER 5 . GEOMETRICAL FABRIC ELEMENTS
..................82
Introduction .......................................
82
Summary of General
Field
Relations
.......................... 83
First Generationof Structures. Dl
..................83
Second Generationof Structures. D2
.................89
Third Generationof Structures. D3
..................91
Later Generations
of Structures.....................
93
CHAPTER 6 . METAMORPHISM.................................
97
Introduction and previous ......................
work
97
Metamorphic Mineral Assemblages
..................... 100
Compositional
layering.
SO
X
..................... 100
General ..................................
100
Kyanite ..................................
100
Andalusite ...............................
103
Sillimanite ..............................
104
Discussion ...............................
107
Other
Minerals
................................. 109
General ..................................
109
Biotite ..................................
109
111
Garnet ...................................
Staurolite ...............................
115
Chloritoid ...............................
126
Cordierite ...............................
126
Other
minerals
................................ 127
summary ..................................
128
128
Garnet-Biotite
Thermobaromet
ry ......................
CHAPTER 7. STRUCTURAL GEOLOGY AND SYNTHESIS
.............133
Introduction .......................................
133
General
statement
.............................. 133
Methods .......................................
136
Geometries of Domains ..............................
13a
Introduction ...................................
138
Ortega
Group
................................... 139
Relict sedimentary structures
.............139
First generation of structures. Dl ........139
Second generation of structures. D2.......149
Aluminum Silicate
Minerals
xi
D3 157
........
Fourth generation of structures.
D4 161
.......
Porphyroblast
microstructures
.............161
summary..................................
167
Vadito
Group
................................... 168
General...................................
168
Relict sedimentary structures
.............168
Third
generation
of
structures.
First
generationsof structures.
.......169
Dl
.......
D2 171
Third generation of structures.
D3 173
........
.......
Fourth generation of structures.
D4 174
Southern granitic rocks
...................174
Second
generation
Porphyroblast
of
structures.
.............175
microstructures
............................... 177
Felsic Schist at .........................
Pilar
178
General...................................
178
First generation of structures.
Dl 179
........
Discussion
Second
generation
of
D2 183
.......
structures.
................................
Eastern
Block
..................................
General ...................................
Discussion
Deformational
fabrics
Alamo Canyon
Deformational
Pueblo
Deformational
in
the
Granite
.........................
fabrics
in
the
........................
in
the
184
184
of
185
Rio
Schist
fabrics
183
Ortega
185
xii
........................... 186
Summary ........................................
186
Contacts Between Lithostratigraphic Domains
.........190
Southern
contact
............................... 190
Northern
contact
............................... 193
Picuris-Pecos
fault
............................ 194
Discussion.....................................
196
Structural
Synthesis
................................ 196
Stratigraphic and Structural Constraints
.......196
Possible
Models
................................ 200
Quartzite
..................................... 211
CHAPTER 8 . PRECAMBRIAN TECTONIC EVOLUTION
...............214
Introduction........................................
214
Depositional
Environments
........................... 215
Vadito
Group
................................... 215
Felsic Schist at .........................
Pilar
218
Discussion
................................... 219
Discussion..........................................
220
Constraints on Tectonic Settings
...............220
Possible Tectonic Settings
..................... 221
Vadito
Group
.............................. 221
Felsic Schist at ....................
Pilar
225
Ortega
Group
.............................. 226
Precambrian Tectonic Evolution
.................227
CHAPTER 9 . CONCLUSIONS ANDSUMMARY ......................
230
Ortega
Group
xiii
APPENDICES
..............................................
Appendix 1. Samples
Picuris
for
geochronology
in
the
............................
Range......
Plutonic Rocks.............................
Granite
of
Granite
of
Cerro
Alto
Alamo
..............
Canyon
Picuris
Peak..............
......
Metadacite..........
Metavolcanic rocks........................
Vadito quartz-eye schist, southern
picuris
Felsic
...................
Pilar cliffs..
........
Range
Schist,
Rio Pueblo Schist, Comales
......................
campground
Metasedimentary rocks.....................
Marquenas quartzite cobbles..........
R6 schist, southern Picuris Range....
Appendix 2. Garnet-biotite
thermometry
and
microprobe data................................
REFERENCES CITED
........................................
LIST OF
FIGURES
Figure
Page
1.1
Location
map
1.2
History of stratigraphic
Precambrian
2.1
Regional
of
Picuris
rocks
tectonic
in
Range.
5
nomenclature
the
map
of the
Picurie
northern
€or
9
Range.
Rio
Grande
2.2
Mexico.
lar.
at
rift.
Generalized
map
showing
Precambrian
rocks
of
Newnorth-central
16
2.3
Photomicrograph of Del Padre Sandstone Formation. 21
2.4
Generalized geologic map of the Picuris Range. 24
3.1
Photograph of Vadito Group feldspathic schist.
3.2
a. Photograph and photomicrograph of quartz-eye
38
schistfelsic
3.3
Generalized
stratigraphic
column
of
the
Ortega
Group.
3.4
52
Summary
cross-section
of
stratigraphy
in
Range.
3.5
Chart
72
of
stratigraphic
relations
in
Range.
Ortega
Picuris
the
Picuris
75
4.1
Summary of geochronology in the Picuris Range. 77
5.1
Photographs
the
of
primary
sedimentary
structures
in
s.
e.
xv
5.2
Photograph
of
transposed
layering
in
Piedra
Lumbre
schist.
5.3
85
Photomicrograph
ofS1 foliation
in
felsic
schist
Pilar.
5.4
87
Photograph
of
down-dip
L1 extension
lineation
in
OrtegaQuartzite,northwesternPicurisRange.
5.5
Photomicrographs of F2 folds
in
Ortega
88
Group
schists.
5.6
90
Photomicrograph
ofS 2 * crenulating
earlier
cleavage.
5.7
Photographs
92
of
third
generation
lineations
Ortega
5.8
Group
in
Photograph
and
photomicrograph S3
of crenulations
Ortega
Photomicrograph
of
kyanite
blades
with
undulatory
102
Photomicrograph of fibrolitic
sillimanite
aggregates
in
6.3
Common
106
metamorphic
mineral
assemblages
in
Group.
the
Ortega
110
6.4
Photomicrographs
of
pre-S2*
6.5
Photomicrograph
of
garnet
biotite
porphyroblasts.
112
containing
differently
orientedinclusiontrailsincoreandrim.
6.6 Photomicrograph of garnet
growth.
in
95
extinction.
6.2
the
Group. 94
Ortega
6.1
at
with
symmetrical
114
secondary
116
m i
6.7 Photomicrograph
of
sector
zoned
staurolite
with
anhedral
rim.
and
euhedral
core
118
6.8 Photomicrograph of staurolite after chloritoid.
6.9 Photomicrograph
of
staurolite
119
porphyroblast
with
growth.
secondary
symmetrical
121
6.10 Photomicrograph
of
llmillipedefl
microstructure
porphyroblast. staurolite
123
6.11 Photomicrograph
of
staurolite
and
garnet
with
concentric
inclusion
quartz
shells.
6.12 Photomicrograph
124
of
staurolite
that
preserves
spaczd
cleavage.
6.13 Summary
in
125
of
elements
timing
and
relationships
porphyroblast
between
growth
in
fabric
the
Ortega
Group.
129
6.14 Microprobe profiles across garnet and staurolite. 132
7.1
Generalized geologic map of the Picuris Range.
7.2
Contoured stereographic projections of
So and L1 for
Group
zite. Ortega
134
Ortega
7.3
141
Photograph of quartz-vein-rich shear zone in Orteqa
Quartzite.
7.4
Photograph of sheath fold in Ortega Quartzite
shear
7.5
zone.
144
Photograph of small shears in basal Ortega
Quartzite.
7.6
143
145
Photomicrograph of recrystallized quartz mylonite
from
147
xvii
7.7
from
Photomicrograph of pristine quartz ribbon
mylonite
7.8
R3.
148
Block-diagram sketch of Rondo syncline showing
competent
Ortega
Quartzite
and
disharmonic
folds
schists.
7.9
151
Contoured stereographic projection and fold sketcl
153
of Soil and L2* relationships in the Ortega Grou?.
7.10 Sketch
cross-section
through
Copper
Hill
7.11 Sketch
cross-section
showing
imbrication
antidin?.
154
of
ortegs
Quartzite
northwestern
in area.
map
7.12 Sketch,
of
limbs
156
photograph, and stereogram of transec'ing
S2*
F2 folds.
7.13 Photograph
of
small
158
near-bedding-parallel orD2D.
shear
Piedra
zone
inLumbre
schist.
162
7.14 Photomicrographs of zoned
containing
two
staurolite
different
Si
porphyroblast
orientations
and
Group
lar.
projections
of
modal
Soil and
S2
170
and
photomicrograph
of
cordierite
in
of
sheared
quartz-eye
in
schist
7.18 Summary
schist
176
7.17 Photomicrograph
at
166
in
Vadito
7.16 Photograph
Vadito
core
rim.
7.15 Stereographic
rocks.
in
felsic
181
of
structural
fabrics
in the
Vadito
Group,
and
felsic
schist
at
Pilar.
Ortega
Group,
188
in
xviii
7.19 Synoptic stereographic projections of fabric
Picuris
189
Range.
theelements
in
7.20
Photographof sheared,
asymmetric
clast
in
mylonitic
MarquenasQuartziteatOrtega-Vaditocontact.191
7.21 Photomicrographs
of
mylonitic
rocks
from
the
Vadito contact in the southern Picuris Range.
7.22 Photographs
Quartzite
of
and
mylonitized
felsic
schist
contact
at
192
between
Pilar
Picuris
Orte-Ja-
in
ortega
the
northzrn
Range.
7.23Sketch
of stratigraphic/kinematicmodelal.
204
7.24
Sketch
of stratigraphic/kinematic
model
a2.
206
7.25 Sketch
of
stratigraphic/kinematic
model
b.
7.26
Sketch
8.1
of stratigraphic/kinematic
model
208
d.
210
Generalized geologic map of Precambrian-cored
uplifts
north-central
in
New Mexico.
223
LIST OF TABLES
6.1
Garnet-biotite geothermometry f o r four samples
from the northern Picuris Range....
.................1 3 1
INTRODUCTION
CHAPTER 1.
Purpose
and
Scope
A n important
the
nature
and
problem
timing
in
of
Precambrian
the
geology
formation,
involves
growth,
and
deformation of continental crust. The recent literature
contains
numerous
successfully
examples
reconstructed
of
the
geologic
studies
Precambrian
that
crustal
have
evolution
of an area (Hoffman and Bowring,
1984; Green et al.,1985;
McLelland and Isachsen,1985; Rivers, 1983).
well-documented
studies
multi-faceted,
consisting
of
detailed
witha variety
of
related
conjunction
that
have
Most of the
examined
this
lithologic
topic
are
mapping
in
studies
includin~~:
U-Pb zircon geochronology, structural geology, geochemistry,
metamorphic analysis, and geophysics.
The
early
Proterozoic
history
of
the
southwestern
U.S.
has been the focus of much recent research. Regional
isotopic studies (Nelson and DePaolo,
1985; Silver and
1981; Silver, 1984;
others, 1977; Van Schmus and Bickford,
Stacey and Hedlund,1983; Gresens, 1975) and geochemical
terrane mapping (Condie,1978a, 1978b, 1982, 1984, 1985)
provide
the
framework
for
more
detailed
investigations
the stratigraphy, sedimentology, petrology, structure, and
metamorphism of rocks within smaller areas.
In New Mexico,
into
2
this
more
detailed,
concentrated
in
mapping-based
the
Picuris,
ranges of northern
New
lithostratigraphic
rock
stratigraphic
Although
eachof these
reconnaissance
Tusas,
Mexico,
areas
manner
and
where
packages
reconstructions
research
had
Truchas-Rio
both
Mcra
local
inter-range
been
by
1960, many
been
distinctive
allow
and
has
correlations.
mapped
basic
in
at
a
least
questions
concerning Precambrian geologic history, environments, and
processes
The
Taos
in
remain
unanswered.
Picuris
Range,
north-central
development of ideas
located
New
about
20 km southwest of
Mexico,
concerning
has
been
Precambrian
important
geology
in
the
in
the
of
northern New Mexico. Several studies have addressed the
problems
Picuris
of
the
Range,
stratigraphy
including
and
structural
mapping
of the
entire
evolution
range
(Montgomery, 1953), structural analysisof the southwestern
portion of the range (Nielsen, 1972), and
a
stratigraphic/structural study of a small
tract
in
Copper Hill area (Holcombe and Callender,
1982).
and
Callender’s
structural
previous
investigation
of stratigraphic
relationships
published
not
large
enough
age
relationships
in
this
interpretations,
to
and
resolve
the
small
but
critical
detailed
Holcmbe
and
area
the
the
disagreed
study
relations
structural
with
area
nor
was
to
history
entire Picuris Range. The unresolved questions raised by
Holcombe
and
Callender
concerning
stratigraphic
relations
define
of
the
3
and
deformational
rocks
provided
A major
much
tenet
difficulties
history
of
of
the
of
these
the
in
nearby
motivation
present
resulted
and
study
part
from
for
was
Precambrian
the
that
present
study.
earlier
unrecognized
struct.ura1
complexities within the previous areas of study. To resolve
these
questions,
stratigraphy;
deformation;
relative
The
the
Range
geologic
style
of
timing
absolute
timing
of
and
objectives
geometry
to
forces
of
thoroughly
history,
paying
have
the
and
primary
and
and
topics
significance
events;
present
following
nature
and
metamorphic
the
of
early
the
various
this
Proterozoic
characterize
special
addressed:
episodes
various
of
fa5rics;
deformational
responsible
of
been
for
and
tectonism.
study
were
to determine
strata
the
attention
in
the
Picuris
Precambrian
to
Precambrian
deformational events and environments. The secondary
objectives
have
been
kinematics
towards
to
apply
understanding
this
the
knowledge
dynamics
of
geometry
(physical
forces) involved in tectonism, and finally, to attempt to
fit
the
Precambrian
geologic
history
of
the
Picuris
Range
into a regional framework within northern New Mexico. These
objectives
were
structural
analysis
of
collection
of
samples
Pb
geochronology,
zircon
Proterozoic
pursued
rock
rocks
in
by
detailed
much
of
for
and
other
by
geologic
the
Picuris
mapping
Range,
petrographic
examination
nearby
and
with
analysis
a?d Uof
localities.
appropriate
and
4
Methods
This
at
dissertation
scales
is
based
on
of
1:6,000 and 1:12,000 of
detailed
geologic
rapping
about165 sq lcm ( 6 5 sq
mi) of the Picuris Range, and laboratory analysis of
structural
Mapping
minute
data
was
and
done
rock
on
topographic
samples
collected
enlarged
U. S. Geological
maps
of
the
in
the
field.
Survey7.5
Carson,SW,Taos
Ranchos
de
Taos, Trampas, Penasco, and Tres Ritos quadrangles in Taos
County (Fig. 1.1)
Field
1986.
.
work
was
mainly
during
the
summers
1982-
of
More than400 oriented and unoriented rock samples
were
collected,
were
recorded
and
at
Laboratory
and
done
New
structural
stratigraphic
data
about
600 station locations.
study
Mexico
and/or
at
Institute
both
the
of
Mining
University
and
of
New
Technology
Mexico
included
the following:(1) geometrical analysisof fabric elemmts
recorded
both
in
the
field
and
from
oriented
laborator7
specimens using stereographic projection;
(2) compilation of
structural
maps
showing
the
distribution
and
variation
of
attitudes of fabric elements;( 3 ) petrographic examination
of
about300 thin
relationships
sections,
between
with
fabric
emphasis
development
on
and
timing
porphyroblast
growth; and ( 4 ) selected electron microprobe analysis 4 of
garnet-bearing samples. Furthermore, 12 rock samples were
selected for U-Pb zircon geochronologic study. Rocks were
6
crushed, ground, and concentrated in facilities of
New thc
Mexico
Bureauof Mines
and
Mineral
Resources
and
the
University of New Mexico. Zircon separates are being
analyzed by Dr.S. A.
Bowring
of
Washington
University
in
st. Louis.
Access, Physiography, Physical Features, and Map Area
Several
towns
are
located
around
the
flanks
thor
of
Picuris Range. These towns are linked by three highways
that
forma triangle around the mountains (Fig.
1.1).
Numerous
graded
parts of the
A Forest
and
ungraded
mountainous
Service
road
area
from
dirt
roads
enclosed
Vadito
provide
by
to
access
these
paved
Picuris
Peak
to
roads.
provides
excellent access to the southeastern Picuris Range. Another
Forest
Warm
Service
Springs
A primitive
from
N.M. 7 5 just west of
road
provides
road
access
from
U.S.
68
to
into
the
Penasco
central
Hondo
to
Picuris
Canyon
Ra?ge.
provid'2s
access to the northern Picuris Range. Several private roads
from U.S. 68 provide
access
to
the
northeastern
Picuris
Range. Also, many unimproved dirt roads off N.M.
7 5 provide
access
to
southwestern
the
Harding
Picuris
Pegmatite
between
the
area
in
the
Range.
Relief of about 1470 m (4800 ft)
occurs
Mine
within
high 3292
of m (10,801 ft)
the
at
range
Picuris
7
Peak
to1230 m (6000 ft)
in
the
westernmost
exposures
of
Precambrian rock near the town of Dixon. The mountains are
rugged,
with
deep,
steep-walled
canyons
separated
by
sharp
ridges. In general, the range rises to the east towards
Picuris
series
Range
Peak,
of
it
drops
north-trending
from
the
Physiography
by
where
narrow
valleys
easterly
in
the
de
half
ridge6
thousand afeet
that
Sangre
western
east-trending
several
separate
Cristo
of
that
the
are
the
to
Picuris
Range.
range
is
underlain
dominated
by
resistant quartzites. To the east, these ridges join the
prominent 3050 m (10,000 ft)
the
eastern
Picuris
Climatic
high
north-trending
ridge
of
Range.
conditions
and
consequently
vegetation
vary
considerably over the range. At lower elevations, and
particularly
along
the
southern
slopes,
precipitation
is
low
and scattered pinon pine and juniper dominate the flora. At
higher
elevations
and
in
the
central
canyons,
ponderos3
pine
and aspen form dense forests. Outcrop exposure likewise
varies
in
from
heavily
Previous
excellent
forested
many
slopes
and
of
the
dryer
regions
to
poor
valleys.
Work
Two major
present
in
in
groups
the
of
Picuris
Proterozoic
supracrustal
Range:
a metaclastic
sequence
rocks
without
are
8
metavolcanic rocks (Ortega Group), a
and
largely
metavolcanic sequence (Vadito Group). Based almost entirely
on
work
done
traditionally
relative
in
the
been
southwestern
two
different
chronologyof these
One
hypothesis
the
Vadito
holds
Group,
Picuris
Range,
hypotheses
sequences
in
that
the
Ortega
whereas
the
other
there
concerning
the
Picuris
Group
is
maintains
have
the
Range.
younger
that
than
the
Vadito Group is younger than the Ortega Group. 1.2
Figure
summarizes
have
the
proposed
additions
for
Early work by
the
and
modifications
stratigraphy
of the
Just (1937) suggested
various
Picuris
that
workers
Rang?.
the
metasedimentary sequence was younger. Montgomery(1953)
reinterpreted the field relations of these rocks.
He
believed
that
the
Vadito
rocks
unconformably
overlay
t\e
older Ortega rocks. Miller et al.(1963) recognized a
similar
stratigraphy
(1972) retained
this
in
the
Truchas
relationship
Mountains,
during
his
and
Nielsen
analysis
of
Ortega rocks in the Picuris Range. Gresens and Stensrld
(1974) noted
thata thick
stratigraphically
northwestern
below
Picuris
metarhyolitic
the
near
Ortega
the
town
sequence
Quartzite
of
Pilar
was
in
as
the
well
the eastern Picuris near
U.S. Hill. They suggested that the
accepted
stratigraphic
relationship
between
Ortega
and
Vadito was suspect. This idea was not accepted by Long
(1976) and
Scott(1980), both of whom followed Montgomery's
original stratigraphy. Subsequently, Holcombe and Callender
as
in
9
Jir.C.
1937
Figure 1.2. History of stratigraphic nomenclature for
Precambrian rocks in the Picuris Range.
10
(1982) made a detailed
to
the
They
structural
Ortega-Vadito
concluded
contact
that
the
analysis
in
the
of
rocks
southern
contacta fault,
was
and
adjacent
Picuris
that
RR?nge.
the
original stratigraphy of Montgomery needed revision. Their
geometric models favored
a younger Ortega Group. McCarty
(1983) confirmed
Pegmatite
The
Range
most
Mine
only
that
major
regional
specifically
findings
in
the
Harding
are
geologic
addressed
those
of
studies
structural
in
the
problems
Picuris
and
Montgomery
(1953) and Nielsen
Montgomery mapped the entire range in
reconnaissance
benefit
these
area.
interpretations
(1972).
of
fashion
ofa geometrical
and
interpreted
analysis
or
the
modern
structure
without
petrofabric
criteria. His accurate field mapping has proven to be
extremely useful during the present study. Montgomery's
rock
descriptions
structures
are
interested
in
and
interpretations
excellent
the
early
and
are
thought
of
large-scale
recommended
on
to
Precambrian
readers
rocks
in
New
Mexico. Nielsen's effort involved compilation and field
checking
of
geologic
Range
compiled
North
Carolina,
by
field-camp
students
and
and
a structural
maps
of the
teachers
western
at
Picuris
the
analysisof a small
University
area
between Copper Hill and the Harding Pegmatite Mine. More
local or topical studies include: 1) Holcombe and Callender
(1982)' who
performeda structural
analysis
in
a portion
Nielsen's map area. Although Holcombe and Callender
of
of
11
obtained
similar
data,
their
interpretation
differed
markedly from that of Nielsen;2 ) Long (1976), who
concentrated
southern
on
the
Picuris
occurrence
Range,
but
of
the
included
a map
granitic
and
rocks
in
the
structural
interpretation of Vadito Group rocks in that area;
3) Scott
(1980), who
conducteda strain
analysis
of
the
Marquems
structure and
Quartzite; 4) McCarty (1983), who mapped the
petrography of Vadito
Group
rocks
near
the
Harding
Pegmatite
Mine; 5) Hurd (1982), who conducteda structural study of
the
central
Hondo
syncline
in
the
northwestern
Picuris
Range; and6) D.A. Bell (1985), who studied the contact
relationships
rocks
in
and
the
Prior
with
one
and
rocks
U-Pb
zircon
Harding
to1985, all
exception,
dated
geochronology
Pegmatite
Mine
geochronology
was
based
included
on
only
granitic
area.
in
the
the
of
the
Picuris
Rb-Sr
Harding
Range,
isotopic
system,
pegmatite
an'l
plutonic bodies southof the Harding Pegmatite Mine. Stacey
et al. (1976), F'ullagar and Shiver(1973), and Long (1974)
all
noted
rocks
that
the
approximate
depositional
age
of
Vadito
was1700 Ma. Based on Rb-Sr geochronology of th's
VBbudo Granites" (Montgomery,1953) south of the Harding
Mine, Gresens (1972), Fullager and Shiver(1973), and
Register (1979) suggested that intermittent intrusion
occurred from about
1670 Ma to 1440 Ma. More recent
preliminary
distinct
work
intrusive
using
U-Pb
events
for
zircon
the
chronology
southern
suggests
plutons
at
two
about
the
12
1680 Ma and1450 Ma (Bell and Nielsen,1985).
(personal
communication
reported a U-Pb
zircon
in
Grambling
date
of
L.T. Silver
and
about
1700 Ma
Williams,
for
1985b)
feldspathic
quartz-muscovite schist in the Tusas Mountains. Grambling
and
Williams
(1985b)
appropriate
below
the
for
Range
similar
Ortega
Mineral
suggested
Quartzite
assemblages
indicate
medium
metamorphism),
with
rocks
in
in
in
this
the
cliffs
near
might
be
Range
the
rocks
of the
(amphibolite
metamorphic
age
Picuris
Precambrian
grade
peak
that
which
lie
of
Pilar.
town
Picuris
facies
conditions
near
53OoC,
3.7 kb (Holdaway, 1978), at or near the A12Si05 triple
point. Regional metamorphism may have peaked about
1350-1450
Ma
ago
over
much
of
northern
New
Mexico
(L.E.
Long, 1972;P.E. Long, 1974; Gresens, 1975; Callender et
al. , 1976)
.
throughout
isobaric
P-T
Recent work by Grambling
(1986) suggested that
centraland northern
surfaces
conditions
this
entire
block
from
in
Proterozoic
Mexico,
rocks
subhorizonl-.al
preserve
of
4 + 0.5 kb, 525 2 5OoC. He proposed
area
may
have
mid-Proterozoic
sedimentation.
New
behaved
a coherent
as
until
the
consistent
that
cratonic
beginning
of
Paleozoic
CHAPTER 2.
Regional
Geologic
The
Rio
generally
New
to
the
structural
Mexico,
west,
basins
linked,
are
flanked
topographic/structural
the
and
that
of
rift
the
lies
Sangre
by
uplifts.
between
Brazx
the
de
Cristo
uplift
to
the
Baltz (1978) stated that although some
elements
reactivation
related
consists
a series
of
down-dropped
east (Fig. 2.1).
by
rift
Precambrian-cored
In northern
uplift
Setting
Grande
north-trending,
GENERAL GEOLOGY
in
of
deformation
the
rift
Precambrian
does
are
and
cross-cut
undoubtedly
younger
some
controlled
features,
earlier
rift-
structlral
trends.
The
Picuris
Range
of
north-central
Precambrian-cored,
fault-bounded,
projects
from
westward
the
New
Mexico
a
wedge-shaped
southern
Sangre
uplift
de
is
that
Cristo
Mountains. This isolated, rugged range lies about
20 Ian
southwest
of
Taos,
by 30
km.
that
separates
and
includes
an
area
The Picuris block forms part of the constriction
the
en
echelon
Espanola
in the northern Rio Grande rift (Fig.
2.1).
southeastern
fault
approximately
15 km
that
margin
separates
of
the
the
San
Luis
east-tilted
west-tilted Picuris block (Baltz,
1978).
and
San
Luis
basins
The
basin
a majoris hinge
basinal
block
from
Prior to rifting,
the
I
P
15
this
region
between
represented
a broad
the
Great
Plains
Laramide
and
the
structural
Colorado
high
Plateau
(Baltz,
1978).
Surrounding the Picuris
Mexico
are
To
2.2).
several
the
north
major
is
Range
in
north-central
New
Precambrian-cored
the
Tusas
Range,
uplifts
and
to
(Fig.
the
northeast
is the Taos Range. South and southeasta large
is
area of
Precambrian
Mora
exposure
including
the
Truchas
Range
and
the
uplift.
Montgomery (1963) and Grambling (1979b) suggested that
the
Picuris
mi)
from
the
block
was
Truchas
horizontally
block
along
offset
the
about
km (2337
north-trending,
dextral, Precambrian Picuris-Pecos fault (Fig.
2.2).
Picuris-Pecos
motion
during
fault
experienced
Paleozoic
1978). Sutherland
may
has
and
early
components
Cenozoic
The
of
times
dip-slip
(Baltz,
(1963) noted that the Picuris-Pecos fault
bethe eastern
margin
of
the
Paleozoic
Uncompahgre
uplift.
Baltz (1978) showed the Precambrian rocks in the
Picuris
area
as
being
high
in
Oligocene
and
Mioc'me
early
time. The present, deeply eroded, Precambrian Picuris block
has
remained
high
since
Pliocene
time.
Rio
~
16
Picuris
OTAOS
0
.
I
N
-
0
50 K M
Figure 2.2.
Generalized map showing distribution
Precambrian rocks innorth-central New Mexico.
of
17
Local
Geologic
Setting
General
To the
north,
west,
and
basement
rocks
are
volcanic
rocks
associated
(Steinpress, 1980).
the
San
Luis
bounded
south
by
of
the
Tertiary
with
the
Picuris
block,
sedimentary
Rio
Grande
and
rift
These Tertiary units are found within
basin
to
the
north,
and
the
Chamisal-Penacco
re-entrant of the Espanola basin to the south. To the east,
basement
rocks
unconformably
of
the
southern
the
Rio
either
overlain
Northwest
deep
are
Sangre
of
Grande
flat-lying
by
the
in
Upper
de
which
Tertiary
contact
Paleozoic
Cristo
Picuris
gorge
fault
sediments
or
are
sedimentary
strata
mountains.
Range
the
with,
is300the
m (1000 ft)
Rio
and
Grandethrxgh
has cut
basalts
of
the
Taos
plateau (el. 2135 m).
Several Quarternary/Tertiary geomorphic surfaces, which
onlap
basement
along
the
rocks
northern
Phanerozoic
and
and
are
dissected
southern
flanks
by
arroyos,
of
the
occxr
range.
Rocks
General. All Paleozoic and Mesozoic sedimentary cover
has
been
eroded
from
Precambrian
crystalline
basement
rock
18
in the main block of the Picuris Range. Locally,
unconsolidated
Paleozoic
rocks
rocks to the
Farther
Cenozoic
rest
east
east,
thicknesses
in
of
deposits
cover
unconformably
the
on
Precambrian
and
southeast
of the Picuris
the
Sangre
Paleozoic
de
Cristo
strata
basement
conceal
rccks.
bas&ment
Range.
Range,
the
tremendous
crystalline
basement. Relatively thin sedimentary rocks near the
Picuris
Range
range
in
age
from
Mississippian
to
Pennsylvanian. The Pennsylvanian terrigenous sediments in
the
area
to the
were
derived
northwest
mainly
from
during
two major
the
Uncompahgre
periodsof uplift
the Picuris-Pecos fault (Miller et al.,
1963).
thicknesses
and
vary
erosion,
(Baltz, 1978)
and
flanks
Paleozoic
deposits
strataon
of
and
of
Sedime7taL-y
Cenozoic
Mesozoic
tectonics
paleotectonics
.
Cenozoic
Paleozoic
considerably to
dueboth
west
highland
the
lie
unconformably
on
the
northern,
western,
Picuris
Range,
as well
as
in
Precambrian
and
and
souther2
topographic
lows
within the range. Miller et al.
(1963) listed the Cenozoic
units, from oldest
to youngest, as the Picuris Tuff, the
Tesuque
and
Ancha
formations
of
the
Santa
Fe
Group,
the
Servilleta Formation, stream terraces, and floodplain
alluvium. In the area in which Steinpress
(1980) made a
detailed
Picuris
the
study
Range
primary
of
Cenozoic
near
Cenozoic
the
stratigraphy
town
unit
of
Dixon,
exposed.
in
the
the
western
Santa
Fe
Group
is
19
Brief
contact
summaries
with
of
the
Precambrian
Phanerozoic
rocks
are
deposits
presented
found
in
in
this
chapter. These include the Del Padre Sandstone
(Mississippian?),
Other
local
and
poorly
Paleozoic
consolidated
rocks
that
are
Cenozoic
not
units.
discussed
include
the Espiritu Santo Formation (Mississipian?), the Tererro
Formation (Mississipian), the Flechado Formation
(Pennsylvanian), the Alamitos Formation (Pennsylvanian), and
the Sangre de Cristo Formation (Pennsylvanian-Permian).
More
extensive
information
may
be
found
in
Miller
et
al.
(1963), Steinpress(1980), and Manley (1976).
Paleozoic rocks. The Del Padre Sandstone
(Mississippian?) isa non-fossiliferous orthoquartzitic
sandstone
that
rests
unconfonnably
on
Precambrian
rocks
(Miller et al., 1963). The type section for the Del P,*dre
is
at
the
junction
of
Rito
del
Padre
and
the
Pecos
River
the Santa Fe Range. East of the Picuris Range, the Del
Padre
Ortega
Sandstone
may
Quartzite,
as
be
easily
the
Del
confused
Padre
is
with
the
highly
Proterozoic
indurated,
orthoquartzitic, locally cross-laminated, locally
conglomeratic
with
rounded
clasts
of
pure
quartzite,
and
well bedded with thin, black, iron oxide-rich layers.
Thicknesses
of
the
Del
Padre
vary
considerably
in
region. Miller et al. (1963) measured
a section 20 m (64
ft) thick in the Rio Pueblo area. The Del Padre is
the
20
conformably
Santo
overlain
by,
and
interfingers
with,
the
Espiritu
Formation.
Thin
sections
southeastern
map
of
the
area
Del
near
Padre
the
collected
Picuris-Pecos
from
fault
the
show
medium-grained, well recrystallized, granoblastic quartz
grains
that
generally
contain
inequant
extinction
(Fig.
Thus the sandstone protolith has recrystallized
a to
2.3).
quartzite,
and
subsequently
experienced
a moderate amo~ntof
strain.
Cenozoic deposits. The Picuris Tuff (Oligocene t-,
lower Miocene) was originally named by Cabot
(1938) f0.r tuff
and
conglomerate
that
occurred
near
the
town
of
Vadito.
This unit ranges from
60 to 360 m (200 to 1200 ft) in
thickness
and
is
composed
of
interlayered
water-laid
t-Iff,
coarse conglomerates, clays, and minor basalt flows (Miller
et al., 1963).
This unit, dated at
24 Ma (Steinpress,
1980), represents
lower
Miocene
the
eruption
volcaniclastics
Steinpress
notedthat the
equivalent
to
and
coincides
The
the
with
Tesuque
the Santa Fe
Group,
was
Eocene
of
Oligocene-
erosion.
Tuff
is approximately
os Pinos
and L
lower
initiation
Formation
deposition
following
Picuris
Abiquiu
the
and
(Miocene
originally
of
to
formations,
Rio
Grande
rifting.
Pliocene),of part
defined
near
Santa
Baldwin (1956).
In the Picuris Range, Miller et al.
(1963)
described
sequence
this
as
buff-colored,
poorly
sorted,
Fe
by
21
Figure 2.3.
Photomicrograph of typical Mississippian(?) Del
Padre sandstone from the eastern Picuris Range. Note
granoblastic texture, strained individual quartz grains, and
single mylonitic grain at center of photograph. Field of
view is 14 m
m
.
22
weakly consolidated, sand, silt, gravel, volcanic ash, clay,
and
breccia
that
ranges
(500 to 3500(?) ft).
to 1065(?) m
thickness150from
in
Much of this unit was derived lccally
from Paleozoic and Precambrian rocks. Lithologies gererally
change
considerably
along
strike.
Miller et al. (1963) considered
be
Quaternary
in
age
and
the
the
Ancha
upper
Formation
member
of
to
the
Santa
Fe
Group. The Ancha consists of fan-deposited sand and gravel
of
several
different
ages
that
overlay
the
Tesuque
Formation. They suggested that this material represen’ed
a
very
large,
separated
dissected,
from
recent
once-continuous
alluvial
sheet
deposits
by
and
could
position
be
an3
consolidation. Manley (1976) thought that the Ancha
Formation
was
appropriate
post-Santa
to
name
Fe
all
such
Long (1976) followed
this
these
gravel
deposits
southwestern
the
Picuris
and
suggested
gravels
suggestion
the
by
deposits
that
it
Ancha
Cejita
not
be
Formation.
informally
of
may
naming
Mesa
in
the
Range.
Long (1976) described younger terrace gravels, which
presumably
Cejita
were
Mesa,
derived
and
brown
directly
from
sand-soil
the
deposits
eolian. The ages of these units are
unknown.
(1980) stated
in
Pliocene
that
and
erosion
Quaternary
has
dominated
time
gravel
due
to
that
deposits
may
have
and
gravel
occurs
presently
a number
in
been
Steinpress
the
Picuris
the
regional
region
uplift
of the southern Rocky Mountains. Erosion and deposition of
sand
of
of
active
23
streams in and around the Picuris Range. Within the
mountain
range,
recent
alluvial
and
colluvial
deposits
are
common.
Precambrian
Rocks
General. All Precambrian supracrustal rocks in the
region
appear
to
be
multiply
deformed,
metamorphosed
to
amphibolite facies, and of Early Proterozoic age. Intrusive
granitic
plutons
in
the
southern
Picuris
Range
range
from about1680 Ma to 1450 Ma ( D . A . Bell, 1985).
In
general,
north,
metasedimentary
rocks
occur
in
the
in
age
and
metavolcanic and plutonic rocks occur to the south. S.-veral
major
high-angle
Basic
Precambrian
The
eastern,
Picuris
faults
disrupt
Stratigraphy
Range
dominantly
can
granitic
be
the
and
section.
Structure
divided
into
two
parts:
an
terrane, aand
western,
dominantly supracrustal terrane. The north-trending, highangle Picuris-Pecos fault separates these two terranes. The
rocks
westof the
Picuris-Pecos
fault
are
the
focus
of
report. The general geology of this area is relatively
simple. Three major groups of Precambrian supracrustal
rocks are present (Fig.
2.4).
The metasedimentary Ortega
this
24
0
1
ca. 1460 Ma granitic r o c k s
ca. 1680 Ma granitic rocks
(Indudes Granite of Picurls Peak)
Granite of Aiamo Canyon
Cerro Alto Metadaclte
2
3
4
6 MILES
f3 Ortega Group
a
I
F e l s l c schist at Pilar
and Rlo Pueblo Schist In east
Vadito Group
-Contact
”_..
Fault
Figure 2.4. Generalized geologic map of Picuris Range,
showing the distribution of the major supracrustal rock
groups and granitic plutons.
25
Group (Long,1976) is the predominant rock group, and
consists
mainly
of
resistant
quartzites,
pelitic
schists,
and various phyllites. To the south, structurally above the
Ortega
Group,
is
the heterogeneous
metavolcanic-
metavolcaniclastic-metasedimentary Vadito
Group,
includes the Marquenas Quartzite (Long,
1976).
into
the
granitic
southernmost
plutons
Vadito
which
range
1450 Ma (D.A. Bell, 1985).
the
Ortega
rocks
in
are
age
at
bedded
Stratigraphic
major
at
least
four
from
1680about
Ma to
Group, aishomogeneous sequence of felsic
schist
Most
Intrusive
To the north, structurally below
metavolcanic-metavolcaniclastic(?) rocks
felsic
which
herein
called
the
Pilar.
rocks
and
dip
temporal
lithostratigraphic
between
40° and
80°
relationships
units
are
to
the
between
uncertain
due
south.
the
three
to
tectonization along their mutual contacts. Although all
rocks
have
histories,
presumably
each
differently,
of
experienced
the
resulting
three
in
the
similar
groups
polyphase
accomodated
generation
of
the
strain
strain
profoundly
different tectonite fabrics, geometries, and apparent strain
paths.
CHAPTER 3.
General
PRECAMBRIAN STRATIGRAPHY
Statement
The
Picuris
Range
metasedimentary,
contains
metavolcanic,
exposures
and
of
metaplutonic
Precambrian
rocks. Exposures consist of about
8 0 percent supracrustal
and
20
percent plutonic. The range can
be divided into
three
general
2.4).
An eastern
minor
exposures
separated
by
the
blocks
separated
major
faults
(see
Fig.
block of dominantly plutonic rock, with
of
supracrustal
froma central,
north-trending
metavolcanic
by
and
rocks
dominantly
Picuris-Pecos
plutonic
rocks
the is
south,
to
metasedimentary
fault.
lie
in
block
Complexly
the
layered
southern
po-tion
of the central block. West and southwest
of the central
block,
across
the
Pilar-Vadito
fault,
is
the
western
block,
composed of supracrustal rocks in the north, and intru;:ive
granitic
rocks
in
Stratigraphic
based
on
that
the
south.
nomenclature
proposed
employed
in
this
paper
is
by (1937),
Just
Montgomery (1953),
Miller et al.(1963), Nielsen (1972), Long (1976), and Scott
(1980).
These authors developed
a well-defined, consistent
stratigraphy
for
the
metasedimentary
rock
sequence
(Or5ega
Group) in the central and western blocks. Although there is
some
nearly
variation
all
of
lithology
formations
and
along
members
strike
are
in
readily
these
units,
27
distinguishable
are
generally
The
throughout
the
range,
and
younging
criteria
consistent.
Ortega
Group
has
been
interpreted
a
as
transgressive sequence (Soegaard and Eriksson,
1985) that
originally
ranged
from
basal
conglomeratic
quartz
sands,
to
pure quartz sands, to interlayered sands and shales, to
black muds, to calcareous muds and shales. The Ortega Group
has
an
thickness3-5ofkm.
apparent
Structurally
Ortega
Group
complexly
above,
in
the
but
stratigraphically
southern
interlayered
Picuris
metamorphosed
Range
a sequenze
is
igneous
rocks named the Vadito Group by Long
(1976).
within
out
this
and
volcanic
group
change
reappear,
rocks
and
considerably
probably
interlayered
and
the
of
sedimzntary
Lithologies
along
represent
with
below,
strike,
felsic
volcaniclastic
and
and
pinch
mafic
clastic
sediments. The minimum apparent thickness of this group is
3-4
km.
In
the
southern
Picuris
Range,
the
volcanic
component
of the Vadito Group is dominantly mafic. Although
feldspathic
metavolcanic
rocks
are
common,
they
are
not
as
voluminous as amphibolites. Thus, the Vadito Group typesection
defined
apparently
by
bimodal
Montgomery
(1953), is a mafic-dominated,
volcanic
sequence
containing
interlayered
volcaniclastics and sediments. Williams et al. (1986) and
Williams (1987) proposed a redefinition of the Vadito Group
in
northern
New
Mexico,
based
mainly
on
rocks
exposed
in
the
28
Tusas
Range
and
Rio
Mora
area
in
the
southern
Sangre
de
Cristo Mountains. They suggested that the Vadito Group be
redefined
asa felsic
quartz-muscovite
volcanic-sedimentary
schist,
pelitic
and
sequence
feldspathic
of
schist,
metaconglomerate, quartzite, and amphibolite correlative
over
much
feature
of
is
northern
New
Mexico,
quartz-muscovite
schist
whose
most
containing
distinctive
feldspar
and
quartz lleyesll. These *Ieyest1 were interpreted to be
phenocrysts
This
in
altered
definition
metatuffs.
is
at
odds
Picuris Range (Montgomery,1953).
relationships
Picuris
between
Range
and
the
with
the
type-section
in
Because stratigraphic
Vadito
Group
the
Vadito Group"
of
described
the
by
southern
Williams
et al. (1986) and Williams (1987) are unknown, Montgom..-ry's
original
nomenclature
Note
that
metavolcanic
the
will
herein
be
retained.
felsic
schist
at
Pilar,
a felsic
the
northern
sequence
exposed
in
Picuris
Range,
is characterized by quartz and feldspar "eyesv1. It is
possible
that
these
rocks
are
equivalent
to
Vaditu
the
Groupt1of Williams et al.(1986). The relationship between
these
rocks
and
the
unknown: possibilities
Until
will
more
precise
continueto use
Montgomery
for
the
The stratigraphic
southern
will
be
regional
the
Picuris
discussed
in
correlations
nomenclature
Picuris
Vadito
is Group
Chapter
7.
are
developed
available,
1
by
Range.
position
of
the
pelitic
schists
and
the
29
phyllites
of
the
Piedra
Lumbre
Formation
has
been
extensively debated. Although the most complete, most
extensive
exposure
east-trending
was
first
in
the
of
Piedra
in
the
strip
Lumbre
northern
described ain
relatively
Copper
Hill
Formation
Picuris
thin
occurs
Range,
sliver
of
as
the
an
unit
exposure
area.
Long (1976) proposed the name Piedra Lumbre Formation
for the
laminated
Marquenas
schist
Quartzite
to
and
the
phyllite
south
that
and
sit
the
between
Pilar
Phyllite
the
to
the north. Previous studies noted that the northern and
southern
exposuresof this
rock
appear
to
lie
in
diffe-ent
stratigraphic positions. Holcombe and Callender(1982)
observed
that
although
the
stratigraphically
similar
was
more
structurally
Piedra
to
rocks
Lumbre
in
appropriately
was
the
Ortega
assigned
Group,
to
the
it
Vadito
Group. They suggested that faults separated the Piedra
Lumbre from rocks on either side. The present shows
study
that
the
position,
sliver
unitin is
the
northern
whereas
on
the
the
correct
southern
southern
stratigraphic
exposure
limb
a minor
of
is
actually
a fault
anticline
which
is
faulted against Vadito Group rocks. The Piedra Lumbre is
the
on
youngest
the
unit
Pilar
Intrusive
Group are at
Metadacite,
the
Ortega
Group,
and
rests
conformably
Phyllite.
only
least
the
in
into
four
the
southern
distinct
Puntiagudo
portion
plutons:
Granite
the
Porphyry,
of
Cerro
the
the
Vadito
Alto
Rana
Quartz
30
Monzonite, and the Penasco Quartz Monzonite (Long,
1976).
These
rocks
are
best
exposed
in
the
southern
half
of
the
western block. Similar rocks intrude Vadito units in the
easternmost
western
block,
and
the
southern
part
of
th?
central block. Most of the eastern block is composed
a
of
plutonic
This
rock
body
appears
southernmost
to
and
in
from
felsic
eastern
more
sedimentological
texture
intrude
partof the
Additional
and
distinct
the
western
schists
in
plutons.
the
block.
detailed
lithologic
interpretations,
are
descriptions
presented
in
Montgomery (1953, 1963), Nielsen (1972), Gresens and
Stensrud (1974), Long (1976), Scott (1980), McCarty (1983),
and Soegaard and Eriksson
(1985, 1986).
Vadito
Group
General
field
relations
and
previous
work
Montgomery (1953) defined the "Vadito Formation" as all
Precambrian
rocks
that
overlie
the
Rinconada
Formation
in
the Picuris Range. Montgomery's conglomerate and schist
members
of
Marquenas
the
Quartzite
amphibolite
Due
Vadito
to
of
the
the
were
redefined
Formation
Vadito
lateral
and
Group,
by
Longas
(1976)
the
unnamed
schist
the
and
respectively.
variability
of
Vadito
Group
units,
31
and
the
different
Quartzite
bottom
in
of
located
the
the
lithologies
northern
Ortega
immediately
and
Group
below
exposed
southern
Picuris
lowermost
Ortega
Range,
Formation.
In the southeastern Picuris Range, the poo-ly
underlying
between
the
heterogeneous,
the
Ortega
more
the
basal
of
pure
contact
of
occurrence
relatively
exposed
orthoquartzite
the
being
herein as
defined
is
the
below
Quartzite
schistose
rocks
Ortega
Quartzite
and
the
of
the
Vadito
Group is sharp and highly sheared. In the southwester2
Picuris
Range,
the
basal
Ortega
Quartzite
is
nowhere
exposed. Instead, Ortega Group schists are in fault contact
with the Vadito Group. In the northern Picuris, the basal
Ortega
Group
is
adequately
exposed
onlythe
along
clifCs
near Pilar. In this area, the contact with underlying
felsic schist at Pilar is also sharp. The fact
verythat
different
north
lithologies
and
south
is
underlie
significant
the
Ortega
and
will
Quartzite
be
in
the
discussed
in
descriptions
of
Chapter 7.
The
the
following
Vadito
Group
southeastern
sections
rocks
Picuris
Southwestern
provide
found
in
brief
the
southwestern
and
Range.
Picuris
Range
General. The area of the Copper Hill-Harding Pegmatite
Mine
in
the
southwestern
Picuris
Range
has
been
the
focus
o
32
numerous Precambrian geologic studies. Rocks are wellexposed
and
easily
accessible,
and
universities
run
several
their undergraduate geology field camps there. Vadito Group
rocks
in
this
area
consist
of
schists
(35% of area),
amphibolites (35% of area), the Marquenas Quartzite
Formation (25% of area), other quartzites and
(2%
metaconglomerates (2% of area), quartz-muscovite schists
of area), and calc-silicates( a % of area).
Schists. Vadito schists containa large variety of
fine-grained,
texturally
and
mineralogically
distinct
schists, phyllitic schists, and schistose quartzites.
HcCarty (1983) performed a detailed
these
rocks
and
separated
out
petrographic
nine
study
of
distinct
schistose
units
in the Harding mine area. She concluded that these rocks
represent a metamorphosed
grained
graywackes
protolith
interlayered
of
fineto medium-
with
impure
sandstones,
shales, volcaniclastic sediments, and basaltic flows that
accumulated
ina relatively
deep-water
Soegaard and Eriksson
(1986) called
vlorthochemical
metasedimentary
basin
these
setting.
schists
rocksll.
AmDhibolites. Amphibolite bodies are scattered
throughout the Vadito Group. The thickest and most
extensive
amphibolite
unit
occurs
just
south
of
the
Pegmatite mine. Associated with amphibolites are biotite
Harding
33
schists, quartz-biotite rock, metadacite and felsic schist,
and metaquartzite and metachert. These rocks were
thoroughly
(1983), who
and
described
by
interpreted
volcaniclastic
Montgomery
(1953) and McCarty
them
sediments
as
interlayered
intruded
by
basaltic
felsic
flows
sills
and
dikes.
parauenas Ouartzite Formation. The Marquenas Quartzite
Formation of Long (1976) is a strip of relatively resistant
polymictic
metaconglomerate
metasandstone
that
is
and
texturally
well-exposed
in
the
immature
western
end
of
the
western block, and poorly-exposed in the eastern end. Scott
(1980) divided the Marquenas into four sub-units.
From
north to south (top to bottom) these are:
1) a northem
metaconglomerate
with
deformed
clastsa micaceous
in
quartz
matrix. Clasts are up to
10 cm long and consist of an
average
of66 percent
metasedimentary
quartzite,
34 percent
felsic schist,and traces of vein quartz (J.A. Grambling,
personal communication,1987); 2) a rippled, crosslaminated,
micaceous
3) a vitreous,
quartzite
with
quartzite
with
isolated
pebble
layers;
cross-bedded massive grey, somewhat micaceous
scattered
pebble
layers;4 ) and
a southern
metaconglomerate with deformed orthoquartzite
(54%), silicic
metavolcanic and quartz-muscovite schist
( 4 1 3 % ) ~and white
vein quartz clasts(J.A. Grambling, personal communication,
1987) in a micaceous quartz matrix. Quartzite clzsts
34
are
up
to
1 m in
or 1:2:6,
1:2:3
diameter,
aspect
with extremes of1:8:16
metaconglomerates
ratios
for
averaging
schistose
clasts
Montgomery (1953) also noted that the
(Montgomery, 1953).
coarsen
Estimates of thickness
to
with
from
have
east
ranged
to
west.
from
410 m ( I m q . 1 9 7 6 )
about610 m (Montgomery, 1953).
Lithologies
and
metaconglomerates
plain
deposits,
sedimentary
suggest
an
accumulated
1986).
(Soegaard and Eriksson,
have
been
plain
channel
with
water
structures
origin
as
during
the
proximal
waning
alluvial-
flood
stages
The immature quartzites may
depositsa perennial
in
depths
in
braided-allu-rial
greater 8 than
m (Soegaard
and
Eriksson, 1986).
The
stratigraphic
position
of
this
unit
has
recently
been disputed. This will be addressed a in
later discussion
section.
Other uuartzites and metaconqlomerates. Numerous thin,
isolated
exposures
quartzite
All
of
are
either
Group,
orthoquartzite
scattered
these
represent
Vadito
are
of
similar
younger
or
south
of
and
the
conglomeratic
Marquenas
lithologically
alluvial
infolded
or faulted
channel
Marquenas
to
Quartzite.
the
Marquenas
deposits
in
the
equivalents.
Felsic schists. Light-colored quartz-feldsparmuscovite
rocks
are
locally
found
interlayered
with
otker
and
35
Vadito Group rocks. Felsic schists commonly contain rounded
to sub-rounded quartz grains (quartz-eyes) and/or euherlral
to
subhedral
feldspar
megacrysts
that
probably
represe’lt
metamorphosed relict phenocrysts. The matrix generally
consists
of
aligned,
fine-
to
medium-grained
muscovite
laths
in mosaics and strung-out patches of fine-grained,
grawlar
quartz, plagioclase, and microcline grains. Textures
locally
suggest
tectonic
grain-size
reduction
of
an
originally coarser-grained felsic protolith. These felsic
schists
probably
related
rocks,
volcaniclastic
represent
interlayered
metamorphosed
rhyolites
with
derived
locally
and
felsic
sediments.
Calc-silicates. Thin, discontinuous calc-silicate
horizons
occur
in
a number
of
places
in
the
northern
part
of
the Vadito section. One of the thickest, described by
McCarty (1983), is 1 to
4
m thick, contains quartz, calcite,
epidote, thulite, and garnet, was
and presumably a
manganese-rich
limestone.
Southeastern
Picuris
Range
General. Vadito Group rocks in the southeastern
Picuris
Range
differ
from
those
in
the
southwest
in
ways. Biotite-bearing feldspathic and pelitic schists are
almost totally lacking in the southeast. White,
several
36
feldspathic,
quartz-muscovite
schists
are
more
abundant
and
locally thick. I1Sheetsl1 of granitic rock are complexely
interlayered
with
the
rounded
masses
the
southwest.
Dominant
the
country
of
generally
lithologies
conglomerate (35% of
rocks
in
in
marked
discordant
this
area
to
granitic
ro:k
in
include
area)
, amphibolite ( 3 5 % of
contrast
quartzite
and
area),
feldspathic quartz-muscovite schist
(30% of area), and
pelitic schist( 4 % of area).
Quartzites and concrlomerates. Quartzose metasediments
in
the
southeastern
equivalents
of
the
Picuris,
Marquenas
possibly
stratigraphic
Quartzite,
are
interlayered
with amphibolites, felsites, and granitic rocks. Quartzite
xenoliths
are
common
in
the
granitic
rocks,
and
stringers
of
granitic rock are common in quartzites. Xenoliths are light
tan
to
white,
fine-grained
rocks
with
very
small
micas,
and
local thin dark bedding layers. Several quartzitic
lithologies are present in this sequence. Massive, vitreous
quartzites
have
cross-beds
defined
by
high
concentrations
hematite. Local conglomeratic layers contain flattened
clasts up to about
4 cm long. Clasts include: rounded,
purple-gray
crystalline
quartzite or vein
quartzite:
quartz;
and
somewhat
dark
gray,
rounded
white
metallic-looking,
fine-grained schist. Dark clasts are most abundant. Some
conglomerates
contain
only
very
sparsely
scattered
quartz
of
37
clasts in micaceous quartzite matrix. Schistose quartzite
crops out locally. There appears toa be
general decrease
in
clast
size
from
west
to
east.
AmDhibolite and biotite schist. Amphibolites are
common,
and
locally
grade
into
black,
medium-grained
biotite
schists. Amphibolites range in texture from rocks
containing
amphibole
large,
matrix,
oriented
to
plagioclase
rocks
with
laths
a blac'c
in
many
small,
somewhat
aligned
amphibole crystals ina white plagioclase matrix. BioL.ite
schists
are
less
common,
well-layered,
biotite-quartz
rocks.
Locally, epidote-rich zones grade into mafic rocks. Mtxt
amphibolites
probably
represent
metamorphosed
mafic
volcanic
flows and sills.
Felsic schists. Although felsic schists appear to make
up a large
these
proportion
rocks
are
not
of
area
in
resistant,
the
and
southeastern
thus
exposures
Picuris,
are
scattered and poor quality. In general, these rocks are
white, fine-grained, fairly dense, quartz-muscovite schists
that
commonly
contain
euhedral
rounded quartz eyes (Fig.
3.1).
Picuris
Range,
metamorphosed
many
of these
phenocrystic
feldspar
megacrysts
As in the southwestern
rocks
felsic
are
interpretedh?to
volcanic
rocks.
and
38
Figure 3.1.
Photograph of white felsic schist (HC-542) with
flattened, rounded quartz-eyes and euhedral feldspars
megacrysts from the Vadito Group, southeastern Picuris
Range.
39
Pomhvroblastic schist below Ortesa.A schistose
horizon
base
commonly
of
underlies
the
quartzite
is exposed
the
Ortega
in
the
Quartzite
where
southeastern
the
Picuris
Range. This schist is generally coarse-grained, pink
to
white, strongly crenulated, and contains large blocky
altered
porphyroblasts
muscovite
of
chloritoid(?) a in
quartz-
matrix.
In one locality (HC-414), poor exposures of gray and
white
banded,
very
fine-grained
rock
with
lens-shaped
feldspar pods lie just below the Ortega Quartzite. Thin
sections
reveala quartz-rich
rock
with
The originof these schists is
unknown.
of
the
rather
textures
than
to
are
clearly
due
to
mylonitic
text-xes.
At least some
shearing
and
metamoqhism
sedimentation.
Andalusite schist. The only pelitic schist identified
in
southeastern
biotite
518).
schist
Vadito
exposuresa distinctive,
is
containing
large
knobs
of
black
andalusite
(HC-
This unit is only
a few meters thick and appears to
pinch out along strike in both directions. Possible
protoliths for this schist include shale, altered
volcaniclastic
rock.
sediment,
or
hydrothermally
altered
volcanic
40
Felsic
schist
at
Pilar
General
Montgomery (1963) defined the Rio Pueblo Schist as the
basal
llmigmatitic
Quartzite)
of
quartziteg1
the
Ortega
of
the
Formation
lower
in
the
quartzite
(Ortega
southeastern
Picuris Range, near the town of Rio Pueblo.
He consid2red
this
feldspathic
unit
to
consist
of
granitic
rock
intruded
and intermixed with quartzite. He noted that
a similar
micaceous
quartzite
crops
out
in
steep
cliffs
near
the
town
of Pilar. In the cliffs near Pilar, stratigraphically and
structurally
quartzite
below
(herein
consists
Ortega
called
ofa sequence
of
Group
the
rocks,
felsic
this
schist
light-colored,
micaceous
at
pink
Pilar)
to
white
to
green quartz-muscovite and quartz-eye schists. With respect
to
the
same
Ortega
structural
Group,
the
position
felsic
as
the
schist
Vadito
at
Pilar
Group
occupies
rocks
in
the
southern Picuris Range. The felsic schist at Pilar may be:
1) laterally
Vadito
equivalent to the Vadito;
2) a portion of
which
is
not
exposed
in
the
southern
Picuris
Range;
or 3) a totally different package of rocks. These rocks may
represent hydrothermally altered, locally reworked, felsic
volcanic tuffs and flows (Gresens and Stensrud,
1974).
It is
rocks
that
important
to
Montgomery
note
that
called
Rio
although
Pueblo
all
Schist
of
the
felsic
(including
the
41
the
felsic
texturally
schist
at
Pilar)
distinctive
from
are
very
Vadito
similar,
Group
they
felsic
are
schists
in
the southern Picuris Range. In this report felsic schists
will
be
treated
Northwestern
separately
Picuris
from
Range
type
(Pilar
Vadito
Group
rocks.
cliffs)
General. The Pilar cliffs are
a 450 m (1500 ft), nearvertical
escarpment
U.S. highway 6 4 .
and
of
the
town
of
Pilar,
along
Excellent exposures of Ortega Quartzite
underlying
kilometers
southwest
felsic
along
the
schist
at
Pilar
occur
for
several
cliffs.
Quartz-muscovite schist. The cliffs near Pilar
ewose
a sharp
contact
Quartzite
and
between
overlying,
feldspathic
right-side-up
quartz-muscovite
Ortega
schists
and
micaceous quartzites of the felsic schist at Pilar. The
base of this felsic schist is nowhere exposed. Thin
quartzite
mylonites
Compositional
occur
layering
in
within
the
contact
zone.
felsic
schists
is parallel
to
the
Ortega-Vadito contact. schists are white, light gray, pink,
and
green,
and
commonly
quartz-eyes (Fig. 3.2).
m thick,
marks
the
contain
rounded
and
flattened
A distinctive pink zone, about
100
schist
immediately
below
the
Ortega
Quartzite contact. This zone, which
is anomalously hiGh in
manganese
and
rare
earth
elements,
contains
such
unusual
42
a.
b.
Figure 3.2.
a. Photograph of typical felsic schist at
Pilar. Note well-developed, anastomosing foliation. Pen
top for scale. b. Photomicrograph of quartz-muscovite,
quartz-eye schist from the Pilar cliffs. Foliation h3s been
deflected around the quartz-eye. Section is cut parallel to
extension lineation and perpendicular to foliation. Field
of view is4
mm.
43
minerals as piemontite, thulite, and Mn-andalusite
(Grambling et al.,1983, Grambling and Williams,
1985).
This
distinctive
Ortega
Quartzite
Mexico,
event
horizon
and
may
during
the
has
been
in
most
of the
be
related
a regional
to
waning
traced
uplifts
of
below
the
northern
New
volcanic
stages
of volcanism,
or
basal
exhalative
deep
lateritic weathering (Grambling et al.,
1983).
In
an
excellent
metarhyolite
study
occurrences
in
that
re-examined
northern
New
possible
Mexico,
Grese?s
Stensrud (1974) suggested that due to alteration, several
metavolcanic
units
in
the
Picuris
Range
had
been
misinterpreted as metasediments. They cited the rocks in
the
Pilar
cliffs
Northeastern
as ofone
the
Picuris
best
examples.
Range
General. Two occurrences of feldspathic quartzmuscovite
quartz-eye
schist
have
been
mapped
in
the
northeastern Picuris Range. Both are very localized
exposures
of
schist
within
Ortega
Quartzite.
Quartz-muscovite schist. The western occurrence
is in
a small
down-dropped
Quartzite,
where
fault
white
to
block
within
the
pink,
quartz-eye
Ortega
schists
are
exposed. About one kilometer east, in
a structurally
complex
area
adjacent
to
the
Picuris-Pecos
fault,
white,
and
44
greenish,
and
pinkish
quartz-muscovite
schists
appear
to be
infaulted and infolded with the Ortega Quartzite.
In the
center
of
the
fold,
very
fine-grained
quartzites,
fine-
grained quartz-muscovite schists, and metacherts(?) are
highly contorted. Both of these exposures are thought
to be
equivalents
of
Eastern
the
felsic
Block
of
schist
the
at
Picuris
Pilar.
Range
General. There are four isolated exposures of qusrtzmuscovite
quartz-eye
schists
in
the
southeastern
Picuris,
on
the eastern side of the Picuris-Pecos fault. Each of these
contains
and
rocks
were
called
considered
lowermost
U.S.
similar
to those
Rio
themto be
Ortega
Pueblo
found
in
the
Pilar
cliffs
Schist
by Montgomery (1963), who
migmatitic
basal
quartzites
of
the
Formation.
Hill. Excellent exposures of well-beddedl white,
gray, and pink quartz-muscovite quartz-eye schist are found
in
open-pit
mica
mines
on
an
isolated
hill
of
Precambrian
rock just west U.S.
of Hill. This rock is composed of up to
60 percent
coarse
white
muscovite
flakesa matrix
in
of
granular quartz and feldspar. Quartz-eyes are abundanL and
are
consistently
North
contact
of
with
flattened
this
hill,
granitic
in
the
similar
rock
to the
dominant
bedded
northeast
foliation
schists
and
are
massive
plane.
in
gray
45
quartzite to the south. Although exposures are poor in the
area, several. important relationships are clear. Granitic
rocks
intrude
illustrated
and
by
crosscut
layering
transitional
in
the
granite-schist
schists,
rock
as
found
along
the contact. A Mn-rich horizon occurs stratigraphically
below
the
quartzite,
porphyroblasts
andalusite
that
are
and
piemontite
might
found
be
locally
and
altered
pseudomorphs Mnafter
along
the
schist-quartzite
It is possible that this contact separates Ortega
contact.
Quartzite
from
analagous
to
underlying
the
felsic
tectonic
schist,
boundary
and
exposed
is
in
therefore
the
Pilar
cliffs.
Two kilometers
northwest
of
this
exposure
a smsll
is
hill composed of poorly-exposed, highly-fractured, vitreous
quartzite
and
light-colored
quartz-muscovite
schist.
Comales camRsround. A large isolated block of
generally
poorly-exposed
Precambrian
rock
lies
in
the
extreme southeastern corner of the Picuris Range. New
Mexico
Highway3 and
the
Rio
Pueblo
pass
through
the
northern end of this block at Comales campground. This
somewhat schistose, slightly gneissic, feldspathic quartzite
was named Rio Pueblo Schist by Montgomery
(1963). However,
these
rocks
could
be highly
sheared
granitic
units
rather
than an originally fine-grained layered rock. White,
feldspathic,
quartz-muscovite
schist
crops
out
in
the
high
46
hills
to
the
south,
below
an
exposed
section
of
massive,
gray, vitreous quartzite. In the contact zone,
porphyroblasts
of
Mn-andalusite
Depositional
and
environments
the
Vadito
It is necessary
felsic
their
the
schist
at
in
of
separate
in
the
the
the
the
differences
which
be
found
in
felsic
float.
schist
at
Pilar
Group
Pilar
lithologic
manner
to
can
Picuris
and
two
Vadito
the
Group
Range
from
due
uncertainty
sections
the
to
both
concerning
relate
stratigraphically. The felsic schist at Pilar probably
represents strained, metamorphosed, and altered rhyolitic
flows and tuffs. Rounded quartz eyes and feldspar
megacrysts may represent relict phenocrysts. Vernon
(1986)
suggested
that
most,
deformed rocks are
if all,
not such
residual
quartz
phenocrysts,
eyes
rather
in
than
porphyroblasts. Thin sections from throughout this unit
consistently
show
very
similar
textures
of
quartz
megacrysts
in a fine-grained matrix of quartz, muscovite, and feldspar.
If
the
m,
isrepresentative
and
present
rhyolitic
proximal
The
thickness
of
tuffs,
toa major
the
these
felsic
heterogeneous
of
this
section,
a minimum
thicknessa pile
of
rocks
volcanic
Vadito
may
have
of300
of
rhyolites
originated
center.
Group
terrane
of
feldspathic
47
and
pelitic
probably
schists,
represents
and
amphibolites
metamorphosed
and
and
related
rocks
complexely
interlayered graywackes, basaltic flows and sills, felsic
tuffs and sills, and volcaniclastic sediments. Long
(1974)
described
original
volcanic
textures
such
as
pillows,
pillow
breccias, and relict vesicles in amphibolites. Duncan and
Shore (1984) suggested
melange
that
representing
some these
of
rocks
young
trench-fill
area
accreted
onto
continental crust. They recognized turbidity flows, d?bris
flows
containinga wide
sediment
folding
variety
(but
no
of
clast
tectonic
sizes,
folding),
soft
and
metavolcanics and metacherts. There is no question th’lt the
Groupis indeed a terrane
Vadito
is a highly
deformed
considerable
melange
additional
should
The
mixed
be
used
appears
environment
of
to
the
Stratigraphic
mixed
terrane,
mapping
to
fluvial-alluvial
Quartzite
of
be
Vadito
and
before
describe
protolith
lithology,
as
such
genetic
blt
it
requir”.-s
terms
s-lch
it.
of
inconsistent
the
Marquenas
with
the
depositional
schist.
Position
of the
Marquenas
Quartzite
Soegaard and Eriksson(1986) suggested that the
Marquenas
Vadito
and
Quartzite
Ortega
Formation
groups,
and
is
as
younger
actually
than
both
formed
from
the
weathering and erosion of the Ortega Quartzite. Several
48
relationships observed in this study refute this. Although
bedded
orthoquartzites
are
abundant
Marquenas
Quartzite,
no
silicates
have
been
recognized
that
very
aluminous
the
quartzite
clasts
clasts
in
Ortega
as
the
in
the
containing
alumino-
Marquenas,
Quartzite
suggesting
a
was
source
not
for Marquenas rocks (J.A. Grambling, personal communication,
Soegaard and Eriksson's suggestion requires
a
1987).
Marquenas
Quartzite
wedge
in
fault
contact
with
Ortega
and
Vadito rocks. Although the northern Marquenas Quartzite
contact
is
presently
clearly
a fault
no
evidence
in
for
the
Copper
a
such
fault
Hill
along
area,
the
thore
is
southlrn
boundary. Furthermore, along strike to the east, possible
Marquenas
Quartzite
metavolcanic
related
equivalents
rocks,
quartzites
suggesting
and
are
interlayered
that
the
with
Marquenas
metaconglomerates
are
and
probably
part
of the Vadito Group. Cross-beds in both the northern and
southern
quartzite
tops
the
to
disparity
alluvial
of
the
suggests
in
north.
Vadito
the
it
Marquenas
should
environments
Quartzite
schist
the
of
However,
sedimentary
Marquenas
that
members
in
and
the
quartzite
the
Copper
and
be
Quartzite
noted
between
deeper
the
schist
are
that
the
fluvial-
water
Hill-Harding
show
graywackes
Mine
area
separated
b.7
an
unconformity ora fault. This however, does not imply that
the
Marquenas
youngest
was
derived
supracrustal
from
the
unit the
in range.
Ortega
Group
and
is
the
49
Ortega
Group
General
The
Group ais
4+ km thick
Ortega
sequence
crop
Statement
of
out
mature
in
terrigeneous
most
of
the
transgressive
metasedimentary
major
rocks
Precambrian-cored
that
uplifts
of
northern New Mexico. Just(1937) defined the Ortega
Quartzite in the
Tusas
Range,
and
correlated aits%ction
to
of similar quartzitein the Picuris Range. Montgomery
(1953) redefined
the
sequence
as
the
Ortega
Formation,
which
consisted of three members: the Lower Quartzite;
Rincmada
Schist; and Pilar Phyllite. Nielsen(1972) further
subdivided
the
the
Rinconada
Formation
Ortega
sequence
Formation
with
witha slate
Long (1976) proposed
status,
and
the
the
Ortega
Quartzite,
six
members,
and
anda muscovite
phyllite
member.
raising
two
into
the
Ortega
of the
members
Pilar
the
Formation
Formation
Pilar
to
group
to
the
Piedra Lumbre and Pilar formations. Although this
nomenclature
has
remained
unchanged,
the
relative
positions
of some of these units has been debated. The most complete
section
of
in
northern
the
The
the
Ortega
youngest
map
Group
in
the
Picuris
Range
is
in
Picuris
is
located
area.
Ortega
Group
unit
the
Range
the Piedra Lumbre Formation. The correct stratigraphic
position
of
the
Piedra
Lumbre
Formation
has
been
uncertain,
50
as the northern
and
southern
exposures
of
this
rock
arpear
to lie in different stratigraphic positions. Montgomery
(1953) mapped
Pilar
the
the
Phyllite
large
Piedra
member
northern
Lumbre
of
the
Formation
Ortega
exposure
with
as
part
Formation,
the
of
and
uppermost
t.he
included
schists
of
the Rinconada. Nielsen (1972) called this unit the
muscovite
phyllite
correlated
thin
it
of
the
Pilar
Formation,
with
the
northern
exposures.
He considered
in
the
northern
outcrops
quartzites
the
member
quartzites
in
the
Vadito
Group
of
as
and
the
equivalents
of
the Pic~ris
southern
Range. Long (1976) proposed the term Piedra Lumbre
Formation
schist
for
and
the
exposures
phyllite
that
near
sit
Copper
between
Hill
the
of
layerel
Marquenas
Quartzite
to the south and the Pilar Phyllite to the north. Holcombe
and
Callender (1982) noted
was
lithologically
was
structurally
that
similar
more
although
to
rocks
the
in
appropriately
Piedra
the
Lunbre
Ortega
assigned
to
Grou?,
the
it
Vadito
Group. They suggested that faults separated the Piedra
Lumbre from rocks on either side. Hurd
(1982) examined a
small
that
no
area
no
in
the
Vadito
faults
northern
Group
existed
Picuris
rocks
between
were
Piedra
Range,
present
Lumbre
and
in
and
determined
the
map
Pilar
area,
units
the map area. Bauer(1984) tentatively proposed that the
unit
in
schist
the
until
north
should
informally
more known
was about
its
be
called
relationship
the
to
Hondo
the
Piedra Lumbre type section.
He also suggested that if the
in
and
51
two
units
between
are
Piedra
correlative,
Lumbre
and
then
the
Pilar
in the
fault
south,
that
does
exists
not.
exist
in the north. Within the map area, the Pilar Phyllite
contact
is
quartzites
gradational
previously
Nielsen (1972) are
up
into
the
Piedra
Lumbre,
as
Vadito
quartzites
interpreted
actually
layered
within
the
and
lower
the
by
Fiedra
Lumbre schist. The present study shows that the northern
unit is in the correct stratigraphic position, whereas the
southern
limb
two
exposure
ofa minor
Group
thirds
of
dips
Because
consistent
of
this
actually
a fault
sliver
on
the
southern
anticline.
Ortega
Regional
is
metasediments
the
are
the
Precambrian
stratigraphy
Group
over
up
most
exposures
consistently
Ortega
section
take
of
in
the
the
no.rthern
range.
between
40° and 80° south.
contains
a remarkably
the
presents
a single
entire
range,
the
remninder
seto f lithologic
descriptions of Ortega Group rocks for the Picuris RanNJe.
A
generalized
stratigraphic
column
of
the
Ortega is
Group
shown in Figure 3.3.
Ortega
Quartzite
Formation
General. The name Ortega was first used by (1937)
Just
for the spectacular
quartzites
of
the
Ortega
Mountains
the Tusas Range. Montgomery(1953) later used Ortega
Formation to include
the
entire
section
of metaclastic
rocks
in
52
FORMATION
AEMBE
THICKa
LITHOLOGY
(m)
Micaceous, cross-beddedqtzite
PIEDRA
LUMBRE
PILAR
Fine-grained,laminazed,
garn e t - s t a u r o l i t e s c h i s r and
phyllite.
Localblack
slanes
and c a l c - s i l i c a t e s .
Black, g r a p h i t i c S h y l l i r e and
Local w h i t es c h i s t
slate.
l a y e r s . B e s a l blue-black
quartzite.
200
400
200
600
-
-
PHY LLlTE
guymu-bi-gr-st1 phyllitic
cnlsr
whine t o
Massive,micaceous,
grayqtzite;bluecrystalline
ctzire.
Red3ish, s m a l l c t - s L 1 s c h i s t
Tan-ignite m i c a c e o . ~ , X-bedsed
otzite.Hassive
blue o r z i z e .
Mu-s-d s c h i s r wi-h l a r a e
twinned stl. Bi-mu-andalusite
0 0
200
7
-
70
0
-
100
-
schis:.
100
20
-
25
:xessive, c l e a n , nmiiuwgrained
c r y s t a l l iw
n ehg,irt a
ey
cross-5edded
quaxtzi=e.
Local
aluminous s c h i s z l a y e r s
ORTEGA
QUARTZITE
"""""_
1000
1200
-
Gray q u a r t z i t e and r e 3 i s h
wea-hering, c o a r s e l y gr2nuler
cross-kedded q l i a x L t e . L o c a l
g"-beering b l a c k qrar=zi:es.
C o n ~ 1 o ~ r a " iat
c hasp_.
Figure 3.3.
Generalized stratigraphic column for the Ortega
Group. Modified from Nielsen, 1972; Long,
1976; and
Grambling, 1985b).
53
in
the
Picuris
Range,
and
called
the
thick
basal
quartzite
the Uower Quartzitell. Nielsen (1972) named this Lower
Quartzite
the
Ortega
In
this
report
divided
into
two
Quartzite
the
Formation.
Ortega
mappable
Quartzite
members
Formation
sub-
informally
is
called
the
lower and upper quartzites. These members are discernable
over
the
entire
The
two
exposed
strike
in
by
Picuris
major
the
two
Range.
east-trending
Picuris
Range
bands
are
northwest-striking
of
Ortega
slightly
faults
Quartzite
offset
that
are
along
covered
by
unconsolidated Cenozoic sands and gravels. Several small,
isolated
exposures
Picuris-Pecos
fault
Exposures
Ortega
of
of
orthoquartzite
in
the
crop
out
east
of
the
southeastern of
part
the rayge.
Quartzite
are
generally
excellent,
due
to its highly resistant nature. The quartzite invaria3ly
forms high ridges and peaks. Thicknesses of Ortega
Quartzite
range,
map
of
except
area,
about
1200 m are
for
where
an
the
fairly
area
in
quartzite
constant
the
has
througholt
northwestern
an
apparent
the
part
of
thicknests
the
of
more than two kilometers. This is inconsistent with the
average
regional
thickness
of
Ortega
Quartzite
of 1.0 to 1.5
km, and is probably due to tectonic doublinga high
along
angle fault. The upperand lower quartzite members are
repeated in this section.No such schist is recognized
elsewhere
within
the
Ortega
Quartzite
of
the
Picuris
Range.
54
Lower auartzite. The lower quartzite generally
consists
of
brick-red
to
brownish-red,
granular,
crystalline, medium- to coarse-grained, weathered-looking
grains of quartz. This unit is commonly highly fractured,
with
coarse
black
iron-oxide
layers
defining
bedding
a?d
cross-laminations. Locally, dark quartzite horizons contain
medium-sized, altered garnet porphyroblasts, and basal
quartzites
contain
small,
rounded
clasts
of
white
vein
quartz, quartzite, and felsic schist. The thickness
of this
unit
is
approximately
500 to 600 m.
Upper quartzite. The upper part of the Ortega
Quartzite
is
gray
to grayish
white,
coarse-grained,
vitreous, massive, clean, cross-bedded quartzite. Laysring
and
cross-bedding
are
defined
by
black
layers
of
iron-oxide
minerals. Aluminosilicate minerals (especially kyanite and
sillimanite)
are
Aluminosilicates
ubiquitous
are
throughout
locally
the
concentrated
quartzite.
in
thin
bedding-
parallel schistose layers. Locally, in the Hondo Canyon
area,
these
aluminous
layers
are
composed
of
75 up
to
percent kyanite crystals. Near Copper Hill, similar
horizons
contain70 percent andalusite porphyroblasts (J.A.
Grambling, personal communication,1987).
minerals
in
the
quartzite
are
Common accessory
ilmenite,
hematite,
tourmaline, epidote, muscovite, kyanite, sillimanite, and
zircon.
55
The
than
lower
the
quartzite
upper
is
quartzite,
less
and
resistant
is
commonly
to
weathering
found
cn-mbly,
as
iron-stained float. The contact between upper and lower
Ortega
Quartzite
is
defineda poorly
by
thick,
schistose
layer
which
exposed,5- tc LO-m
generally
forms
canyons
cr
saddles.
Rinconada
Formation
General. The Rinconada~ Formation consistsa of
sequence
that
of
lie
Nielsen
interlayered
pelitic
stratigraphically
(1972)
schists
above
the
and
Ortega
orthoquartzites
Quartzite.
divided this formation into the R1 schist, R2
schist, R3 quartzite, R4 schist, R5 quartzite, and R6 schist
members. Because R1 and R2 appear to differ only in
metamorphic
mineralogy,
(198633) has
suggested
not
in
bulk
that
a better
chemistry,
Grambling
terminologyis Rsl, R q l ,
€22. For
Rs2, Rq2, Rs3; where
Rsl includes R1 and
this
report, five members Rl/R2, R3, R4,andR5,
R6 will be used.
The
members
consistent
in
of
the
thickness,
Rinconada
Formation
lithology,
and
are
texture
remarkably
over
the
two
east-trending bandsof exposure in the range. Quartzite
occurs
are
locally
invariably
Protoliths
for
in
schist
interlayered
the
members,
in
Rinconada
and
quartzite
were
thin
pelitic
schists
members.
probably
sands
and
stales
56
that
accumulated
in
deltaic,
fluvial,
and
shallow
marine
settings (Soegaard and Eriksson,
1985).
R1/R2 member. Because these schists are easily
weathered and eroded, exposures are generally poor. The
best
exposures
in
the
northern
band
occur
where
Hondo
Canyon
cuts across the strike of Rinconada units (Plate
1). The
best
exposures
in
the
southern
band
occur
in
the
Copper
area. Montgomery (1963) estimated thickness of60-105 m
(200-350 ft)
for R1, and
60-150 m (200-500 ft) for R2.
Traditional
presence
of
muscovite
definitions
large
lrsalt
staurolite
of
andalusite
and
in
and
R2
depended
porphyroblasts
pepper"
porphyroblasts
R1
R1
R2
schist,
in
and
on
biotite-
garnet
muscovite-biotite
the
and
schist.
In general, especially in the Copper Hill area, these
criteria
are
there
is
these
units.
consistent:
some
In the
mineralogical
Copper
into R1 schists
however
in the
Hill
and
northern
textural
area,
the
Ortega
overa 5-m-thick
zone
of
Picuris
variability
Quartzite
grades
intercalated
quartzite and schist. The lowermost schists of R1/R2
contain
abundant
knobby
andalusite
and
cordierite
porphyroblasts up to
6-7 cm across. Andalusites are less
abundant
upwards,
where
schists
consist
of black
in
biotite
books in muscovite-quartz matrix. R2 begins where
porphyroblasts of staurolite coexist with andalusite. R2
Hill
57
schists are relatively uniform in texture. Large
twirrned
staurolite
crystals
weather
outa medium-grained
of
muscovite-quartz-garnet-biotite schist. Locally near Copper
Hill, the basal R 2 contains a 1 m thick
resistant,
gray
quartzite horizon. Garnets are small and euhedral, and
although
they
remain
constant
in
size,
they
become
more
abundant up-section. R 2 schists become more quartz-rich at
the
topof the
quartzite
member,
they
grade
into
R3
the
member.
R3 member.
that
where
range
The northern band of
R 3 contains exposures
from
excellent
in
the
west
to
average
in
thz
east. Good exposures occur along most of the southern band.
The R 3 quartzite
commonly
forms
From
1) tan
is
resistant
prominent,
bottom
to
highly
to
white,
to
steep-sided
top, Rthe
3 member
micaceous,
weathering,
an'l
ridges.
consists
cross-bedded
of:
quartzite:
2)
massive, vitreous, crystalline, bluish-gray quartzite;
3)
friable brown quartzite;
4)
slabby
gray
5 ) quartzite
to
white
quartzite;
containing thin layers of staurolite-garnet-
muscovite-quartz
schist
and
knobby
grey
plagioclase
schist;
6) friable
tan
to
brown
quartzite,
which
grades
up
R 4 to
schists.
In
the
Copper
Hill
area,R3-R4
thecontact
is
marked aby
the
58
massive,
1-m-thick
Thicknesses
of
R3
layer
of
dark
blue-black
range
from
60 to 100 m.
quartzite.
R4 member. Northern exposures of the R4 schists are
fair to poor. Exposures around Copper Hill are excellent
and extensive. The R4 schists tend
to underlie canyons and
saddles
between
the
bounding,
more
resistant
schists
are
characteristically
quartzite
members.
R4
fine-
to
medium-
grained, reddish weathering, gray to silver, garnetstaurolite-muscovite-biotite-quartz schists. Quartz-rich
versus biotite-rich layers are common. Small euhedral
garnets
are
staurolite
Thicknesses
concentrated
in
porphyroblasts
range
thin
are
reddish
scattered
layers,
wherens
throughout.
from
25 to 70 m.
R5 member. The R5 quartzite member crops out
extensively
over
most
of the
two
bands
of Rinconada
Formation. This rock forms prominent ridges and steep
slopes. From bottom to top, the R5 consists of:
1) massive,
2) massive
micaceous, white to gray quartzite;
blue
crystalline
quartzite;
3) massive, resistant gray-black quartzite and white
quartzite;
4) massive, gray
to white,
quartzite
with
thin
locally
cross-bedded
staurolite
schist
vitreous
horizons;
59
5) pure
quartzite, interlayered tan, cross-bedded quartzite
and massive, blue quartzite;
6) tan
micaceous quartzite, massive blue quartzite, friable
quartzite,
schist
massive
quartzite
which
grades
up
into
R6
member.
In general,
the
R5
member
has
more
localized
cross-bed9
and
is more massive than the R3 member. Thickness range
80 from
to
200
m.
R6 member. Although the R6 member is easily weathered
and
eroded,
extensively
it a
isprominent
over
both
lithology
the
northern
that
and
crops
out
southern
bands
of
In general the R6 is well banded, gray
r?-d,to
exposure.
quartz-muscovite-biotite-garnet phyllite to schist. This
rock type is commonly strongly crenulated.
In the norkhern
exposures,
medium-sized
porphyroblasts
of staurolite
are
commonly concentrated in layering. Interlayers of garletrich
and
quartz-rich
rock
contain
small
red-brown
euhedral
garnet porphyroblasts. Garnet forms up 40
to percent of
some of the garnet-rich layers. Cleavage surfaces in
muscovite-rich
phyllitic
horizons
exhibit
a characteristic
gray to silver sheen. Good graded bedding occurs locally.
Basal
R6
Although
numerous,
schists
minor
this
contain
local,
thin
textural
and
unit
lithologically
is
black
mineralogical
quartzite
variations
distinctive
beds.
are
with
respect to the other Rinconada schists. The upper contact
60
of
theR6 with
very
the
overlying
distinctive,
several
Pilar
meter
Phyllite
thick,
is
marked
a
blue-black
by
garret
quartzite. Thicknesses of R6 range from4 0 to 100 m.
Pilar
Phyllite
Formation
Just (1937) originally included
the Pilar Phyllite
Formation as the Hondo Slate. Montgomery
(1953) renamed the
Hondo
Slate
as
the
Pilar
Phyllite ofMember
the Ortega
Formation. Nielsen (1972) separated a slate member froma
muscovite
phyllite
member
(later
to
become
the
Piedra
Lumbre
Formation) in the Pilar Formation. Long (1976) propos?d
group
status
Formation
The
for
and
the
Piedra
Pilar
Ortega,
Lumbre
Formation
and
broke
out
the
Pilar
major
tsro
and
Formation.
is
exposed
in
two
minor localities in the Picuris Range. Lithologies ars
identical
of
in
somewhat
all
areas,
more
with
phyllitic
the
rock
exception
in
parts
of
of
the
the
occurrence
southwNsstern
exposure. The Pilar is a distinctive, black, compact, very
fine-grained,
quartz-rich
carbonaceous
slaty
phyllite.
Brown to yellow iron-staining commonly occurs locally. In
some areas, small, elongate, flattened cavities appear on
cleavage surfaces, and thin white, schistose layers, which
probably
represent
phyllite. McCarty
phyllitic
layer
bedding,
cut
the
homogeneous
black
(1983) mapped a 15-m-thick, brick-red
in
the
southwestern
exposure.
61
On
the
average,
the
Pilar
consists
of50 about
percent
small (less than
0.1 mm) streaky quartz grains and lerses
and
small
grains
veins
that
of
are
coarser
quartz,
25% very
fine
very
graphite,25%and
mainly
opaqce
fine-grained
(less than0.05 mm) aligned muscovite laths. Relict
porphyroblasts occur in some rocks. Minor minerals include
of Pilar
sphene and traces of biotite. Apparent thicknesses
Phyllite
range
Most
workers
carbonaceous
of
oxygen-starved
northern
The
Piedra
Copper
most
carbonaceous,
porphyroblasts
syncline
rocks
were
occurs
area.
represent
deposited
Range,
the
Formation
consists
as
of
muscovite-biotite
and
local
most
as
sliver
of exposure
Lumbre
area
that
complete,
Formation
thin
Hill
these
Hondo
in
reduced,
some
extensive
an
exposure
east-trending
in
in
unit
was
first
describzd
south
of
Copper
Hill.
originally
described
dark-colored,
phyllite
calc-silicate
with
and
Although
tectonic
this
may
rather
be
true
than
in
original
much
of
in
fine-grained,
garnet
thin
quartzite
Callender (1982) suggested that layering in the Piedra
is
of
strip
horizons. Based on transposition structures, Holcombe and
Lumbre
type
Formation
Picuris
a relatively
the
basin.
the
Lumbre
that
shales
Lumbre
Although
Piedra
agree
black
Piedra
the
from
200 to 600 m in
sedimentary.
theorighal
section,
the
62
graded bedding is locally preserved. In the Copper Hill
area,
the
contact
between
Piedra
Lumbre
andis Pilar
very
sharp, with Pilar rocks appearing highly deformed.
Ir the
Hondo
syncline,the contact
calc-silicates
transition
dark,
gradational
unusual
with
lithologies
numercus
marking
t.he
zone.
Piedra
more
and
is
Lumbre
diverse
lithologies
than
at
Copper
in
the
Hill,
Hondo
and
Syncline
probably
are
include
more
of the original stratigraphic section. The thickness of the
Piedra
Lumbre
folding
in
anda lack
the
of
syncline
marker
is
not
horizons,
measureable
and
no
due
to
stratigraphic
younging features are preserved. Most of rocktheinPiedra
Lumbre
consists
garnet-
and/or
of
thinly
banded,
staurolite-bearing
fine-
to
medium-grained,
quartz-muscovite-biotite
schist, phyllite, and metasiltstone. Excellent graded beds
are found locally. Lithologies are variable along strike,
but one
50-m-thick
section
measured
up
from
the
Pilar
Phyllite consists of: gray, laminated schists: laminats.-d
schistose
beds:
quartzite
gray
and
schist;
phyllitic
yellow,
schist
cross-bedded
with
small
crI3ss-
quartzite5 cm
with
cross-beds; gray phyllitic, garnet-bearing schist; and
massive
quartzite
with
finely
laminated
garnet-plagioclase
bands. The contact zone between Pilar and Piedra Lumb-e
contains
interlayered
black
slate
and
phyllite,
fine
schists, metasiltstones, cross-bedded knobby plagioclase
schists,
quartz-rich
black
metasiltstone,
amphibole
schist,
63
dark
calc-silicates,
quartzites,
Montgomery (1953) made
the
calc-silicate
complete
and
and
various
petrologic
related
rocks
phyllite,c.
descriptiors
from
the
of
Piedra
Lumbre
Formation in the Copper Hill area. Thicknesses of Piadra
Lumbre
sections
are
difficult
to
estimate
due
to
extremely
high amounts of internal deformation. The apparent
thickness
range6
of
from200 m to
Formation
these
Piedra
has
are
In
400
been
minimum
general,
Lumbre
m.
Formation
The
removed
top
by
of
in
the
the
Piedra
weathering
Aondo
and
syncline
Lumbre
erosion,
so
estimates.
schists
of
the
Piedra
Lumbre
contain
less
garnet, are more resistant to weathering, are darker, and
are
more
heterogeneous
than
the R6 schist
member
of
th?
Rinconada Formation. Piedra Lumbre protoliths include
laminated, thinly interlayered quartz sandstones and
siltstones, and carbonaceous shales.
Sedimentology
The
most
Precambrian
of
thorough
rocks
in
Eriksson (1985) on
the
the
Ortega
Group
sedimentological
New
Mexico
Ortega
was
Group
analysis
made
of
by
done
in
Soegaard
northern
New
and
Mexico.
Based on facies analyses in the Tusas, Picuris, Truchas, and
Pecos
areas,
they
concluded
accumulated on a shallow
marine
that
the
shelf
Ortega
that
Group
sloped
gently
the south and southeast.No evidence of subaerial exposure
to
64
was
found,so sediments
were
considered
subtidal
shelf
deposits. These rocks represent
a transgression in which
sediment
ended
input
with
initially
the
drowning
matched
basin
subsidence,
of
outer
shelf
the
and
but
which
deposition
of
black basinal muds (Pilar and Piedra Lumbre formations).
No
Proterozoic
Lumbre
supracrustal
have
been
rocks
identified
younger
in
any
than
of
the
these
Piedra
ranges.
Soegaard and Eriksson
(1985) found that sedimentary
structures
were
dominated
by
storms
on
the
outer
shelf,
and
tides on the inner shelf. Very low shelf slopes are
inferred
due
to
the of
lack
shallow-water
turbidites.
Soegaard and Eriksson
(1985) noted that depositioyal
models
for
mudrocks
accumulation
such
as
the
of
thick
Ortega
quartz
Group
are
arenites
not
well
and
lesser
constrained,
because no modern analogs have been found. In the geologic
record,
very
to
Lower
the
thick
sandstones
Paleozoic,
the
and
quartzites
Proterozoic,
and
are
restricted
the
late
Archean. Most thick, pure quartz sandstones probably
form
in a relatively
long-lived,
tectonically
stable
setting
during a major sea-level rise. Influx of mature
terrigeneous
relative
quartzose
sea-level
sediment
rise.
must
be
balanced
by
the
65
Plutonic
Rocks
General
The
Range
suite
was
of
granitic
originally
rocks
named
the
in
the
Dixon
southern
Picuris
Granite (1937)
by Just
during his reconnaissance mapping. Montgomery
(1953) called
these
rocks,
and
similar
rocks
to
the
south
in
the
Truchas
Range, the Embudo Granite.In a detailed field, petrologic,
and geochemical study of these rocks, (1976)
Long subdivided
four
major
intrusive
youngest,
Cerro
Alto
Porphyry,
Rana
Quartz
are
Picuris
exposed
range.
which
Metadacite,
he
named,
Puntiagudo
Monzonite,
and
from
to oldest
Granite
Penasco
Quartz
In addition to these four units from the
Monzonite.
southern
units
Range,
in
the
at
map
least
area
in
two
other
the
intrusive
eastern
half
b2dies
of
the
In this report, these are informally called th,?
Granite
of
Alamo
Canyon
and
the
Granite
of
Picuris
two
large,
Peak
outcrop distributions in Fig.
2.4).
The
East
south
Granite
of
the
elongate,
plutonic
rock
of
Alamo
Canyon
Picuris-Pecos
partially
which
are
fault
are
fault-bounded
separated
a strip
by
exposures
of
no-th-
of
Cenozoic
sedimentary rocks. In many of the best-exposed, non-faulted
(see
66
areas,
thin
zones
of steeply-dipping
Precambrian
orthoquartzite and Pennsylvanian sandstone, limestone, and
shale
separate
surrounding
both
textural
resemble
Granites
Cenozoic
Although
show
the
one
of
Alamo
Canyon
from
the
sediments.
the
eastern
variation
another
from
and
and
place
are
western
to
granitic
place,
probably
the
these
bcdies
rocks
result
a singleof
intrusive event. The eastern block generally consists
of
pink to white, medium-grained, weathered looking, mica-rich
granitic rock with euhedral megacrysts of feldspar. The
western block is more heterogeneous.
In the northern half,
black
flattened
amphibolite
lenses
float
within
a pinkish,
fine- to medium-grained granitic rock. These granitic rocks
are always weathered looking, fairly equigranular, and
commonly crumbly. In places, this rock could easily
b=
mistaken for an arkose.In the south, this unit appears
intrusive into Rio Pueblo quartz-eye schist. The Granites
of Alamo
Canyon
Pegmatites
exposures
are
just
All of the
nowhere
cut
voluminous
basal
in
the
Ortega
Group
southernmost
quartzites.
granitic
northwest U.S.
of Hill.
granitic
rocks
east
of the
Picuris-Pecos
fault contain at least one tectonic foliation. commonly
three
closely-spaced,
to weather
into
orthogonal
small,
angular
joint
blocks.
sets
cause
this
rock
67
The
Granite
of
Picuris
Peak
In the southeastern Picuris Range, on the west side of
the
Picuris-Pecos
rocks,
herein
interlayered
Blocks
of
fault,
called
with
medium-
the
to
coarse-grained
Granite
of Picuris
supracrustal
orthoquartzite
Vadito
within
Peak,
Group
the
granitic
are
country
plutonic
rock
rack.
suggost
an
intrusive relationship between the two. Contacts between
granitic
rock
and
parallel to bedding
supracrustal
in
the
The
Granite
of
textures
ranging
from
rock
country
Picuris
invariably
trend
east,
rock.
Peak ashows
variety
coarse-grained,
pink
of
loyal
feldspar-rich
rock to white, quartz-rich rock. In thin section, these
rocks
show
interlocking
mosaics
of
microcline,
plagioclase,
quartz, biotite,and iron-oxide minerals. Most samples
exhibit
considerable
Cerro
The
Alto
Cerro
foliated,
alteration
of
feldspars
and
mica.
Metadacite
Alto
stock-like
Metadacitea fine-grained,
is
gray,
body
Vadito
that
sits
within
Group
supracrustal rocks. Montgomery (1953) included this rock
with
the
unit
he
mapped
as
"Vadito
Conglomerate-felsitetl
(Vcf). Long (1976) described isolated sills of metadacite
in
nearby
amphibolites,
and
found
xenoliths
of
metadacite
nearby granitic rocks. Granitic rocks appear to cut the
in
68
main
metadacite
metadacite
Picuris.
that
the
oldest
of
the
Cerro
Porphyryor the
the
and
thus
Long
granitic
concluded
body
in
that
the
the
southerr
Bell (1985) found no evidence for later
D.A.
intrusion
Granite
is
body,
Cerro
Alto
Metadacite
Rana
Quartz
body
was
Alto
by
the
Puntiagudo
Monzonite,
and
concluded
contemporaneous
with
the
Vadito
amphibolite.
Puntiagudo
Granite
Porphyry
Long (1976) has described the Puntiagudo Granite
Porphyry as consisting of subequant, subhedral, Carlsb3dtwinned, 1 cm-long, microcline phenocrysts, and rounde-l
quartz phenocrysts ina plagioclase, k-feldspar, biotite,
muscovite matrix. Modally, the rock ranges from quartz
monzonite
The
to
granodiorite.
pluton
is
massive,
and
cross-cuts
Vadito
Grou?
schists and amphibolites along sharp contacts. The
intrusion nowhere penetrates Ortega Group rocks.
D . A . Bell
(1985) found
schists,
Rana
The
that
but
does
Quartz
Rana
the
Puntiagudo
not
intrude
intrudes
the
the
Vadito
Vadito
amphibolite.
Monzonite
Quartz
Monzonite
is
the
most
widely
exposed
the plutonic rocks in the southern Picuris Range. Contacts
of
69
with the other
(1976) noted
granitic
inclusions
bodies
of
are
poorly
Puntiagudo
in
exposed,
the
but.
Long
Rana,
suggesting that the Rana is younger. The Rana is generally
foliated,
medium-grained
biotite
quartz
monzonite
to
granodiorite, witha discontinuous fine-grained, gradational
border zone (Long,1976).
This body also is strongly
discordant
Group
with
A s with
suspected
Vadito
country
rocks.
the Puntiagudo pluton,
D.A. Bell (1985)
that
this
unit
intrudes
Vadito
schists,
but
not
Vadito amphibolites. In the one area where the Rana pluton
is
in
border
D.A.
contact
zone
with
that
the
Vadito
everywhere
amphibolite,
else
rims
the
the
fine-grained
pluton
is
ab3ent.
Bell (1985) proposed that this contact is an
unconformity
separating
older
Vadito
schists
and
granitic
plutons, from younger Vadito amphibolites. The
PuntiapdoRana contact is
a fault in most places. In unfaulted areas,
the
Rana
pluton
Penasco
intrudes
Quartz
Puntiagudo
rocks.
Monzonite
Long (1976) found the Penasco Quartz Monzonite to be
the
youngest
The
Penasco
plutonic
is
body
generally
in
the
southern
conformable
with
Picuris
Vadito
Range.
Group
in its northern exposure. Penasco rocks are less foliated
than
the
sphene
other
quartz
granitic
monzonite
rocks
to
in
the
area.a biotiteIt is
granodiorite,
that
locally
rocks
70
contains abundant, large (up 9to
cm long) megacrysts of
Carlsbad-twinned
microcline,
and
mafic
xenoliths
alonq
its
borders.
D.A. Bell (1985) found that the Penasco pluton irtrudes
the
Cerro
Vadito
Alto
Metadacite,
the
Vadito
schists,
and
the
amphibolites.
Pegmatites
Pegmatites
are
common
in
the
southwestern
Picuris
Range, and cut all granitic rock types. Long
(1974) divided
them into five groups:1) small, simple pegmatites with
microcline, quartz, albite, muscovite, and green beryl:
2)
larger,
variably-zoned
bodies
with
unusual
mineralization
(e.9. the Harding Pegmatite):
3) small, zoned bodies
of
albite, quartz,and muscovite; 4) small dikesof pegmatiteaplite: and 5 ) medium-sized dikes of K-feldspar,
plagioclase, quartz, and tourmaline. These pegmatites
commonly
occur
cut
widely
across
in
abundant,
most
rock
deformed
quartz
veins
that
types.
Several, 2 to .I-m-thick, concordant, simple pegmatites
of
quartz-microcline-albite-muscovite
have
intruded
Pilar
Phyllite in the west-central map area. These pegmatites are
aligned
within
the
themselves folded.
hinges
of
macroscopic
folds,
but
are
not
71
Summary
of
The
General
Picuris
Field
Range
Relations
contains
three
supracrustal
rock
sequences and at least five different plutonic bodies. In
all
rocks,
compositional
layering
and
foliations
a? dip
average of60° south. Cross-beds in Vadito Group quartzites
suggest an overall younging to the north. Thus, as
illustrated
bya cross-section
the
Group
Vadito
through
structurally
overlies
underlies the Ortega Group (Fig.
3.4).
Pilar
structurally
and
the
Picuris
and
RanTe,
stratigraphically
The felsic schist at
stratigraphically
underlies
the
Ortega Group. Thus, the Vadito Group and the felsic s"hist
at
Pilar
occupy
identical
stratigraphic
positions
with
respect to the Ortega Group. The nature of the
relationships
The
among
Vadito
all
Group
three
is
of
these
extensively
terranes
intruded
is
by
unk?own.
synte-tonic
granitic plutons. No plutons intrude Ortega Group roclcs or
the
felsic
schist
at
Pilar
(however,
the
base
of
the
f'?-lsic
schist is not exposed). The pre-tectonic Cerro Alto
Metadacite
intrudes
the
amphibolite
unit
of
the
Vadito
Group, but not the schist unit. The syntectonic Puntiagudo
and
Rana
plutons
intrude
the
schist
amphibolite unit ( D . A . Bell, 1985).
Penasco
pluton
intrudes
both
unit
but
not
the
The post-tectonic
the
schist
and
amphibolite
units of the Vadito Group. These relationships suggest that
the
Vadito
Group
is anot
simple,
continuous
stratigraphic
72
a
a
VADITO FELSIC SCHIST
1460 M a GRANITIC
ROCKS
PIEDRA
LUMBRE
1 6 8 0 ~a GRANITICROCKS
-C o n t a c t
-
Ductilefault
Bedding-parallel
s h e a r zone
"
0
0
PILARPHYLLlTE
0
V A D ~ T ORUASTZITE
SCHIST
VADITO
RINCONADA
ORTEQA
OUARTZITE
VADITO
AMPHIBOLITE
FELSIC SCHIST AT PILAR
Figure 3.4.
Generalized south-north cross-section
through
the Picuris Range. No vertical exaggeration.
See text for
discussion.
73
package,
but
separating
The
part
rather
different
quartz-eye
of
the
of
Range
are
found
in
Santa Fe
faults
and/or
lithostratigraphic
felsic
schist
at
unconformities
units.
Pilar
is
similar
to
redefined
Vadito Groupll that Williams et al.
(1986) described
Parts
contains
the
in
the
more
mafic
similar
the
to
Tusas
Range
Tusas,
(Pecos
and
Rio
Mora
Group
in
older
mafic
metavolcanic
(Moppin
greenstone
the
areas.
Vadito
the
Range
Taos,
metavolcanic
belt),
and
southern
sequences
series),
the
Picuris
Taos
the
Rqnge.
Williams (1987) suspected that each of these may be se?arate
older greenstone-type terranes of different ages. If mafic
rocks
is
in
the
probably
because
it
southern
best
to
Picuris
maintain
may aberock
group
Range
the
are
a terrane,
such
original
without
it
Vadito
Group
name
stratigraphic
equivalents.
The
Vadito
metavolcanic
None
of
Group aas
whole
packages,
and
older
mafic
the
orthoquartzites
such
as
the
is
unlike
felsic
terranes
the
both
the
pre-Ortega
contain
Marquenas
mafic
packal-ges.
thick
Quartzite
Formation
of
the Picuris Range. None of the other felsic packages are
intruded by granitic plutons. Thus the Vadito Group of the
Picuris Range is anomalous.D.A. Bell (1985) suggested that
a major
unconformity
separated
the
Vadito
schists
from
the
Vadito amphibolites. He suggested that this unconformity
was subsequently folded. Early, near-bedding parallel
ductile faults cut Vadito and Ortega rocks to the north. It
74
is possible that the contact that
D . A . Bell (1985) suEpected
was a folded
unconformity
is
actually
a folded
faultcr
series of folded faults in the Vadito Group.
so, If
these
faults
may
have
juxtaposed
rocks
from
totally
different
tectonic terranes (i.e. the amphibolite, schist, and
metaconglomerate/quartzite units
Furthermore,
represented
that
the
the
by
very
Vadito
boundary
of
different
schist
between
Marquenas
schist
ora bedding-parallel
The
Picuris
Vadito
sedimentary
and
unconformity
southern
the
and
Group).
environments
Quartzite
suggest
metaconglomerate
is
an
fault.
Range
Vadito
Group
may
consist
fragments of various lithostratigraphic sequences. This may
be
the
reason
characteristics
Alternatively,
that
of
the
both
there
are
Vadito
Group
types
of
two
other
possesses
pre-Ortega
terranes.
possibilities
to
aclount
for the incongruity: 1) there is more lithologic variety to
either
the
previously
the
felsic
been
southern
or
mafic
terrane
recognized; 2)
or the
Picuris
Range
is
(or
entire
entirely
both)
than
has
Vadito
Grxp of
distinct
from
b'3th
the felsic and older mafic metavolcanic terranes. For this
paper,
these
rocks
will
be
considered
a single
as
called the Vadito Group, sensu stricto 3.5).
(Fig.
package
of
75
PRECAMBRIAN STRATIGRAPHY
PICURIS RANGE, NEW MEXICO
E
Pilar Phyllite
Rinconada
Ortega Quartzite
I
”””_
FAULT
\
/
?\
FAULT?
/?
Felsic schist
~Zq+?f-PTJ
R
Granite
M
SOUTHERN PICURIS
NORTHERN
PICURIS
Figure 3 . 5 . Possible stratigraphic relationships among the
Ortega Group, Vadito Group, and the felsic schist at Pilar
in the Picuris Range. The felsic schist at Pilar may be
equivalent to some of the Rio Puebloof Schist
Montgomery
(1963). Vf = Vadito Group felsic schist. Va
= Vadito
amphibolite. Vs = Vadito schist. Vq = Marquenas Quartzite
and similar Vadito Group quartzites and metaconglomerates.
CHAPTER
4.
GEOCHRONOLOGY
Introduction
Prior
to
this
crystallization
Range.
No U-Pb
of the Vadito
absolute
Group
study,
ages
of
zircon
Group
ages
because
Several
recent
plutonic
rocks
are
no
attempts
supracrustal
ages
of
have
the
for
metavolcanic
zircon
the
dates
been
in
Picuris
exist
have
to
Picuris
for
any
unit
Range,
and
no
of
the
Orteya
in
the
section.
been
Picuris
made
the
published
deposition
rocks
southern
have
rocks
been
southern
available
U-Pb
in
no
fo’r
reported
Range,
and
one
study
has examined detrital zircons in the Ortega Quartzite. The
remaining
geochronology
in
the
Picuris
Range
has
been
limited to the K-Ar and Rb-Sr isotopic systems. L.T. Silver
(personal
communication
reported a U-Pb
zircon
in
age
Grambling
and
of
1700
about
Williams,
Ma
for
1985b)
felsic
metavolcanic rocks in the Tusas Mountains. Grambling and
Williams (1985b) suggested that these rocks could be
correlative with the felsic schist at Pilar. All previously
published
Picuris
Figure 4.1.
Range
geochronology
is
date
summarized
in
77
Ma
U-Pb zircon
Rb-Sr whole. rock
1200
,234263 Embudo (miner
.23S&Z3 isochron) (1)
235519 a i o t i t e , SE of
Trangas
(3)
1300
335 Muscovite in
s i q l e peg
270
5
(3)
30 Harding Peg.
(6)
1400
1450 Penasco ( 7 )
1470 + 20 Penasco
430 Penasco (2)
440 L 160 Rana ( 6 )
(4)
1500
550
5
140 Puntiagudc
(6)
1600
16 30 Cerro
Alto
(7)
1700
1665 "Rio P u e b l o S c h i s t '
16 73 + 5 Rana (7)
1684 + 1 Puntiagudo ( 7 )
570 Rana
709
(2)
-+ 41 "Embudc
Granire" (1)
1800
1830
-+ 2 1 d e r r i t a l
z i r c o n s , Orcega
Quarrzite (5)
1900
Figure 4.1.
Summary of previous geochronologic work in the
Picuris Range. Ages recalculated using decay c0nstant.s of
Steiger and Jager (1977).
78
Previous
Work
The
first
Picuris
geochronologic
Range
consisted
and
lepidolite
muscovite
et al., 1958).
investigation
of
K-Ar
from
the
and
of
model
Barding
rocks
Rb-Sr
in
the
study
Pegmatite
of
(Aldrich
Samples yielded an average value
of
approximately 1300 Ma for the age of the pegmatite. Upon
reinterpretation of these data, Long
(1976) proposed that
the
Rb-Sr
mineral
isochron
suggested
that
the
pegmatite
may
be as old as1375 Ma. Register (1979) found an Rb-Sr age of
1370
& 30 Ma
for
the
Harding
Gresens (1975) determined
for
pegmatitic
Fullagar
isochron
muscovite
and
Pegmatite.
an
in
Rb-Sr
the
of
of
southern
(1973) produced
Shiver
analysis
age
the
Wmbudo Granite"
an
in
about
1335 Ma
Picuris
Rb-Sr
the
Range.
whole-rock
south'zrn
Picuris Range. Long (1976) suspected that the resulta?t age
of 1673 5 41 Ma
was
actually
a composite
age
of
several
different plutons. Upon reinterpretation of field relations
(1976) deduced poorly constrained
of these samples, Long
ages
of1673 Ma
for
the
Rana
Quartz
Monzonite,
1400and
Ma
for the Penasco Quartz Monzonite. Fullagar and Shiver also
derived
mineral
isochrons 1208
of & 63 and 1212 & 23 for
"Embudo Granites". Fullagar and Shiver(1973) suggested
that a thermal
event
may
have
reset
the
system
at
about
1200
Ha. Long (1976) also susected that the significance of
these
ages
is
uncertain
due
to
partial
or
total
resetting
of
79
the
Rb-Sr
and
K-Ar isotopic
Gresens (1975) found
in
rocks
southeast
of the
systems.
K-Ar
biotite
town
of
dates 1235
of k 19 Ma
Trampas.
Gresens (1975) proposed the following history for
Precambrian rocks in the southern Picuris Range: intrusion
of
theVmbudo Granite"
metamorphism
at1673 Ma: tectonism and
at1425 Ma: a thermal
event
at
1350 Ma: a?d
tectonism and hydrothermal activity 1250
at Ma. Long (1976)
postulated
that
the
overlap
of K-Ar
represented a thermal
In a Rb-Sr
and
Rb-Sr
dates
1220-1250 Ma.
at
event
geochronologic
study
of
granitic
rocks
in
the southern Picuris Range, Register
(1979) reported d%tes
of 1550 k 140 Ma
1440
+
for
160 for
the
Two workers
the
Rana
have
Puntiagudo
Quartz
reported
Granite
Porphyry,
and
Monzonite.
U-Pb
zircon
crystallization
(1976) sam?led
ages for rocks in the Picuris Range. Maxon
detrital
zircons
in
the
Ortega
Quartzite
from
two
localities
in the cliffs near Pilar. Several different shapes
of
zircon populations were recognized. Concordia diagram:
contained
upper
and
lower
of 1830 + 21 and 436
intercepts
Ma, respectively. The age of 1830 21
of crystallization
unknown source
Ortega
of
area,
zircons
in
and
a maximum
Ma is a minimwn age
some
age
igneousfxom
rock
some
for
deposition
of the
Quartzite.
D.A. Bell (1985) performed U-Pb zircon chronology on
the
four
granitic
plutons
in
the
southwestern
Picuris
Range.
80
He reported
Porphyry, 1673
1630 Ma
the
of
1684 2 1 Ma
dates
for
the
Puntiagudo
Granite
3 Ma for the Rana Quartz Monzonite, about
Cerro
A l t o Metadacite,
the
Penasco
for
Quartz
and
about
1450 Ma
for
Monzonite.
Discussion
These
Rb-Sr
crystallization
and
K-Ar dates
ages
due
may
to
be
minimum
isotopic
estimates
resetting
of
during
thermal overprinting. Resetting is not
a problem with the
U-Pb
zircon
dates
reported
in
recent
investigations.
Therefore, 12 samples for geochronology were collected for
the
present
study.A preliminary
U-Pb
zircon
date
has
thus
far been generated for only one sample.
A sample of Rio
Pueblo
the
Schist
from
southeastern
the
type-section
Picuris
Range
at
Comales
campgroxnd
in
yielded
a relatively
concordant age of about
1665 Ma (S.A. Bowring, personal
communication, 1986).
The rock dated is texturally
different
felsic
from
the
schist
at
Pilar,
and
may
not
be
equivalent. Ages of the remaining samples willrep’xted
be
in a future
Ten
of
publication.
these
crystallization
samples
for
various
should
yield
volcanic
ages
and
of
plutonic
units
the Picuris Range. The other two samples are from
metasedimentary rocks. One is designed to investigate
detrital
zircons
in
quartzite
clasts
from
the
Marquenas
in
81
Quartzite
Formation,
investigate
the
and
the
of
peak
age
other
was
collected
metamorphism
in
to
the
Ortega.
Group.
The
timing
Precambrian
of
rocks
metamorphism
in
northern
and
New
deformation
of
Mexico
has
previously
been
attempted with the Rb-Sr system. Rb-Sr whole-rock and
mineral
isochron
studies
structural-metamorphic
absolute
ages
of
in
work
conjunction
have
been
with
used
structural-metamorphic
detailed
to
events
determine
in
multiply
deformed-metamorphosed rocks (Black et al., 1979;
Marjoribanks and Black, 1974). Deformational events that
cause
strong
penetrative
cleavages
are
sufficient
to
homogenize Rb isotopes over
a large scale. By carefully
collecting
samples
schistosity,
whose
whole-rock.
fabrics
and
are
mineral
dominated known
by some
isochrons
can
yield
data
on the thermal-structural historyof the rock. Ward and
Grambling (1985), who
rocks
Ma
in
for
northern
successfully
New
Mexico,
development aofstrong
cleavage
in
schists,
applied
this
reported
an
syn-metamorphic
technique
age
of
to
around
crenulation
and
a cooling rate of 2-3OC/Ma at
depth. Although this strong crenulation cleavage may
correspond
Range,
toa similar
concurrence
of
fabric
in
schists
development
of
of
these
the
Picuris
fabrics
is
not
required.
for
Appendix 1 describes
the
the
and12 the
geochronology
present
study
zircon
separation
method
samples.
used
1425
CHAPTER 5.
GEOMETRIC FABRIC ELEMENTS
Introduction
Multiple
are
microscopic
developedto various
and
mesoscopic
degrees
in
tectonite
most
of
the
fabrics
Proterozoic
rocks in the Picuris Range. Correct identification and
interpretation
of
these
fabrics
is
critical
to
the
and describes the
structural analysis. This chapter defines
various
Range.
fabric
elements
in
Precambrian
rocks
of
the
Picuris
It is important to note that the character of the
fabrics
themselves
varies
between
areas,
and
between
lithologies within an area. In these cases, range-wid?
correlation
The
of
specific
defonnational
characteristic
set
of
fabric
episodes
suggest
that
the
that
structures
identified asDl, D2, and D3.
to
elements
area
becomes
difficllt.
produced
a
and
fabric
elements
are
This terminology is not meant
was to
subject
distinct
pulses
of
deformation. There may have been considerable temporal
overlap of formation of these 'Igenerations1l of structures.
Folding events are denoted
Fl, Fz, etc., with
associated cleavages SI, SZ*, S2, etc.
intersection
lineations
of
surfaces
L10 and L20 are
designated
by
subscripts. L1 and L2 are extension lineations formed
during Dl and D2 respectively.
83
Compositional Layering,SQ
In most rocks, compositional layering is thought to
represent original stratigraphic layering. Orthoquartzites
in
both
the
Ortega
Group
and
the
Vadito
Group
commonly
contain well preserved cross-laminations (Fig. 5.la).
Schists of the
graded
of
Piedra
bedding
the
in
pelitic
Lumbre
which
portion,
Formation
metamorphism
locally
has
and athus
"reverse"
preserve
caused
coarsening
grainsize
grading (Fig. 5.lb).
In some areas, compositional layering does not
correspond to sedimentary layering. Holcombe and Callender
(1982) suggested
Piedra
Lumbre
that
exposed
much
of the
layering
south
of Copper
Hill
in
the
was
slivlr
of
structurally
transposed. This is clearly the case locally inof many
the
schistose
unitsof the Picuris Range (Fig.
5.2).
Most quartzose rocks fracture along compositional
layers rather than later cleavages. Over most
of the range,
SQ strikes
First
east,
and
dips
around
60'
south.
Generationof Structures, Dl
The earliest recognized cleavage,
SI, appears asa
schistosity in quartzites and micaceous quartzites. Within
the
matrixof most
schists,
S1
appears
to
have
been
84
a
b.
Figure 5.1.
Photographs of primary sedimentary structures
a. Overturned
in Precambrian rocks of the Picuris Range.
cross-bedsinOrtegaQuartzite.
b. Overturnedgraded
bedding in the Piedra Lumbre Formation.
85
Figure 5.2. Photograph of transposed layering and F$(?)
folds in Piedra Lumbre Formation, southwestern Picurls
Range.
86
destroyed by subsequent cleavage formation. Porphyrotlasts
in
some
schists
contain
inclusion
trails
interpreted
to be
S1 is generally parallel
remnants of SI. In the quartzites,
or
sub-parallel
responsible
to
for
compositional
the
mica
sheen
layering,
visible
and
on
is
bedding
surfaces
in quartz-rich rocks. In the felsic schist at Pilar, the
dominant
foliation
is
bedding-parallel,
and
is
interpreted
In these schists, S1
is defined by
as S1 (Fig. 5.3).
aligned muscovite, opaque minerals, inequant quartz grains,
and flattened quartz and feldspar megacrysts. This S1
corresponds
to
the of
S1 Nielsen and Scott
(1979) and
Holcombe and Callender (1982).
The earliest imposed lineation, L1, is generally
a
down-dip
mineral
quartz-eye
5.4).
elongation
schists
near
best
the
seen
in the
quartzites
and
contact
(Fig.
Vadito-Ortega
Commonly, elongate grains of kyanite, sillimanite,
and quartz define this extension lineation.
In Marquexas
Quartzite
Formation
orientation
This
of
lineation
Nielsen
and
metaconglomerates
stretched
pebble
does
correspond
Scott
not
(1979),
who
the
clasts
to
defined
down-dip
may
represent
L1.
the
composite
L1
L1
as
the
intersection
between S1 and S2.
F1 folds are uncommon. None have been unequivocally
identified in the map area. Small, intrafolial, isoclinal
folds in the
Piedra
Lumbre
Formation
south
of
of
Copper
Hill
may be F1 folds. These F1 folds correspond to the rootless,
a7
Figure 5.3.
Photomicrograph of SI foliation in quartzmuscovite, felsic schist at Pilar. Field of view is 3.4 m
m
.
88
Figure 5.4. Photograph of down-dip L, extension lineation
on SI surfaces in Ortiga Quartzite; n k h w e s t e r n Picuris
Range.
89
intrafolial
foldsof Nielsen and Scott
(1979) and McCarty
(1983).
Second
Generation
of
The S2 cleavage
Structures,
D2
appears
as
either
a crenulation
cleavage or a schistosity. S2 is rarely the dominant
cleavage in rocks of the Picuris Range. In most schistose
rocks, S2 has been totally destroyed D3
bystructures. In
quartzose
rocks,
which
do
not
preserve
cleavagesS2 well,
appears locallyas a crenulation cleavage. S2 is generally
at a low angleto compositional layering. ThisS2 does not
correspond to the
S2 of either Nielsen and Scott
(1979) or
Holcombe and Callender(1982). Instead, the presently
defined S2 is
intermediate
The L2 extension
to
theL1 lineation
between
lineation
and
is
contains
their
S1 and S2.
parallel
or sub-parallel
aligned
biotite,
quar?z,
kyanite, sillimanite, and tourmaline. TheL20 intersection
lineation
is
extremely
difficult
to
separate
from
later
intersection lineations. These lineations do not correspond
to
theL2 and L20 intersection
lineations
of
Nielsen
and
Scott (1979) and Holcombe and Callender
(1982).
F2 folds
are
common,
range
from
tight
to
isoclinal,
range in size from microscopic to map-scale 5.5).
(Fig. In
quartz-rich rocks, S1 is folded byF2 folds. Axial surfaces
and
90
a.
b.
Figure 5.5.
F2 antiform
the Piedra
disharmonic
Photographs of F2 folds. a. Photograph cf open
with well developed axial plane cleavag,? from
Lumbre Formation.
b. Photograph of tight,
F2 folds in the Piedra Lumbre Formation.
91
are
generally
from
40°
are
identified
reclined
to 80°.
to
the
north,
with
dips
southerly
Commonly, at the outcrop scale,
F2 folds
only
by
the
occurrence
a strong
of S3
cleavage that transects both limbs of the fold. F2These.
folds
and
correspond
to
F2
thestructures
described
by
Nielsen
Scott(1979) and Holcombe and Callender
(1982).
Third
Generation
of
The S2* cleavage
Structures,
D3
is
the
dominant
cleavage
in
nearly
all
rocks of the Ortega Group. Most schistose rocks break along
the s2* surfaces. S2* is generally oriented within
20° of
compositional layering. In schists,S2* is well-developed,
and is commonly the youngest visible cleavage. In mor?
quartzose rocksS2* may be a crenulation cleavage. Rarely,
quartz-mica
cleavage
schists
preserve
formation,
microfolded
and
in
an
intermediate
by
differentiated
aligned
of
whichS2 an
schistosity has bee?
into
mica-rich War%and
rich zones of an
S2* crenulation (Fig. 5 . 6 ) .
defined
stage
muscovite,
biotite,
S2*
is
opaque
minerals,
tourmaline, and flattened quartz grains. This
S2*
corresponds
to
the
S2 of Nielsen and Scott
(1979) and
Holcombe and Callender(1982).
L2* is a moderately
in
some
schists,
well-developed
defined
primarily
extension
by
aligned
lineation
biotite
and/or
92
Figure 5.6.
Photomicrograph of vertically spaced S 2 * which
has crenulated an earlier oblique S 2 ( ? ) cleavage.
93
tourmaline grains (Fig. 5.7a). L2*0 is the dominant
intersection
lineation
parallel
to
the
L ~ * ois
most
in
most
of F2*
hinges
evident
as
rocks,
folds
and
within
is
any
generally
local
compositional on
bands
S2*
area.
cleavage
surfaces in thinly layered schists (Fig. 5.7b). The L2* and
L ~ * olineations
(1979)
and
correspond
the
L20
of
to
Nielsen
the ofL2
Holcombe
and
and
Scott
Callender
(1982).
F2* folds are rare. In some cases, at the outcrop
scale,
where
the
fold
contains
an axial
plane
schistosity
but lacks the lttransectinglt S2* cleavage, F2 and F2* folds
cannot be told apart. Locally, on the overturned limb of
the
Hondo
syncline,
north
of
Warm
Springs,
it
appears
that
.
F2* folds have overprinted F2 structures (Plate
1)
These
F2*
by
folds
Nielsen
do
and
not
correspond
Scott
(1979) and
to
any
Holcombe
fold
and
generation
desxibed
Callender
(1982).
Later
At
Generationsof Structures
least
two
generations
of
non-penetrative
(on
an
outcrop scale) structures post-date D3 structures in the
region. Strain accumulation associated with these
structures appears to be minor. The major late structure
appears as local, weak, cross-cutting crenulations in some
schists (Fig. 5.8).
Associated with these are intersection
~
94
a.
b.
Figure 5.7.
Photographs of second generation lineations in
the Ortega Group. a. L2* bi2tite extension lineation on S 2 .
Scale bar is in cm. b. L2 0 intersection lineation on S 2
surface.
95
b.
Figure 5.8.
a. Photograph of S-, crenulation cleavage in
Piedra Lumbre schist. b. Photomicrograph of S3 crenulation
m.
cleavage in Rinconada schist. Field of view is 8 m
96
lineations and very open folds. In
a regional sense, these
features
trend
north-south,
approximately
perpendicukr
to
the earlier structures. These structures correspond
to the
S3, L3, and F3 structures of Nielsen and Scott (1979), and
the S3, L30, and F3 structures of Holcombe and CallenZer
(1982).
Within the map area, none of the fourth generation
ductile
offset
recognized.
features
of
Nielsen
and (1979)
Scott were
CHAPTER 6.
Introduction
and
Although
has
have
never
agreed
regionally
in
rigorously
that
Ortega
index
and
all
rocksof the
Work
metamorphism
metamorphosed
Metamorphic
groups,
Previous
the
been
METAMORPHISM
Orteqa
rocks
examined,
and
to
minerals
three
of
all
Vadito
middle
are
aluminum
to
high-grade
the
silicate
have
throughout
both
polymorphs
are
dynamo-thermal
history
regional
of
metamorp’lism,
isograds separating sillimanite, kyanite, and staurolite
zones. These isograds were coincident with lithologic
contacts.
related
intensity
of metamorphism to
timing of deformational events. He found that prograde
regional
metamorphism
deformations,
and
found
Group.
metamorphism. In the Ortega Group, Montgomery traced
(1972)
been
facies.
widespread hydrothermal metamorphism, and retrograde
Nielsen
Range
studies
rocks
amphibolite
abundant
Picuris
previous
group
Montgomery (1953) described a metamorphic
medium-
in
peaked
between F2
the
and F3
that
a regrogressive
peak
occurred
after
the F4 event.
Holdaway (1978) performed
a metamorphic study of Ortega
Group rocks. He described A12Si05 triple point conditions,
98
and
documented
andalusite
+
kyanite
kyanite,
+ sillimanite, chloritoidf staurolite, an?
that
the
Long
+
assemblages
of chloritoid
andalusite. Based on these assemblages, he
concluded
kb in
key
metamorphism
Picuris
(1976)
had
peaked
at
about 3.7
53OoC,
Range.
reported
on
conditions
of metamorphism
of
Vadito Group rocks
in amphibolites, pelitic schists, and
calc-silicates. Various mineral assemblages permitted him
to deduce
upper
temperature
and
pressure
limits
of
about
6OO0C and 3.7kb. Long also developed
a model for the P-T
path with respect to the deformational history. In this
model,
temperature
pressure,
towards
increased
the
at
aluminum
about
4OoC
per
kb of
silicate
triple
point
(Holdaway, 1971), until achieving peak conditions between
the
F2
and
Following
F3
the
retrogressive
deformational
metamorphic
stagesof the
peak,
P-T
(1976), in which P and T varied
scale
during
an
overall
events
of
Nielsen
several
path
were
considerably
isothermal
(1972).
highly
conjectlral
proposed
by
Long
on
a small
pressure
decrease.
McCarty (1983) also examined metamorphism in Vadito
Group rocks. She concluded that
P and T increased during F1
and
F2
deformations,
until
peaking
during
or
after
the growth of biotite, andalusite, and cordierite(?)
porphyroblasts. Peak temperature was between 525OC and
6OO0C, and peak pressure was
3.7
kb.
These estimates are
compatible with Holdaway's(1978) estimates in the Ortega
F2
with
99
Group. McCarty noted that because andalusite may not be
pure
3.7
A12Si05,
kb.
triple
point
pressures
may
be
slightly
above
Peak conditions were followed by retrogressicn and
alteration
of
andalusite
muscovite+chlorite,
and
to
muscovite,
growth
of
cordierite
garnet
and
to
staurolite
porphyroblasts. After garnet growth, further retrogression
resulted
in
alteration
of
biotite
to
chlorite.
Grambling and Williams (1985a) investigated aluminum
silicate
phase
relations
in
Precambrian
rocks
of
north-
central New Mexico. In the Picuris Range, they found
that
kyanite-andalusite-sillimanite
assemblages
Hill
of3.8 2 0.5 kb and 505 2
regionpreseme conditions
in
the
Copp?r
3%.
All of
these
rocks
suggest
triple
point
the
that
estimates
both
conditions
Picuris
Peak
P-T
for
groups
after
the
were
Ortega
and
metamorphosed
mar
the oftime
major
folding
Vadito
to
in
Range.
P-T
conditions
and
the
various
mineral
assemblages
in Precambrian rocks in the Picuris Range are well
established, so rather
assemblages
following
and
mineral
Ortega
possible
sections
distribution
of
growth
Group,
Relative
than
of
and
chapter
silicate
respect
possible
timing
previous
metamorphic
this
aluminum
with
repeat
to
P-T
criteria
work
reactions,
will
paths
between
the
emphasize
polymorphs,
fabric
on
the oftiming
development
for
the
these
in
rocks.
porphyroblasts
and
the
100
matrix
are
based
mainly
on
those
described
by
Vernon
(1978),
Olsen (1978), T . H . Bell (1985), and Bell et al. (1986), Bell
and Rubenach (1983), Wilson (1971), Williams and Schoreveld
(1981), Zwart (1962), and Spry (1969).
Metamorphic
Mineral
Aluminum
Assemblages
Silicate
Minerals
General. Kyanite, andalusite, and sillimanite are
common
minerals
in
quartzites
and
schists
of
the
Ortega
Quartzite and Rinconada formations. In many rocks two of
these polymorphs appear to stably coexist. Near Copper
Mountain (M.L. Williams, personal communication, 1987) and
in a few
locations
in
the
northwestern
map
area,
all
tllree
may stably coexist. Holdaway (1978) also noted that all
three coexist in the Hondo syncline area. The occurre?ce of
these
minerals
variations
seems
toa function
be
of
both
litholoqr
and
inP-T topology.
Kvanite. Kyanite is a common mineral in the Ortega
Quartzite and Rinconada formations. Along the northern
exposure
of
Ortega
coexists
with
Quartzite,
sillimanite,
or
kyanite
occurs
generally
either
alone
quartzites
in
aluminous schist layers within quartzite.
Less commonly,
and
101
kyanite
map
coexists
area,
Along
kyanite
the
kyanite
with
coexists
southern
either
andalusite,
with
exposure
coexists
in
one
andalusite
of
with
and
Ortega
locality
and
or
the
sillimanite.
Quartzite
andalusite,
in
is
Formation,
found
alone
in quartzites. Where kyanite is found in quartzites a7d
schists
it
of
occurs
the
Rinconada
either
alone,
Kyanite
grains
are
Formation
or
in
coexisting
generally
the
central
with
elongate
map
area,
andalusite.
subhedral
blades
or euhedral prismatic crystals. Grains range in length from
less than0.5 mm to greater than
15
mm.
Inclusions in
kyanite are generally not abundant. Some grains have been
partly
altered
Kyanite
to
sericite
grains
are
or
pyrophyllite.
typically
aligned
in
dominmt
the
foliation in the rock. Typically, in quartzites, kyaniterich
layers
are
parallel
and
spaced
between
quartz-ric’l
matrix. In schistose quartzites, foliations wrap arou?d
isolated kyanite grains. Nearly all kyanites show domninal
and/or undulatory extinction (Fig.6.1).
kyanite
grains
define
a strong,
down-dip
In many rocks,
extension
lineation. In other rocks, kyanites have overgrown
an13
preserved
preexisting
inclusions
The
growth
of
occurred
coincided
(S,*)
defined
by
small
aligxed
iron-oxide.
evidence
cleavage
foliations
suggests
prior
to or
that
during
most,
if
formation
not
all,
kyanite
of
the
dominant
in these rocks. Kyanite growth may have
with
the
Dl strain
that
produced
a strong
down-dip
102
.
.
~
.
.
Figure 6.1. Photomicrograph of strained kyanite blades
~aligned in SI and L1.
Section is cut parallel to L1 ard
perpendicular to SI. Field of view is 10
mm.
103
extension. Kyanite appears to have remained stable (or
persisted
metastably)
metamorphic/tectonic
during
most
of the
historyof the
subsequent
rocks.
Andalusite. Large porphyroblasts of andalusite are
found
in
many
of
the
the Rinconada
schistose ofunits
and
Ortega Quartzite formations. In particular, andalusite is
abundant
in
the
R1/R2
schist
that just
liesabove
the
Ortega
Quartzite Formation. Andalusite occurs in the northern and
southern
bandsof Rinconada
Formation,
and
local
pelitic
Vadito schist beds in the southern map area. Andalusite is
the
only
common
aluminum
silicate
mineral
in
Vadito
Group
rocks. Montgomery (1953) mentioned coexisting andalusite
and
sillimanite
southern
in
Picuris
an
unlocated
area
in
Vadito
rocks
of the
Range.
Typically, in schists, andalusite porphyroblasts are
dark,
8
vaguely
rounded,
poikioblastic
knobs
that
can
be
up
cm in diameter. Rounded quartz inclusions are invariably
abundant within andalusites. Commonly, these inclusions are
prolate, and are aligned in
a relict foliation. Other
common included minerals are biotite, muscovite, iron-oxide
mineral, and garnet. Most commonly, andalusite is
accompanied by kyanite. No rocks containing andalusite and
sillimanite without kyanite have been found. Alteration
of
andalusite is rare.
Andalusite
porphyroblasts
generally
have
equant
shapes,
to
104
and
are
therefore
not
aligned
or
flattened
in
the
dominant
foliation. The schistose matrix of most rocks warps around
andalusites. Well-defined inclusion trails of e1ongat.e
quartz
typically
foliation, S z * .
appears
to
therefore
are
continuous
with
the
surrounding
nratrix
In one rock, this included relict foliation
have
been
a crenulation cleavage. Andalusite has
trapped
an
intermediate
stage
of S2* development.
Other andalusites contain relict folds (F1 or F2?) defined
by quartz inclusions or opaque minerals. Most andalusites
are
optically
textures.
continuous
in
spite
of
their
sponge-like
In most rocks where andalusite and kyanite
coexist,
relative
timing
relationships
between
the
two
are
ambiguous.
Andalusite
Sz?)
, and
has
overgrown
coincides
foliation
in
with
an
early
development
foliation
(SI o r
of
the
dominant
S2*
schists.
Sillimanite. The occurrence of sillimanite is mo-e
restricted than that of either kyanite or andalusite. In
the
map
area,
sillimanite
is
found
only
in
the
northern
exposure of Ortega Quartzite Formation. Within these rocks
it
has
grown
in
both
quartzite
and
aluminous
schist.
Grambling and Williams (1985a) also reported sillimanite
occurrences
the
town
in
of
the
Ortega
Group
east
of
Copper
Hill
and
Pilar.
Sillimanite
occurs
as
both
fibrolitic
aggregates
of
near
105
radiating
acicular
bundles (Fig. 6.2).
crystals
and
strongly
lineated
fibrolitic
Aggregates range in size from
1.C to
3.0
mm, whereas individual crystals are generally lese than
0.4
mm long. The cores of these bundles are pure, coarse
fibrolite,
whereas
the
rims
contain
only
minor
amounts
of
long, needle-like fibrolite. Sillimanite occurs either as
the
lone
although
aluminum
on0
silicate
thin
polymorph,
section
from
the
or
with
kyanite,
northwestern
map
area
contains all three polymorphs. In some thin sections, where
sillimantite
extend
thin
coexists
across
with
kyanite
sections
where
kyanite,
grain
they
sillimanite
boundaries,
coexist,
whereas
sillimanite
needles
in
oth?-r
grains
dl
cross kyanite grain boundaries. Sillimanite and andalusite
without kyanite are not found in the map area.
s o m In
rocks, sillimanite is replaced by fine, fibrous grains
presumed
to
Although
oriented,
be
pyrophyllite.
some
most
sillimanite
show
a slight
bundles
flattening
are
in
randomly
the
plane
of the
dominant S2* foliation. Within and around sillimanite
aggregates, sillimanite crystals invariably cross quartz
grain boundaries. Many sillimanite crystals show some
amount
of
Some
strong
internal
strain
well-lineated
through
samples
alignmentof sillimanite
of
grains
undulatory
Ortega
in
extinction.
Quartzite
a
show
the
L1 extension
direction. These grains are interpreted to have grown preto S p - D l .
not
106
a.
b.
Figure 6.2. a. Photomicrograph of fibrolitic sillimanite
aggregates strongly aligned in Ll, from Ortega Quartzite,
northern Picuris Range. Section is cut parallel to L1 and
perpendicular to SI. Field of view is 14 m
m. b.
Photomicrograph of fibrolitic sillimanite bundles in Ortega
Quartzite. Field of view is 3.4 m
m
.
107
It appears
Quartzite
However,
that
sillimanite
throughout
because
much
the
Dl
of
has
the
strain
grown
in
the
deformational
is
Ortega
history.
heterogeneous,
absolute
timing relationships are uncertain. Vernon
(1987) noted
that
if
sillimanite
aggregates
grow
in
regions
of
low
strain
(e.g. quartz-rich rock), then their growth is not related to
strain
in
terms
of
the
bulk
heterogeneous
shortening
model
of Bell (1981). Random bundles do not necessarialy imply
post-deformational
growth
on
scales
larger athan
thin
section. Therefore, in these quartzites, sillimanite may be
pre-
or
syn-kinematic
unstrained
due
Alternatively,
the
early
to
with
the
respect
to
D3,
but
may
remain
heterogeneity
of strain.
ifDl structures developed throughout m-lch of
deformational
history,
the
lineated
sillimanite
could have grown at almost any time. Although timing
relationships
remain
stable
much
of the
over
unclear,
sillimanite
deformational
was
probably
history.
Discussion. There appears to be
a difference in the
occurrence
southern
of
aluminum
exposures
of
silicate
Ortega
phases
Quartzite
in
the
northern
Formation
in
and
the
area. Stratigraphy and lithologies are identical across
the northern and southern limbs of the Hondo syncline. On
the southern limb, kyanite coexists with andalusite. On the
northern limb, kyanite coexists with sillimanite, and
locally with both sillimanite and andalusite. Grambling and
map
108
Williams (1985a) suggested that these variations correspond
to
variations
horizontal
in
elevation
isograds,
with
across
kyanite
an
area
occurring
containing
cubat
the
highest
elevations, andalusite(+ kyanite) at intermediate
elevations, and sillimanite( 2 kyanite and andalusite) at
the
lowest
elevations.
Andalusite,
kinematic
kyanite,
with
respect
and
to
probably
sillimanite
dominant
s ~ foliation
*
the
are
in
pre-
most
rocks. Kyanite grains are generally aligned ain
first
generation extension lineation (Ll), and exhibit undulose
extinction. Andalusite typically has overgrown and
preserved an early foliation
(Sz?).
relationships,
kinematically
late
kyanite
with
is
interpreted
respect
syn-kinematically
Based on these
to
or
Dl,
as
and
having syngrown
andalusite
post-kinematically
with
has
grown
resp3ct
to
D2 *
In strongly
lineated
Ortega
Quartzite,
sillimanit?
appears to have grown pre- to syn-Dl. In other samples,
sillimanite appears to have grown syn- to post-D3. Beoause
Dl
structures
different
probably
areas,
the
developed
absolute
outside of small
areas
is
In the
area,
kyanite
map
at
different
timing
of
times
mineral
in
growths
uncertain.
grew
during
shearing
of
the
Ortega Quartzite and Rinconada formations. Although folding
of the
the
Hondo
present
syncline
large-scale
may
have
post-dated
variations
in
kyanite
aluminum
growkh,
silicate
109
distribution
probably
across
due
to
the
axis
of the
variations
Hondo
in
syncline
elevation
are
of the
rocks
two on
limbs. Sillimanite is exposed in the deeper rocks at lower
elevations on the
northernlimb, whereas
in
rocks
the
shallow
at
higher
kyanite
is
exnosed
elevations
on
the
southern
limb.
other
Metamorphic
Minerals
General. Schists and quartzites of the Ortega Group
contain a large
variety
of
shapes
and
sizes
of
many
different mineral phases. This section will concentrate on
relationships
between
tectonite
fabrics
and
porphyroblastic
phases found within the map area. Common metamorphic
mineral
assemblages
found
in
Ortega
Group
rocks
are
given
Figure 6.3.
Biotite. Biotite is common in schistose rocks
throughout the range. Most Piedra Lumbre and Rinconada
formation schists contain
10 to 15 percent biotite. GTain
sizes
range
from
0.1 nun to
about
average sizeof about 0.6
euhedral
to
subhedral,
mm.
and
more
than
5.0 mm, with
an
Most biotites are blocer,
do
not
contain
significant
amounts of inclusions. Commonly, biotite grains cut across
boundaries between other minerals. Locally, chlorite is
intergrown
with
biotite.
in
110
r T
PIEDRA
LUMBRE
"cc
li
.I
Et
I
.
z
I
t
.I
4
ORTEGA
.e
I
I.
t
I
Is
)I
If
t
%
0 .
I
I
.
Figure 6.3. Common metamorphic mineral assemblages found in
rocks of the Ortega Group.
111
From
the
sample
orientation
to
sample,
of
biotite
there
is
grains
considerable
with
respect
ranre
to
in
strc.in
fabrics. This probably reflects multiple stages of biotite
growth duringthe deformational history. Several thir
sections
display
two
generations
of
biotite
growth
distinguished by orientation and grain size. Typically,
large
biotite
porphyroblasts
are
flattened
in
the
dominant
S2* foliation. These grains tend to show undulose
extinction,
ends,
and
Numerous
have
lie
strong
within
small,
quartz-rich
the
elongate
anastomosing
biotite
generally aligned obliquely Sto
z*.
small
In
grains
one
biotite
with
sample
of
respect
Piedra
porphyroblasts
in
pressure
shadows
S2*
crystals
at
both
foliation.
in
the
matrix
are
The timing of these
to
the
Lumbre
larger
grains
is
Formation
schist
(HC-62),
fine-grained
micaceous
uncertain.
matrix
have
been flattened and sheared (Fig. 6.4). In another highly
folded
layer
Piedra
has
Lumbre
been
sample,
a biotite-rich
folded
compositio?al
around
a strong S2*(?) axial plane
cleavage. Biotite grains in this layer are spaced and
aligned
At
in
the
least
cleavage.
one
period
of
biotite
growth
was
pre-kinematic
with respect to S2*. Large biotite grains are aligned
within an anastomosing S2* cleavage. Some appear to be
sheared as well. Other smaller biotites may be either
predate
or
postdate
biotite
grains.
growth
of
the
large
pre-
to
syn-S2*
112
Figure 6.4. Photomicrograph of sheared biotite fish in
Piedra Lumbre schist, showing dextral shear. Field of view
is 4 nun.
113
Garnet. Porphyroblasts of garnet are abundant
throughout
Garnets
the
are
Piedra
Picuris
Range
especially
Lumbre
well
Formation
in
schists
developed
and
the
R6
and
in
some
schists
member
of
quartzites.
of
the
the
Rinconada
Formation. Several horizons in the Piedra Lumbre Formation
contain
garnet
thin
layers
of schist
that
are
up60 to
percent
porphyroblasts.
Garnet
crystals
more than4 . 0
mm.
range
in
size
1.0 mm
to
less
than
from
Although most garnets are euhedral, some
grains, especially in quartzites, are
sub- to anhedral. In
general,
garnets
from
Ortega
Group
rocks
in
the
map
ar2a
tend to be relatively inclusion-free. Typically, garn3ts
that do contain
inclusions
tend
to
have
relatively
inclusion-rich cores and inclusion-free rims. Quartz is the
most abundant inclusion. Other included minerals are
biotite, chlorite, and zoisite. Alterationof garnet is
uncommon. In garnet-staurolite schists, garnets are
commonly
found
within
the
larger
staurolites.
In schists, the dominant foliation
(S,*)
is generally
warped around the equant garnet porphyroblasts. Rarely,
quartz
inclusion
trails
within
garnets
show
complex
microstructures. In the cores of garnets, inclusions define
a relict
foliation
that
is
oriented
a high
at
angle
to
the
foliation in the matrix. In some samples, the core
foliation
is
bent
into
a rim
orientation
the matrix foliation (Fig.6.5).
that
merges
These inclusion
with
114
a.
b.
Figure 6.5 a. Photomicrograph of garnet porphyroblast
containing different orientations of quartz inclusions in
core and rim. Section is cut normal to L1. Field of view
is 3.4 m
m
. b. Sketch.
115
microstructures
are
more
suggestive
of
non-rotational
inclusion patterns (T.H. Bell,1981, 1985; Bell and
Rubenach, 1983; Bell et al.,1986) than the garnet
porphyroblast
rotation
patterns
described
(1968, 1970), Schoneveld (1977), and
by
Rosenfeld
Powell and Vernon
.
(1979)
An
unusual,
relatively
is shown in Figure
6.6.
foliation
oriented
bracketed on two
the
relict
uncommon
microstructure
A garnet core containing
a relict
normal
sides
garnet
the
matrix
foliation
is
by
a secondary
garnet
growth
which
is at
foliation
to
right
angles
to
the
in
core
foliation. This secondary growth foliation is parallel to,
and merges with the matrix foliation. This oftype
porphyroblast structure has b’een described by (1931)
Bell as
indicative
ofa deformational
history
involving
bulk,
inhomogeneous shortening.
Most
as
pre-
garnet
porphyroblasts
to
syn-D3, but
due
to
appear
their
to
equant
be
at
shapes,
least
and
as
the
general scarcity of good internal microstructures, relative
ages of growth remain uncertain. Locally, there is evidence
for two stages of garnet porphyroblast growth. The first
stage
second
preserveda second(?) generation foliaton, whereas the
stage
preserved
a third
generation
foliation.
Staurolite. Staurolites are abundant in schists of the
Piedra Lumbre and Rinconada formations. They occur
old
116
Figure 6.6.
Photomicrograph of garnet porphyroblast
containing symmetrical secondary growths. S z * in matrix is
horizontal. Garnet core contains llmillipedefl
microstructures defined by quartz inclusion trails. Section
is cut normal to L1. Field of view is 4 m
m
.
117
throughout
the
porphyroblasts
Picuris
and
Range
and
porphyroblast
form
the
most
spectacular
microstructures
of ary
mineral. Smaller, less spectacular crystals occur in
quartzites
ofthe Ortega
Staurolite
to
more
crossed
prisms
simple
twins
range
Formation.
in
length
1.0 mm
less
than
from
mm. Superb twinned staurolite crystals
than
25.0
including
Quartzite
twins,
are
found
compound
twins,
and
inR4the
member of the
right-angle
Rinconada
Formation in the northern part of the map area. Sectorzoned
patterns
of
inclusions
are
also
common
in
many
of
the
larger staurolite crystals in the Ortega Group. Commonly,
the
large
poikioblastic
staurolites
are
by multiple stagesof growth (Fig. 6.7).
concentrically
zoned
Between succqssive
growth zones, sizes, densities, and textures of inclusions
vary considerably. In general, cores are euhedral, less
inclusion-rich,
and
contain
larger
inclusions
thansubthe
to anhedral rims. Quartz and iron-oxide minerals are the
most commonly included minerals. Other inclusions conTist
of smaller
Several
amounts
of
staurolites
garnet,
appear
to
prisms of chloritoid (Fig.6.8).
staurolite
grained
porphyroblasts
carbonaceous
biotite,
have
muscovite.
replaced
small,
euh,?-dral
One sample contains
that
layer
and
and
have
the
overgrown
a folded
fine-
coarser-grained
quartz-
muscovite matrix (HC-457A). Staurolite crystal morphologies
are profoundly different for the two growth mediums. In the
graphitic layer, staurolites are euhedral, whereas in the
118
a.
b.
Figure 6.7. Photomicrographs of staurolite porphyroblast
with sector-zoned inclusion patterns in euhedral core
mantled by secondary subhedral rim. Field of view is 14 nun.
a. Plane light. b. Cross polarized light.
119
Figure 6.8. Photomicrograph of staurolite after chloritoid,
in biotite-staurolite-garnet R4 schist. Field of view is 14
mm.
120
matrix, staurolites are anhedral. The reason for this is
unknown.
Staurolite
porphyroblasts
relationships
Most
with
staurolite
respect
crystals
dominant foliation (S2*).
upon
the
ends
of
exhibit
a narrow
to
are
matrix
range
of
tectonite
generally
not
fabrics.
aligned
in
the
Matrix foliations generally abut
staurolites,
and
slightly
wrap
around
the
sides of staurolites. Relict foliation trails in the cores
of
staurolite
grains
merge
with
trails
in
the
rims
of
grains. In turn, these rim trails merge with the matrix
foliation. Successive orientations of included trails
generally
lie
at
Staurolites
unusual
high
have
angles
also
microstructures
as
to
been
one
found
described
another.
that
above
contain
similar
for
the
garnets
with two-staged growth. A euhedral, zoned staurolite
porphyroblast is lrbracketedll by symmetrical secondary
anhedral staurolite growth R4
in schist (€IC-450) (Fig. 6.9).
The
euhedral
part
of
the
staurolite
contains
an
inclusion-
free core surrounded by an inclusion-rich rim. Quartz
inclusions
that
is
in
the
nearly
secondary
perpendicular
growth
to
define
a relict
the
foliation
dominant
matrix
foliation (S2*). Inclusion trails in the secondary growths
define a relict foliation (Si) that is oblique to both the
euhedral staurolite rim foliaton and
S2*.
This foliation is
truncated
the
byS2* where S2* abuts
(Fig. 6.9b).
against
staurolite
In the strain shadows, where
S 2 * is only
12 1
a.
b.
Figure 6.9. a. Photomicrograph of large staurolite with two
stages of growth and complex fabric relationships. Field of
view is 12 m
m
. b. Close-up view of left side of staurolite
porphyroblast in cross polarized light. See text for
discussion. Section is cut normal to L1. Field of view is
4 m
m.
122
weakly
developed,
matrix
foliation.
A sample
of
the
secondary
Piedra
Lumbre
growth
Si
merges
garnet-staurolite
with
schist
the
in
the central map area
(HC-64) contains staurolite crystals
with
an
included
foliation
oriented
at 30°
about
from
One possible
vertical matrix schistosity (Fig.6.10).
interpretation of this
rotation
after
microstructure
overgrowth
of
the
the
is
of staurolite
included
foliation.
In one schist sample of Rinconada Formation
(HC-377),
approximately
equal
sized
staurolite
and
garnet
porphyroblasts coexist. Both minerals contain
a conce?tric
ring
of
quartz
inclusions
at approximately
from the porphyroblast edges (Fig.
6.11).
both
and
porphyroblasts
for
some
inclusions
cleavage
the
unknown
together
reason
duringa certain
Several
cleavages.
grew
staurolites
equal
This implies that
(at
same
incorporated
stage
of
preserve
distances
growth
the
rates?),
quartz
growth.
relics
of
spaced
In the staurolitesthe originally quartz-rich
segregations
originally
are
mica-rich
now
quartz-inclusion-rich,
cleavage
quartz-inclusion-poor (Fig. 6.12).
segregations
are
whereas
now
In most of these
examples, the relict spaced cleavage (stage
4 to 5 of Bell
and Rubenach's model of cleavage formation,
1983) is
parallel to the
dominantS 2 * schistosity
(stage6 of Bell
and Rubenach's model of cleavage formation,
1983) in
the
rock matrix. These relationships suggest that an earlier,
1 23
Figure 6.10. Photomicrogr2ph of staurolite porphyroblast
with Si at about 30° to S2 in matrix. This geometry may
indicate rotation of the porphyroblast. Piedra Lumbre
Formation schist. Section is cut normal to L1. Field of
view is 6 m
m
.
124
Figure 6.11. Photomicrograph of R6 schist with staurolite
and garnet porphyroblasts. Both minerals contain concentric
quartz-rich inclusion trails about 0.4 mm from rims. See
text f o r discussion. Section is cut n o m a 1 to L1. Field of
view is 6
mm.
125
Figure 6.12. Photomicrograph of staurolite at extinction*
that preserves relict spaced cleavage. Well-developed S2
schistosity in matrix is parallel with Si. Note musco-rite
beard around edge of staurolite. Section is cut normal to
L1. Field of view is 4.5 m
m
.
126
pre-porphyroblast
cleavage
that
stage
has
of
not
S2* awas
differentiated
been
reoriented
spaced
during
evolution
into
a schistosity.
Most
staurolite
porphyroblasts
seem
to
have
grown
pre-
Staurolites contain inclusion trails that
or syn-D3.
represent
either
pre-S2*
foliations,
or
intermediate
stages
of S2* orientation and development.In some rocks, distinct
stages
of
staurolite
growth
are
demonstrated
by
rims
and
cores with distinctly different textures.
In a few ro",ks,
primary
growth
of
staurolite
was
followed
closely(?)
b:r a
secondary staurolite growth. Microstructures in these rocks
are similar to structures described
T.H.byBell (1985' and
Bell et al.(1986) as indicative of strain partitioning and
cleavage
reactivation.
Chloritoid. Chloritoid porphyroblasts are uncommon in
Ortega Group rocks in the map area. The Piedra Lumbre and
the
Ortega
Quartzite
Chloritoid
to
more
than
2.0
grains
formations
range
in
contain
size
some
from
chloritoid.
less
0.1than
mm
mm. In calc-silicate rocks of the Piedra
Lumbre Formation, chloritoids are unstrained, poikioblastic
grains with calcite(?) inclusions. In Ortega Quartzite
Formation, poikioblastic chloritoid contains quartz and
opaque inclusions. Although the timing of chloritoid qrowth
in
these
replaced
rocks
is
chloritoid
uncertain,
in
pseudomorphic
Rinconada
Formation
staurolite
tas
rocks.
127
Cordierite. Small amounts of cordierite are found in
the Ortega Quartzite Formation within the map Most
area.
cordierite
grains
are
anhedral
and
contain
abundant
small
inclusions. Commonly, these inclusions definea relict
foliation that is parallel to that of the matrix. This
cordierite
is
A Vadito
interpreted
schist
as
unit
having
formed
syn-D3.
northeast
of
the
Harding
Pegmatite
Mine contains enormous (up 15
tocm long) subhedral
prophyroblasts of cordierite. The dominant schistosity in
this rock is bent around these porphyroblasts.
In thin
section,
cordierite
microstructures
is
seen
including
to
either
tectonites (Berth& et al.,1979).
cordierite
has
grown
contain
complex
multiple
relict
o’r S-C
foliations
In this locality
priorD3.to
Other metamorwhic minerals. Plagioclaseis commol in
Vadito
Group
and
felsic
schist
at
Pilar
units,
lesi; and
common in Ortega Group rocks of the Picuris Range. In
Ortega Group rocks small, anhedral plagioclase grains
preserve a relict
foliation,
appear
to have grown pre-
and
D3.
Iron-oxide
minerals,
lithologies,
exhibit
relationships
with
Piemontite
Pilar,
and
in
the
in
entire
range
respect
crystals
the
common
to
occur
most
of
Ortega
timing
deformational
in
southeastern
the
map
Group
fabrics.
felsic at
schist
area
U.S. near
Hill.
128
Grains
are
small,
euhedral
prisms
that
are
invariably
aligned within the dominant foliation. Assuming that this
foliation
isS1, then
piemontite
is
preto syn-Dl.
Summarv. Within the map area, the distribution of many
pelitic
minerals
sediment
appears
composition
to
than
be
more
related
to
original
variations
in P-T
lateral
topology. Within the map area,
700+ m of local relief is
present. Less resistant, porphyroblast-rich pelitic szhists
tend
to
occupy
topographic
porphyroblast-poor
lows,
quartzites
and
whereas
slates
more
tend
resistant,
to
occup:~
topographic highs. Where pelitic schists of the Rincoyada
and
Piedra
Lumbre
formations
do
occur
at
high
elevatioys,
biotite, garnet, and staurolite porphyroblasts persist,
suggesting
that
what
local
relief is
there
has
not
aff'5cted
pelitic mineralogy. Porphyroblasts of biotite, garnet, and
staurolite
grew
repeatedly
before
and
during
the
D3 phase
of
deformation. Other porphyroblasts such as chloritoid,
cordierite,
A summary
and
and
of
plagioclase
timing
porphyroblast
may
also
relationships
growth
for
Group is given in Figure
6.13.
have
between
various
grown
prior
D3.
fabric
rocks
of
to
elements
the
Ortega
The usefullness of this
diagram is limited by two factors. Timing relationships are
only
style
useful
within
a local
structures
development
area,
probably
and
the
overlapped
ofD2 and D3 structures.
in
developmentDl-of
time
with
s1
s2*
s2
s3.
KYANITE
ANDALUSITE
SILLIMANITE
BIOTITE
QARNET
STAUROLITE
CHLORlTOlD
CORDIERITE
JLAGIOCLASE
FE-OXIDE8
I
CHLORITE
I
I
I
"
"
MUSCOVITE
QUARTZ
Summary of t i m i n q r e l a t i o n s h i p s between f a b r i c
elementsandporphyroblast
grbwth f o rr o c k si n
t h e Crtega
Group.
Figure 6 . 1 3 .
13 0
Garnet-biotite
Most
Thermobarometry
rocks
in
the
map
area
show
minimal
retrograde
metamorphic effects. Four porphyroblastic samples
from the
Ortega
Group
were
selected
for
quantitative
geothermobarometry and P-T path analysis. Chemical anslyses
were
performed
microprobe
by
M.L.
Williams
on
the
Departmentat the
inthe Geology
automated
electron
Universityof
New Mexico. Samples yielded garnet-biotite temperatur2s of
about 5OO0C, at pressuresof around
Microprobe
profiles
in
traverses
across
Figure
6.14, suggest
in P-T conditions
have
kb (Table 6.1).
4
garnet,
that
no
as
shown
t’le by
significant
interrupted
a continuously
jumps
changing
P-T path. This quantitative finding supports the petrologic
observations
and
metamorphism
were
porphyroblast
changes.
interpretations
continuous,
growth
rate
that
deformation
punctuated
changes
only
and/or
and
by
strain
rate
131
64
2.86
377
3.24
385
454
0.148
,19.30
4
777
0.146
22.21
4
498771
3.03
0.157
19.27
793 4
2.46
0.124
19.80
4
593
520
729
455
Table 6.1. Garnet-biotite geothermometry results from four
samples of garnet-biotite schist in the map area.
Calculations arebased on those described by Ferry and Spear
(1978).
132
I1
21
3:
41
51
61
71
8:
9:
10:
Ill
12:
13:
141
15!
16:
17 I
181
19:
20 I
21 I
22:
23 I
24 I
2s I
I./
1:
21
31
4:
5:
61
71
li
a:
91
lo I
11 I
12:
131
f
.
*
I
L
c
c
Figure 6.14. Microprobe profiles across porphyroblasts in
Ortega Group schists. Data are courtesy of M.L. Williams.
-
CHAPTER 7.
STRUCTURAL GEOLOGYAND SYNTHESIS
Introduction
General statement
Deformational
styles
vary
considerably
within
Precambrian rocks of the Picuris Range. These variations
occur to some
degree
within
individual
lithostratiqraphic
groups, but more noticeably among rock groups. For this
structural
synthesis,
the
Picuris
Range
has
been
divid2d
into three domains. These domains are the Ortega Grou.?,
which
comprises
Range;
the
Ortega
Group;
most
Vadito
of
Group,
and
the
the
central
which tolies
the
felsic
schist
the north of the Ortega Group (Fig.
7.1).
contact
Pilar
from
separates
rocks,
Vadito
Ortega
and
a northern
rocks
contact
south
at
of
th%
Pilar,
which
to
lies
A southern
from
felsic
separates
schist
at
Orteqa
r'xks
rocks.
A domainal
deformational
the
portionPic~ris
of the
approachis essential
style,
and
because
possibly
litholoqr,
tectonic
setting
differ
among the Ortega, Vadito, and felsic schist sequences, and
it is possible
that
none
of
the
three
are
related
in
original stratigraphic manner. The southern and northern
domainal
contacts
are
characterized
by
ductile
shear.
an
./
I
;I
i
i
i
!
!
!
I
I
I
I
i
135
Within
each
domain,
the
various
generations
of
structures have been catalogued.It is importantto note
that
although
generations,
structures
Because
the
is
of
formed
the
strain
probably
this,
together
structures
Regardless
groups
are
of
will
be
sub-divided
responsible
due
to a progressive
and
seemingly
domains,
all
for
these
that
to have
seem
referred
to asa generation
the
into
deformation.
fabrics
than
a deformational
of
the
are
history
structures
rather
histories
structures
of
event.
diverse
three
reconcileda single
in
structural
major
supracrustal
rock
comprehensive
kinematic model for the entire range. This model involved
a
complex
interplay
mid-crustal,
metamorphic
of
ductile
progressive
folding
environment
conditions,
and
is
under
and
shearing
a
in
medium-grade
presented
at
the
end
of
this
chapter.
Two of
the
most
persistent
questions
regarding
th?
1) what are
Precambrian geology of the Picuris Range are:
the stratigraphic relationships, if any, between major
lithostratigraphic
are
the
Answers to
evolution
range
rock
Precambrian
these
and
packages
structural
questions
tectonic
in
the
histories
lead
to models
setting
range;2) and
what
of
of
these
outlining
the
range
packages?
ths
and
place
the
ina regional framework.
Although
structure
of
previous
studies
Proterozoic
rocks
of
the
in
stratigraphy
the
Picuris
and
Range
arz
13 6
numerous,
detailed
models
of
the
kinematics
and
dynamics
are
few. This is due mainly to uncertainties and
inconsistencies
concerning
the
basic
geology
of
rocks
in
the
range. This in turn is largely due to the heterogeneity of
strain
within
and
between
presence of previously
major
rock groups,
unrecognized
and
the
and
ductile
faults
shear
zones. The most important Precambrian shear zones in the
Picuris
Range
are
bedding-parallel,
and
are
accordingly
difficult to recognize and interpret. Some workers have
described
have
relatively
described
It has
complex
recently
complications
Precambrian
simple
are
strain
polyphase
become
in
whereas
ochers
histories.
apparent
characteristic
uplifts
histories
of
northern
New
that
these
all offive
the
Mexico
problens
and
is?lated
that
have
b?en
mapped and analyzed in some detail (Williams,
1987:
Grambling et al., in prep.). In each range, workers
hwe
described
been
early,
folded
ductile
and
shear
zones
which
this
study
include
have
subseq-lently
faulted.
Nethods
Methods
mapping,
Thin
and
employed
and
sections
both
thin
were
parallel
in
section
and
generally
and
normal
microstructural
cut
to
normal
to
intersection
detailed
analysis.
the
or
foliation,
extension
lineations. All stereograms are lower hemisphere equal-area
137
projections,
contoured
Serious
problems
to
1 percent
can
of
arise
area.
when
correlating
of structures between domains
(P.F. Williams, 1985).
lithologies
and
stratigraphies
differ,
the
generations
If
character
cf
imposed structures and fabrics may also differ. If terranes
have
experienced
identical
somewhat
structural
conditions,
it
different
histories
may
be
structural
under
difficult
histories,
different
to
match
or
structural
corresponding
structures and fabrics. In the Picuris Range, previous
interpretations
events,
and
concerning
relative
the
timing
numberfabric-€oming
of
of
those
events
have
varied
study to study. Overprinting relationships are the best
criteria for determining structural history (P.F. Williams,
1985).
Foliation overprinting relationships are not
a=
reliable as fold interference patterns. Other useful
criteria
in
certain
areas
are
deformational
style
and
orientation patterns. Because fold interference patterns
are
rare
structural
in
the
history
Picuris
among
Range,
the
most
three
correlations
domains
depend
of
on
foliation overprinting, deformational style, and orientation
patterns.
from
Geometries
of
Domains
Introduction
The
three
major
stratigraphic/structural domains
in
the
Picuris Range (Vadito Group, felsic schist at Pilar, a.nd
Ortega Group) are shown in Figure
7.1.
that
intrude
only
the
Granitic plutcns
southern ofpart
the
Vadito
Group
are
included in the Vadito Group domain.
A fourth, poorly
understood
domain
consists
of
all
of
the
rocks
that
crop
east of the Picuris-Pecos fault. This eastern block is
considered a separate
stratigraphic
domain,
equivalents
of
even
though
one
or
it
more
may
of
contain
the
other
domains.
It is not clear how this domain fits into tho
regional
picture,
and
it
will
not
be
emphasized
in
this
dissertation.
The
Ortega
sequence
Group ais
well-layered
dominated
by
a >1000m thick,
metasedimentary
resistant
basal
quartzite. The Ortega Group is structurally characterized
by
major
folds
which
are
overprinted
a slightly
by
oblique,
penetrative, reactivated cleavage.
The
layered
This
felsic
feldspathic
unit
parallel
The
schist
is
at
quartz-muscovite
structurally
foliation
Vadito
Pilar
a homogeneous
is
seque?ce of
which
is
"quartz-eye"
dominated
a single,
by
probably
Group ais
complexly
schists.
layer-
mylonitic.
interlayered
out
139
metavolcanic-metasedimentary sequence
by
heterogeneous
units
that
which
thicken,
is
charact.erized
thin,
and
pinch-cut
along strike. Vadito Group rocks contain
a strong foliation
that
is
axial
Ortega
planar
to
tight,
moderately
sized
folds.
Group
Relict sedimentam structures. Well-preserved crossbeds
in
many
phyllites
quartzites,
provide
and
excellent
graded
beds
stratigraphic
in
some
control
schists
in
and
the
Ortega Group. With the exception of the southern Copper
Hill
area
and
local
regions
of
minor
folding,
younging
criteria are consistent with the major fold structure. On
the
northern
young
to
the
syncline,
In
Hill
the
limb
of
south.
strata
Copper
anticline,
dip
the
On
Hondo
the
to
southern
the
Hill
area,
beds
dip
south
on
and
syncline,
the
young
First creneration of structures.Dl.
generation
of
characterized
structures
by
fairly
in
the
limb
and
to
of
young
southern
the
dip
the
to
an3
Hondo
the
limb
of
north.
the
Copper
south
7.1).(Fig.
The earliest
Ortega
small-scale,
strata
Group
is
near-bedding-parallel,
high shear-strain features. These structures are most
evident
in
quartzites
and
quartz-rich
rocks
at
or
basal Ortega Quartzite Formation. Within the map area,
near
the
140
folding
does
not
appear
to
be
an
important
component
Dl
cf
strain.
Nielsen (1972) and Nielsen and Scott
(1979) suggested
that
because
stratigraphic
and
structural
facings
were
consistent, the first-generation of structures represents
one limb of
a large recumbent fold.In the Picuris Range,
no
evidence
The
on such a fold
closure
of
only
has
penetrative
Dl feature
been
recognized.
isa bedding-parallel
schistosity (S1) in micaceous quartzites (Fig.7.2).
Commonly
associated
with
this
schistosity
a strong
is
south-
plunging extension lineation(L1) on SI surfaces (Fig. 7.2).
L1 is
defined
by
either
elongate
grains,
or aligned
quartz
crystals of sillimanite, kyanite, and tourmaline. This
lineation generally plunges down-dip. Bedding-parallel
foliations
are
common
in
rocks
from
multiply
deformed
Proterozoic terranes (Holst,
1985; Hobbs et al.,1976), and
debate
continues
over
whether
they
are
the
result
a
of
primary depositional fabricor a tectonite fabric. Holst
(1985) reported
sucha fabric in metasediments of the
Proterozoic Thomson Formation of Minnesota.
He concluled
that
it
was
a tectonic
foliation
clearly
related
to
other
early tectonite structures. TheS1 fabric in the Picuris
Range
and
is
is
also
to
considered
a tectonic
structures
The
related
are
Ortega
described
Quartzite
other
early
foliation.
in
the
contains
deformational
These
following
features,
other
D.
_.
paragraphs.
one-meter-thick-or-less,
141
Figure 7.2. Contoured, equal-area stereographic projection
of L1 and S0/S1 in Ortega Group rocks.
142
bedding-parallel
zones
of
deformed
vein
quartz
surrourded
by
an anastomosing aluminous schist matrix. Highly contcrted
quartz layers and aligned quartz pods (Fig.
7.3) indicate
that these features are zones
of high shear strain. It is
not known whether
the
concentrated
these
in
vein
quartz
zones
and
during
aluminous
schist.
deformation
or
were
pricr
to
deformation. Metamorphosed aluminous %hale drapes" are
commonly
found
within
Eriksson, 1985).
relationship
the
Ortega
Quartzite
(Soegaard
and
Roering and Smit(1987) reported a clear
between
bedding-parallel
the
shear
of vein
occurrence
zones
in
quartz
quartzites
and
from
the
Witwatersrand Supergroup, South Africa. In these zones,
quartz
veins
In
one
vein-rich
developed
locality
shear
during
in
zone
the
shear
deformation.
northwestern
contains
a fold
mapa q-jartzarea,
structure
interpreted
to bea sheath fold. This fold in only exposed in two
dimensions,
but
is
consistent a
with
sheath
fold
profile
7.4).
perpendicular to the transport direction (Fig.
folds
in
lineation
which
are
1984; Cobbold
the
common
fold
in
axis
is
mylonite
cut
Sheath
parallel
to the extension
zones
(Bell
and
Hammo?d,
and Quinquis,1980; White et al.,1980;
Carreras et al.,1977; Quinquis et al.,1978; Berth6 a?d
Brun, 1980).
The
contains
basal
Ortega
Quartzite
zonesof abundant,
small,
in
the
concave
Pilar a mcliffs
a
shear
are sub-parallel to compositional layering (Fig.
7.5).
planes
that
14 3
Figure 7.3.
Photograph of bedding-parallel, quartz-veinrich shear zone in the Ortega Quartzite.
Quartz pods are
surrounded by anastomosing, aluminum silicate-rich schist.
144
a.
"
-----
-- - - - -
""
""
"""""
""""-
b.
Figure 7.4.
a. Photograph of sheath fold in Ortega
Quartzite shear zone. Rock face on which fold is exposed is
normal to the extension lineation. b. Sketch.
145
Figure 7.5.
Photograph of small shears in basal Ortega
Quartzite from the Pilar cliffs, northwestern Picuris Range.
146
These
arcuate
features
are
defined
by
reduced
grain
size
rather than by compositional variations. Sense of shear is
not discernable. These features are similar to narrow shear
zones
found
in
non-foliated
quartzites
in
the
Witwatersrand
Supergroup (Roering and Smit,
1987).
Mylonite zones are important Dl structures. Quartz
mylonites
are
found
locally
in
the
Ortega
Quartzite
(HC-366)
(Fig. 7.6). These rocks appear to have undergone grain size
reduction by ductile processes. All contain an intense
foliation and well-developed extension lineation. Such
structures
generally
imply
conditions
of
high,
localized,
simple shear strain (Tullis et 1982).
al.,
Pristine
quartzites
quartz
within
ribbon
the
mylonites
Rinconada
are
Formation
found
and
locally
the
in
basal
Ortega Quartzite in the Pilar cliffs. These mylonites show
no
evidence
of
or annealing
coarsening
post-kinematic
quartz grains (Fig.7.7).
of
These types of structures are
only recognized in extremely pure quartzites. Althoug’l in
style
and
other
Dl
orientation
structures,
these
the
quartz
lack
of
ribbon
mylonites
annealing
suggests
resemble
thst
they developed late in the metamorphic history. Perha,?s
these
fabrics
are
Dl mylonites
that
were
found
in
reactivated
s:mor
pOSt-D3.
All
of
these
features
quartz-rich
lithologies
suggest a (early?) deformational history of near beddingparallel
progressive
simple
shearing
along
discrete
zones.
147
Photomicrographof
Figure 7.6.
mylonite from the Ortega Quartzite.
recrystallized quartz
Field of view is 16 nun.
148
Figure 7 . 7 . Photomicrograph of pristine quartz ribbon
mylonite from R3 quartzite in the north-central Picuris
Range. Section is cut parallel to the extension lineation
and normal to the mylonitic foliation. Field of view is 16
m
m
.
149
It is
was
reasonable
also
to
assume
partitioned
evidence
of
these
into
that
the
features
considerable
Dl shear
schistose
has
been
strain
lithologies,
destroyed
where
by
later
deformation. Some schists however do contain evidence
Dl of
fabrics
preserved
as
microstructures
in
some
pre-D2
porphyroblasts. These features will be described a in
following
section.
Strain
these
conditions
Dl-type
D2 time.
suitable
for
structures.presumab1y
the
formation
continued
at
of
some
least
of
into
This overlap is well illustrated by occurrences of
the Dl extension lineation. In some areas these lineations
are
folded
whereas
in
around
F2
folds,
other
adjacent
suggesting
areas
that
they
sit
they
are
within
pre-FZ,
axial
of D2 folds, suggesting that they are syn-Fz. Other
structures
suchas the
everywhere
follow
have
formed
quartz
folded
vein
shear
compositional
layering,
which
may
o?ly
early Dl.
in
Second qeneration of structures,
D2.
expression
zones,
of
the
second
generation
The dominant
of
deformation
is
F2 folds are now upright, tight
folding on all scales. Most
to isoclinal, variably-plunging structures. On
a
macroscopic
scaleD2 is
best
illustrated
by
the
Aondo
syncline. This fold is the dominant structure in the
mountain range, and affects the entire Ortega Group.
It is
a tight,
shallowly
west-plunging
fold
with
axial
surface
planes
150
dipping due south at about
65O.
ranges
from
an
Dip values of modal
‘CO
average66O
ofon
the
northern
limb60°toon
the southern limb. Ona map scale, as illustrated by the
shape
of
the
thick,
mechanically
stiff
Ortega
Quartzite
Formation, this fold geometry is simple. In detail hcwever,
thinly
interlayered
differing
units
competency
above
contrasts,
and noncylindrically (Fig.1.8).
the
Ortega
Quartzite,
with
have
folded
disharmonically
Folding mechanisms range
from nearly pure buckling (Hudleston,
1986) in the Ortega
Quartzite, to dominantly passive folding (Hudleston,
1986)
in subordinate schists and phyllites. Several map-scale
minor
folds
and
fault-related
complications
are
evident
in
the Hondo syncline. In particular, these structures are
concentrated
southern
on
limb
the
of
overturned,
the
fold
tectonically
in
the
thinned,
Rinconada
and
Pilar
Phyllite formations. Most of the faults the
in area n x t h
of
Warm
Springs
associated
with
complexities
in
are
bedding-parallel,
abundant
local
Outcrop-scale
minor
mapping
folds
in
folds,
of
the
and
because
they
cause
they
enormous
stratigraphy.
Ortega
Group
are
extrenely
common. F2 folding is especially spectacular in the
colorful, finely laminated schists, metasiltstones, ani
phyllites
of
the
Piedra
Lumbre
Formation
in
thethenose
of
Hondo syncline. Although small-scale F2 folds show
significant
consistently
variation
lie
in
within
hinge
the
are
orientation,
plane
of
hinge
lines
compositional
laywing.
15 1
Figure 7.8. Block-diagram sketch of Hondo syncline shl3wing
thick, competent Ortega Quartzite fold, and dishamtonic,
noncylindrical folds-in less competent schists. Fold is
approximately 6 km across.
152
This
effect
is
illustrated
by
stereographic
projectiors
of
compositional layering and L2*0/l lineation (Fig. 7.9).
One
fold
of
particular
interest
is the
Copper
Hill
anticline inthe southwestern Picuris Range. This structure
has
been
that
it
the
is
focus
the
of
only
numerous
large
studies,
anticline
and
is
unique
recognized
in
in
the
Orteg
Group of the Picuris Range. The structure plunges 20°
about
to
the
west,
and
folds
a tectonite
fabric
along
with
compositional layering. Thin sections of Rinconada schist
from
the
hinge
area
shows
an
axial
plane
crenulation
cleavage cutting an earlier schistosity
(SI?) and
compositional layering. In style and orientation the Copper
Hill
anticline
The
resembles
Copper
mineralization
Hill
anticline
localized
is
F2
anticline
Quartzite (Williams, 1982).
Hill
other
in
folds.
contains
the
upper
extensive
portion
of the
copp?-r
Ortega
It is possible that the C-pper
separated
from
adjacent
rocks
by
hig’l-
angle reverse(?) faults as shown in Figure 7.10. The Copper
Hill
the
structure
Hondo
The
folding
may
therefore
be
an
uplifted
minor
fold
of
syncline.
axial
is
plane
generally
cleavage
(S2) associated with F2
either
obscured
by
the
S2*
fabric,
or
not developed. Where exposed, S2 is difficult to
distinguish from the later S2*.
As will be discussed in the
following
planar
to
section,
F2
although
folds
in
the
S2*
is generally
Ortega
not
Group,
axial
it
was
probably
153
ORTEGA GROUP
ORTEGA QROUP
,/
QReAT CIRCLE QIRDLE AT BB.180
\
30.002
b.
C.
Figure 7.9. a. Contoured stereographic projection of So/S1
in Ortega Group. b. Contoured stereographic projectio7 of
L2*0 in Ortega Group. c. Sketch of F2*fold geometry in
Ortega Group that illustrates So/S1-L2 0 relationships. See
text for discussion.
154
Figure 7.10
Sketch cross-section through the Copper H i l l
area, showing possible relationship between the Copper Hill
anticline and adjacent rocks.
155
developed
late
in
the
F2
where
cleavage
the
can
Within
process,
overprinting
S2* fabric
be
the
observed
axial
northwestern
thickness of the
Ortega
and
is
therefore
Identification of S2 is certain
S2.
called S2* rather than
only
folding
map
Quartzite
is
weak,
planar
to
area,
the
and
an
tte
F2
S2
fold.
apparent
Formation
approaches
two
kilometers. Everywhere else in the range, this
fomat.ionis
not much more than one kilometer thick. In this area,
Ortega
Quartzite
schist
horizon
stratigraphy
that
is
repeated
across
a bictite
base
approximately the
where
sits
of
the
quartzite should occur. The mica foliation in the biotite
schist is oriented at 72O 235O.
to
in
Ortega
biotite
Quartzite
schist,
and
dips
Compositional layering
about
50° south to the
south
of
the
about
65O south to the north of thr!
biotite schist. This area probably represents an
imbrication
and
tectonic
thickening
of
the
Ortega
Quartzite
Although the timing of faulting
is unkno'm,
(Fig. 7.11).
its
style
suggests
deformation.
Very
similar
imbrication
described
features
by
in
Williams
the
(1987)
Ortega
Quartzite
in
Tusas
the
have
been
Range,
and
b
: J.A.
Grambling (personal communication, 1987) in the Rio Mora and
Guadalupita
areas.
A minimum
Group,
made
Quartzite
estimate
by
with
of
D2 shortening
comparing
an
unfolded
the
fold
within
the
Or%ega
profile
of
the
reconstruction,
yields
Orteg,s
valu4?s
of
156
S
N
Figure 7.11
Sketch cross-section showing imbrication of
Ortega Quartzite in northwestern map area. Age of faulting
is unknown, but may be related to D2/D3.
157
about 65 percent shortening. This estimate neglects the
shortening
effects
of
tangential
longitudinal
shortening
and
shearing.
The third
D3.
Third seneration of structures,
generation of structures
is
dominated abyslightly
oblique,
penetrative foliation (S2*) that has overprinted earlier
features. S2* is
the
dominant
cleavage
in
nearly
all
szhist
S2* appears as both
a
outcrops in the Ortega Group.
schistosity anda crenulation
cleavage
locally,
and
typically is not axial planar to F2 folds. Figure 7.12a
illustrates
the
manner
in
which
S2*
transects
these
structures. Because S2* is generally the dominant
'clewage
in
many
outcrop,
and
lies
measurements
vergence
slightly
of
the
relationships
oblique
to
D2
bedding-cleavage
across
the
Hondo
axial
surfaces,
structural
syncline
are
potentially misleading. In order for the bedding-clea-rage
rule to be
useful,
folding
and
cleavage
formation
must
be
synchronous (Borradaile, 1978). In terms of the major fold
structure,
stratigraphy
vergence
measurements
requires
synclinal
show
that
closure,
most
although
vergence
the
data
indicate anticlinal closure. Evidence of this be
canfound
at
the
Phyllite
outcrop
scale
Formation
are
where
minor
transected
F2
by
folds
an
in
the
Pilar
overprinting
S2*
cleavage (Fig. 7.12b). If a well-developed S2 cleavage ever
existed,
it
has
been
destroyed
or reactivated
by
S2*.
158
s2*-
925
b
a.
F i c p r e 7.12.
a. Sketchandstereogramshowing
t h en a t u r e
S 2 t r a n s e c t i o no f
Fz f o l d s . b. Photograph of S 2 *
t r a n s e c t i n g F2 f o l d I n t h e
P i l a r Phyllite.Blackmarker
l i n e s p a r a l l e l t h e t r a c e of s2*.
of
159
Borradaile (1978) proposed a method
of
describing
the
nature of fold transection. According to his scheme, the
Hondo
syncline
is
angle
between
the
15O.
an
example
fold
of l1I,
Ifcase
with a dihedral
hinge
and
cleavage aplane
b o ~ t10-of
There is debate concerning the interpretation
of the
relative
timing
of
a transecting
cleavage
and
fold
development. Powell (1974) and Borradaile (1978) notel that
transected
folds
development
are
not
uncommon,
involvesa time
lag
and
between
that
the
one
method
inception
of
of
folding and development of the cleavage. Alternatively,
Duncan (1985) stated that there existed no evidence to
suggest
that
transected
folds
are
not
just
the
effect
of
superimposed deformations. He noted that to prove
transection,
formation
it
were
must
be
formed
shown
that
folding
and
cleavaq?
during
a single defomational event,
and not duringa polyphase deformational history. Although
geometrical
permit
fabric
unambiguous
of F2 folds
.and
microstructures
relationships
conclusions
the
S2* cleavage,
described
below
deformation duringD2 and D3.
transecting S2* cleavage
immediately
the
the
regarding
Picuris
the
Range
absolute
porphyroblast
suggest
a continuum
of
This implies that the
formed
either
lateF2 in
or
afterF2 folding.
Unambiguous
Examples
in
of
southern
examples
apparent
limb
of
of
F2* folds
are
F2* folds
map-scale
the
Hondo
not
common.
possibly
syncline,
exist
on
within
R6 unitthe
do
not
timing
160
of
the
Rinconada
Formation,
and
the
Pilar
Phyllite
Formation. Two sets of complex folds, on the southern limb
of
the
Hondo
syncline,
exhibit
the
wrong
sense
of
asymmetry
for the major west-plunging Hondo syncline structure. Axial
surfaces
of
these
folds
parallelS2*the
cleavage
orientation. Plunges of these folds are unknown. If they
plunge
eastward,
then
they
are
consistent
with
majx the
structure.
The
dominant
intersection
lineation
in
Ortega
Gro-lp
rocks appears to L
be~ * o . This is speculative because
L20
and L ~ * oare essentially identical in style. The
stereographic
projection
of
L20/L2*0
girdle
which
corresponds
7.9b).
The point maximum 35O
at to 242O suggests that many
to
defines a great-circle
compositional
layering
(Fig.
measured F2/F2* folds plunge southwest, consistent in
orientation
Minor
with
fold
the
axes
larger
structures.
commonly
develop
oblique
to
regional trend of major folds (Sanderson,
1973).
This
effect
strain
does
not
imply
that
the
principal
the
axes
rotated
during folding. Sanderson (1973) found that oblique fold
axes
can
develop
under
conditions
of
constant
principal
strain axis orientation. Ina highly deformed Proterozoic
terrane
in
rotated
towards
great-circle
Larue, 1985).
the
southern
the
bulk
distribution
Lake
Superior
region,
extension
direction,
on
a stereogram
(Sedlock
fold
axes
and a plot
and
Such progressive reorientations of fold axes
have
as
161
are
common
in
deformed
terranes
throughout
the
world
(Pfiffner, 1981; Speed and Larue, 1982; Miller et al., 1982;
Williams, 1978), and such an interpretation is propose3 for
rocks
in
the
Ortega
Mesoscopic,
Group
near
of
the
Picuris
bedding-parallel
shears
Range.
represent
another important component of D2/D3 strain. In the Piedra
Lumbre schist, such shears truncate small, Fz-style folds
(Fig. 7.13).
map
area,
In the Warm Springs area of the west-central
numerous
near
bedding-parallel
shears
disrup?
stratigraphy. It is suspected that these structures are
more
abundant
than
can
be
demonstrated
by
mapping
of
stratigraphic relationships. Because some of these shears
appear to cut
in
time
F2
with
folds,
D3
they
may
than
D2.
rather
be
more
closely
associated
Fourth qeneration of structures D4. Several late,
nonpenetrative,
cross-cutting
sets
of
structures
have
locally overprinted earlier fabrics. These are
characterized
by
generally
north-trending
kink folds in schists (see Fig.
5.8).
in
these
D2, and
events
D3
are
relatively
crenulations
and
The strains involved
small
in
comparison
Dl,
to
strain.
Porvhvroblast microstructures. Large porphyroblasts of
staurolite, garnet, biotite, plagioclase, and andalusite are
abundant
in
schistose
units
of
the
Ortega
Group.
162
Figure 7.13.
Photograph of small near-bedding parallel, D2
or D3 shear in Piedra Lumbre Formation.
Shear cuts F2(?)
fold.
163
Microstructures
staurolites
Lumbre
and
common,
garnets
and
especially
from R6
the
schist
useful,
and
the
in
Pielra
Formation.
Certain
as
are
porphyroblasts
inclusion
trails,
clearly
earlier
overgxow,
fabrics
that
and
may
pres3rve
have
bee?
destroyed within the matrix. Typically, interpretatiom of
these microstructures are ambiguous. Nonetheless,
porphyroblast/microfabric relationships
information
One
of
timing
that
the
of
may
most
not
be
obtainable
intriguing
metamorphism
and
yield
subjects
the
important
by
any
involves
various
other
the
method.
relative
generations
of
structures. Do the generations of structures recognized in
the
rocks
several
represent
pulses
of straining
Although
of
microstructures
Group
that
space
timing
can
strain
during
common,
consecutive
during
evolving
staurolites
answer
such
one
orogenic
conditions?
porphyroblast
and
garnets
questions,
from
it
absolute
relationships
across
orogens,
methods
for
establishing
events
can
only
be
porphyroblasts,
which
range
deformational
must
the
should
because
of
episodes,
orogeny,
a continuum
or
metamorphic
well-preserved
in
help
separate
vary
be
in
the
applied
Ortega
noted
time
and
relative
locally
(P.F. Williams, 1985).
Euhedral
from
less
garnet
than
1 mm to 5 mm are
found
in
a variety
in
size
of
lithologies of the Ortega Group. In many units, garnet.s are
concentrated in the most aluminous layers. Generally,
164
garnets are relatively inclusion-free. Where inclusions are
present,
concentrations
are
greatest
in
garnet
cores.
Typical inclusions include small grains of quartz, biocite,
plagioclase, muscovite, staurolite, and Fe-oxide. Inclusion
trails representative of relict foliations are rare. Where
present, these trails are generally curved. At garnet rims,
the
inclusion
trails
merge
into
and
are
continuous
wit\
dominant mica foliation in the matrix (see 6Fig.
.5).
These
types
describN?-d
of
inclusion
trails
are
similar
to
those
the
by
T.H. Bell (1985) and Vernon (1978) which are representative
of a foliation
that
has
been
rotated
or
reactivated
a
by
subsequent foliation (T.H. Bell,1985; Vernon, 1978).
Inclusion
trails
everywhere
seem
to
merge
into
the
younger
orientations, rather than to be truncated. These phenomena
imply
that
during
and
deformation
slightly
Staurolite
than 1 mm to
and
after
metamorphism
porphyroblast
porphyroblasts
greater
than
3 cm
ranging
are
in
were
continuous
growth.
size
from
in
Ortega
abundant
less
Group
schists. An enormous variety of staurolite shapes exists.
The
most
anhedral
crystals
are
typically
highly
poikioblastic with rounded quartz inclusions. Staurolites
are
commonly
twinned
and/or
sector
zoned,
and
contain
aligned quartz inclusion trails (see Fig. 6.7). Most
staurolites are unstrained; some are slightly strained. In
schists, the dominant foliation
(S,*)
wraps around the
randomly oriented staurolite porphyroblasts. Zoned crystals
165
typically
contain
inclusions
euhedral
surrounded
cores
by
a sub- to
with
small
anhedral
quartz
mantle
that
contains larger quartz inclusions.
A number
exist
of
between
intriguing
matrix
microstructural
foliations,
growth
relationships
zones
in
staurolites, and internal foliations. The most revealing
staurolite porphyroblast(HC-450) is a zoned crystal that
contains
The
two
core
orientations aofrelict foliation (Fig.7.14).
contains
a high density of small quartz incluisions
that define straight foliation trails. The rim of the
porphyroblast
quartz
containsa lower
inclusions
that
density
of
larger,
elongate
define
a slightly curved foliat.ion
trail. At the edge of the porphyroblast, these curved
trails
merge
into
a coarse
matrix
foliation,
which
is bent into the dominant rock foliation
(S,*).
perpendicular
staurolite
to
foliation
rim,
and
trails
parallel
to
in
turn
This S2* is
preserved
foliation
in
the
trails
preserved
in the staurolite core. Close examination reveals that the
core
foliation
actually
bends
into
and
merges
with
the
foliation across the sharp core-rim boundary. One
interpretation
porphyroblast
successive
of
these
rotated
apparent
and
microstructures
grew
in
is
that
stages,
the
preserving
orientations a of
relatively
constant
foliation orientation. A second possible interpretation of
these
microstructures
foliation
with
involves
successive
respect a to
non-rotational
reorientation
staurolite
of
rim
a.
b.
Figure 7.14. Photomicrograph of large, zoned staurolite
porphyroblast in R6 schist. Core contains horizontal
inclusion trails that bend into vertical rim inclusion
trails. S2* in matrix is horizontal. Field of view is 14
m
m
. b. Close-up of upper left portion of staurolite uyder
cross polarized light. Section is cut normal to L1. Field
of view is3.4 mm. See text for discussion.
167
porphyroblast. In this scheme, the staurolite growth is
punctuated by orthogonal reorientations of foliation. The
relative
rates
of
porphyroblast
growth
and
strain
might
be
important in producing these microstructures. In eithzr
case, the relict
included
foliations
may
or
may
not
represent successively earlier foliations (i.e.
S2 and SI).
Both
rock
of
sample
these
of
interpretations
Piedra
defomational history
Similar
was
microstructural
Ortega
Group
These
support
presented
progressive
at
rather
relationships
observations
quantitative
in
metamorphism
Formation
that
are
for
this
least
of thepart
than
episodic.
observed
in
other
schists.
petrologic
the
Lumbre
suggest
and
garnet-biotite
Chapter
6~thatsuggests
were
porphyroblast
continuous,
growth
rate
interpretations
P-T
that
deformation
punctuated
changes
analysis
only
and/or
and
by
strain
rate
changes. All porphyroblasts showa tendency for quartz
inclusions
to
progressively
coarsen
from
core
to
rim
to
matrix. This implies that minerals were continually
recrystallizing
and
growing
during
prograde
metamorphism.
Summary. Field and petrologic evidence from the Ortega
Group
suggestsa deformation
near
bedding-parallel,
that
evolved
which
in
turn
into
history
localized
large-scale,
evolved
into
of
early,
progressive
upright
strong,
low-angle,
simple
folding
slightly
shear
and
oblique,
faulting,
168
overprinting foliation formation. There probably existed
considerable
temporal
overlap
of
structural
development
associated with changing deformation conditions.
In
particular, Dl shearing
persisted
dominated byD2 folding,
well
development
into
of
the
time
the
S 2 * cleava-Je
probably occurred late in
F2 folding, and mylonites
fomed
in
some
quartzites
Vadito
as
late
as
or
later
D3.
than
Group.
General. The present structural investigation
concentrated
more
on
Ortega
Group
rocks
than
on
Vadito
Group
rocks. This section on Vadito Group
stkctures and fabrics
will
therefore
draw
mainly
on
previous
studies
by
Nielsen
and Scott (1979), Holcombe and Callender(1982), and McCarty
(1983).
Relict sedimentam structures. Structurally useful
sedimentary
the
structures
Marquenas
are
Quartzite
limited
and
mainly
other
to
smaller
cross-beds
in
orthoquartzite
bodies scattered throughout the Vadito Group.
All beds dip
to
the
where
south
overall
and
most
young
to the
stratigraphic
north.
younging
is
The
only
region
inconsistent
between
the Ortega and Vadito groups is in the Copper HillAs area.
noted
earlier,in this
area
Ortega
Group
rocks to
young
the
169
south. Thus, at least locally, the contact between th?
Ortega
and
Vadito
groups
must
a shear
be
zone.
First seneration of structures, Dl. Rocks in the
Vadito
Group
contain
an
S1
bedding-parallel
foliation
defined mainly by aligned micas in quartzose rocks.
S1 was
recognized by Nielsen (1972), Nielsen and Scott
(1979),
Scott (1980), Holcombe and Callender
(1982), and McCaP-y
(1983).
Associated with S1 is
a locally well-developeA
down-dip extension lineation (L1) defined by aligned quartz,
muscovite,
by
and
aligned,
biotite
grains S1onsurfaces,
constricted
Mylonitic
quartzites
clasts
in
and
highly
and
probably
metaconglomerates.
strained
rocks
are
common along the Ortega-Vadito contact. In recrystallized,
mylonitized
as
has
Marquenas
asymmetric
moved
Although
up
the
Quartzite,
clasts
and
indicate
to
timing
of
the
kinematic
that
north
onset
of
the
indicators
Marquenas
relative
this
such
Quartzite
to G~oup.
the Ortega
shearing
is
unknown,
is consistent with D1-D2 conditions.
Stereographic
(1982) and
projections
from
Holcombe
and
Callender
McCarty (1983) show the orientation So/l
of in
the western Vadito Group (Fig. 7.15a).
In
the
relatively
poorly
exposed
Vadito
Group
rocks
in
the southern map area, no F1 folds were recognized. Vadito
Group F1 folds have only rarely been described. McCarty
(1983) reported
rare intrafolial "hooksg1 in amphibolite
it
170
+
0
EGA
DIT0
Figure 7.15. Stereographic projection of modal S O and modal
S2 in Ortega and Vadito Group rocks. From a) Holcombe and
Callender (1982)*and b) McCarty (1983). This S2 may be
equivalent to S 2 in this report.
171
lenses which may represent relict F1 folds. Although
Nielsen (1972) proposed that evidence for large, recum5ent
isoclinal
folds
folded SO, no
is
present
megascopic
or
locally
where
macroscopic
S1
F1
cuts
folds
tightly
have
been
reported. As in the Ortega Group, the Dl deformation in the
Vadito
Group
parallel
is
thought
shearing,
rather
to
be
than
characterized
by
by
near
bem-lding-
folding.
Second senerationof structures, D2. A well-developed,
regionally
penetrative
foliation
is
the
dominant
tectoxite
fabric in most rocks of the Vadito Group. This foliation is
a schistosity
in
some
rocks aand
crenulation
cleavage
in
others. Although in both style and orientation this
foliation
Group,
have
is
consistent
previous
reported
workers
that
this
with
in
S2*
the
described
in
southwestern
cleavage
is
axial
the
Ortega
Picuris
planar
Range
to
F2
folds
in the Vadito Group. Mesoscopic F2 folds are relatively
common
in
the
spectacular
Vadito
Group,
as
those
in
Pegmatite
Mine
area,
where
exposed,
several
workers
the
though
not
Ortega
Group.
the
have
Vadito
described
as
abundant
In
rocks
the
are
map-scale
or
Harding
best
folds.
Montgomery (1953) mapped a tight, upright synclinal and
anticlinal pair. McCarty (1983) described two generations
of fold
structures,
including
rare
mesoscopic
F2
structures
that occur as tight, upright folds in schists. These are
equivalent to Montgomery's map-scale fold pair. McCarty
172
defined
six
megascopic
F2 folds
by
structural
vergence
relationships and lithologic similarities. D.A. Bell (1985)
did
find
either
opposing
side
of
cross-beds
the
Vadito
similar to those
of
the
Vadito
mesoscopic
orientation
Ortega
to
in
small
quartzite
amphibolite,
and
bodies
reported
on
folds
McCarty.As described bythese wo-kers,
Group
folds
the
F2 structures
are
similar
in
described
above
in
cleavage
correlation
style
and
th?
Group.
The
question
of
post-DI
betw%en
Ortega and Vadito group rocks remains unresolved.
In the
Ortega Group,S2* transects F2 folds, andS2 is either weak
or reactivated. In the Vadito Group,
F2 folds are cut by an
axial
planar
Group S2*.
cleavage
(S2) that
is
identical
to
the
Ortega
Is S2* in the Ortega Group equivalent S2toin
the Vadito Group? Although further analysis of Vadito Group
rocks is required
to
resolve
microstructural
relationships
some
clues.
important
this
in
question,
Vadito
interpretative
schist
may
pro-ride
Holcombe (in prep.) has described and interpreted an
important
setof microstructures
in
oriented
thin
sections
of Vadito Group quartz-muscovite-biotite schist. Biotite
porphyroblasts
such a way
have
that
overgrown
individual
earlier
generations
fabric
of
elements
strain
can
examined. Holcombe found that
a relatively minorD2 coaxial
bulk
shortening
produced
an
axial
plane
cleavage
(S2) at 23O
to anS1 schistosity. S2 is a schistosity rather than
a
in
be
173
crenulation cleavage. He interpreted these features to mean
that S2 was
pirated
from
S1 by
grain
microfolds were modifiedF1 folds.
This
described
history
in
is
Ortega
changes,
an3
F2
S2 transects Fl folds.
remarkably
Group
shape
similar
to that
rocks
within
previo-isly
the
map
area,
with
one important difference. In Ortega rocks,
a third
generation cleavage(S2*) has reactivatedS2 and transxted
F2 folds.
Is it possible that in the Vadito schist, tke
strong S2 schistosity
has
obliterated
evidence Dlof
fabrics? If so, then the folds in Holcombe’s thin section
could be F2, and the transection cleavage
S2*. Further
evidence
that
these
folds
are
actually
F2 folds
is
based
on
fold style. F2 folds are common, tight to isoclinal, and
upright, whereas F1 folds are rare and intrafolial. The
folds
F1.
described
by
Holcombe
are
more
to
F2
than
If this is the case, then at least some Ortega and
Vadito
group
strain
histories
rocks
have
probably
experienced
evidence
Group
to
rocks
least
on
suggest
is
identical
in
D2 and D3.
Third creneration of structures.
D3.
At
similar
that
the
equivalent
the
There exists some
dominant
S2 cleavage
in
toS2*the
microscopic
the
scale,
the
in
Ortega
Vadito
Group.
dominant
cleavage
transects F2-style folds. Ifso, then S2 in the Vadito
Group
is
equivalent
to
the
third
generation
S2* in
the
Ortega Group. To date, no good evidence of an Ortega Group
to
174
S2
equivalent
has
Stereographic
and
in
been
projections
McCarty ( 1 9 8 3 ) show
the
recognized
western
from
the
Vadito
in
ductile
structures
in
rocks.
and
Callender
(1982)
of 1rS211
the cleavage
(Fig.
7.15b)
Fourth seneration of structures,
D4.
of
Group
Holcombe
orientation
Group
Vadito
Vadito
The latest phase
Group
rocks
are
identical
to
the fourth phase in the Ortega Group. These consist
of
cross-cuttingl
schistose
non-penetrative
crenulations
and
kinks
in
rocks.
Southern crranitic rocks.Two distinct ages of granitic
plutons
intrude
Vadito
Group
rocks
in
the
southern
Picuris
Range. These are the Funtiagudo Granite Porphyry
(1684 2 1
Ma)
and
Rana
Monzonite
(1674 2 5 Ma) , and
Quartz
the
Quartz Monzonite (about1 4 5 0 Ma) ( D . A . Bell, 1 9 8 5 ) .
Penasco
A
fourth intrusive body, the Cerro Alto Metadacite, is locally
exposed in the southernmost Picuris Range. Both
of the
older
granitic
rocks
are
strongly
foliated
and
contain
anastomosing zonesof high shear strain. Foliation trends
of
east
to
northeast
are
consistent
with
the
regional
trend
of S2 in the Vadito Group. The younger Penasco Quartz
Monzonite
is
also
contains
consistent
only
a weakly
in
developed
orientation
foliation
with
the
which
regional
S2 in
the Vadito Group. The Cerro Alto Metadacite contains
a
strong
east-trending
penetrative
foliation
defined
mainly
by
175
aligned
biotite
grains.
Interpretation
intrusion
of
such
variables
of
crystallinity,
Nonetheless,
relative
granitic
by
yield
of
as
plutons
and
emplacement
and
possible
comparisons
approximate
timing
of
deformation
depth,
on
style
pluton
is
grain
deformation
based
timings
relationships
between
complicated
size,
degree
mechanisms.
and
orientation
emplacement
relative
to
strain. The Rana and Puntiagudo plutons were certainly
emplaced pre-D~and
The
Penasco
possible
pluton
was
as
early
probably
as
pre- .. to
intruded
syn- to
syn-D..
late-syn-
D3. The Cerro Alto body was emplaced at least pre-D2, and
possibly
pre-Dl.
Pomhvroblast microstructures.
exposed
west
of
the
Harding
An
unusual schist
Pegmatite
Kine
area
contains
abundant, large (up 15
to cm in diameter) rounded, subbedral
porphyroblasts of cordierite. Thin sections across th'zse
cordierites
show
complex
microstructures
outlined
by
qiartz
inclusions (Fig. 7.16). These inclusions delineatetwl3
obvious
relict
foliation
trails
that
unfortunately
are
not
clear enough to show relative timing relationships. The
cordierite
porphyroblasts
are
pre-kinematic
with
respect
the dominant foliation (S2) in the rock matrix. One
possible
represent
interpretation
relict
of
the microstructures is that
successive
overprinted
foliations
they
that
formed prior to growth
of the cordierite. The inclusion
to
17 6
a.
b.
FigcIre 7.16.
a. Photograph oflarge,rounded
cordierite
)hyroblasts
in
Vadito
schist,
southwestern
Picuris
Range.
POrz:
b. I?hotomicrograph of complex microstructures in cordierite
PorE~hyroblast defined by elongate quartz inclusions. Field
of v.iew is 3.4 m
m. See text for discussion.
177
trails
are
straight,
so porphyroblast
growth
was
rapid
relative to the strain rate. Alternatively, they may
represent relict Dl S-C planes (Berth6 et 1979)
al., 0’7
shear bands (White et al.,
1980).
microstructures
these
In either case, these
suggesta complex pre-D2 (or D3)
histo7 for
rocks.
Discussion. Although interpretation of structure,;. in
the
Vadito
Group
is
limited
by
poor
exposures
and
complex
volcanic/sedimentary stratigraphy, the
DI-Dz-D~strain
history
is
comparable
with
that
recorded
in
the
Ortega
Group. Vadito rocks lack the major folds found in the
Ortega
many
Group:
the
strain
bedding-parallel
unrecognized
The
due
contact
to
D2
instead
faults
the
present
absence
between
the
probably
of
Ortega
is
in
expressed
Vadito
reliable
Group
by
rocks,
stratigraphy.
(Piedra
Lumbze
Formation) and the Vadito Group (Marquenas Quartzite
Formation) is a near
south
of
Copper
bedding-parallel,
Hill,
and along
strike
mylonitic
to
the
shear
zone
east, of
south
Copper Mountain. South of Copper Hill, near state highway
75, a two-meter-wide
podo f Pilar
Phyllite
is
caught
up
in
the shear zone.To the east, near the Picuris-Pecos fault,
quartzose
mylonites
lie
at just
andbelow
the
Ortega-Vadito
contact.
McCarty (1983) found that the orientation of So/S1 in
the
Vadito
and
Ortega
groups
differed
consistently
14O inby
yet
178
the contact region south of Copper Hill. Holcombe and
Callender (1982) and McCarty (1983) noted that the S2
cleavage
that
overprints
So/S1
is
similarly
oriented
in
both
groups. They suggested that the apparent structural
vergence
change
S2 cleavage
across
the contact
measured
overprintinga preexisting
was
So/S1
due
to
the
orientation
difference. This implies that the faulting responsible for
juxtaposing
right-side-up
Ortega
rocks
against
overturned
Vadito rocks occurred post-Dl and prior to D3. One
possibility
late
Dl
is
shear
Felsic
that
ductile
and/or
schist
the
at
faulting
major
D2
was
associated
with
the
folding.
Pilar
General. The felsic schistat Pilar isa homogeneous
sequence
of
feldspathic
quartz-muscovite
schist
and
megacrystic quartz-muscovite Ifquartz-eyeff schist expost?d in
the cliffs near Pilar. These rocks may correlate with the
Rio
Pueblo
Schist
located
in
several
isolated
outcrops
in
the southeastern corner of the range. These rocks are welllayered ona range of scales. Ona mesoscopic scale, thick
sections
of
pink
schist
are
interlayered
with
light
green
schist and white schist. On
a finer scale, individual
layers
differ
proportion
of
of
and
quartz
in
the
white
shade
of
megacrysts,
muscovite.
pink
and
or
the
green
or
relative
white,
in
proportions
t
179
The
upper
anomalously
pinkish
portion
of
this
concentrations Mn,
of Fe,
high
earth elements (Codding et al.,
1983).
zone
may
seawater
have
originated
during
subsequent
the
of
Mn on
deposition
Al,
contains
and
many
stages
and
rare
This geochemical
hydrothermal
Mn enrichment
by
waning
unit
of
of
volcanism,
with
clay
with
minerals
(Williams, 1987).
Although
schist
at
it
is
Pilar
metasediments,
possible
are
no
that
locally
primary
portions
reworked
sedimentary
of
the
felsic
volcaniclastic
structures
have
been
recognized. The contact with the overlying Ortega Quartzite
is
abrupt,
and
no
transitional
rocks
First seneration of structures.
Dl.
tectonite
fabric
in
the
felsic
are
present.
The dominant
schist
at
a
Pilar
well- is
developed, somewhat anastomosing foliation
(SI) this is
parallel
to
compositional
layering
in
the
overlying
Ortega
Group. This is the earliest fabric recognized in these
rocks, and in many areas is the only foliation visible.
A
south-dipping
quartz,
muscovite,
S1 surfaces.
fabric,
extension
in
and
lineation
(L1) defined by elongate
tourmaline
AlthoughS1 has
thin
section,
the
grains
is
ubiquitious
on
appearance aofmylonitic
quartz
and
muscovite
crystals
are
recrystallized. It is possible that this fabric formed
through a process
White
of
megacrysts
dynamic
of
recrystallization.
quartz
and
rare
feldspar
in
tte
180
schist
are
Although
interpreted
locally
euhedral,
all
as
these
relict,
grains
megacrysts
show
metamorphosed
may
appear
some
evidence
phenocTsts.
relatively
of
internal
strain. Typically, the grains are flattened in the
foliation
plane,
and
elongate
in
the
extension
direction.
In the extreme cases, quartz megacrysts are highly flattened
with
distinct
asymmetric
quartz grains (Fig.7.17).
isolated
in
relatively
tails
of
dynamically
recrystallized
These quartz porphyroclasts lie
homogeneous
matrix
of
fine-grained
quartzandmuscovite.Suchasymmetricarereliable
kinematic indicators of shear (Simpson and Schmidt,
1933;
Simpson, 1986; Lister and Snoke,1984).
classification
system
of
According to the
Passchier
and
Simpson
(1986), these
porphyroclasts are sigmaa-type. This type structun
of
is
thought
to
represent
recrystallization
conditions
rate
is
large
(Passchier and Simpson,1986).
as
sense
symmetry,
of
shear
matrix
porphyroclast
size,
deformational
history
size
matrix
is
which
relative
the
to
the
strain
rate
Such structures are usl..-ful
indicators
grain
in
if
they
is
small
fabric
is
simple,
and
have
with
monoclinic
respect
to
homogeneous,
the
thin
section
is
parallel to the extension direction. The felsic schist at
Pilar
All
samples
of
section
the
from
seem
to
asymmetric
schist
satisfy
all
porphyroclasts
samples
in
the
of
these
recognized
Pilar
requirements.
in
cliffs
thin
suggest
sinistral shear as viewed towards the west. This implies
cut
18 1
Figure 7.17. Photomicrograph of aymmmetric quartz-eye in
felsic schist from the Pilar cliffs. Thin section is
oriented with south to the right and
to the
up top. Sense
of shear is top to the south (dextral). Section is cut
parallel to the extension lineation and normal to the
foliation. Field of view is 15 nun.
182
that
the
Ortega
Quartzite
has
moved
southward
over
the
felsic schist at Pilar. The timing of this movement is
unknown.
Immediately
near
the
below
top
of the
the
Pilar
base
of
cliffs,
the
Ortega
lie
a variety
of
Quartzite,
pink
and
gray pure quartz mylonites. The mylonitic foliation is
bedding-parallel
and
well-developed
quartz
extension
lineations plunge gently to the south. These structurzs
differ
from
those
in
the
underlying
schistose
rocks
,
in
an
important way. Whereas the schists are well recrystallized
with
granoblastic
textures,
the
quartzites
show
no
evidence
of post-kinematic annealing. The quartz mylonite
stm-tures
may
be
younger
Alternatively,
for
some
than
they
unknown
the
may
shear
have
reason
structures
formed
the
in
the
synchronously,
quartzites
schists.
whe7eupon
successfully
resisted
later coarsening during peak metamorphism. Preliminayr work
on
sense
Ortega
of
shear
Quartzite
in
moved
these
mylonites
southward
over
suggests
the
felsic
that
schisf
the
at
Pilar.
No unequivocal F1 folds
were
found
in
the
felsic
schist
at Pilar. In one locality on the Pilar cliffs, dark, thin
interlayers
SI.
of
tourmaline-rich
rock
pinch
out parallel t o
It is not known if these represent intrafolial,
isoclinal F1 folds, original depositional pinch-outs, or
local
hydrothermal
alteration.
183
Second seneration of structures,
D2.
dominant S1 schistosity is overprinted
Locally, tho
bya slightly
oblique
crenulation cleavage(s2). Intensities of ~2 range from
gentle
warpings
of
S1 to a strong,
of SI.
No F2-style folds were recognized in the felsic
schist
at
Pilar,
apart
from
penetrative
these
crenulation
gentle
warpings
of
layering.
Discussion. The contact between Ortega Quartzite and
felsic
shear
schist
at
Pilara zone
is
strain. A large
portion
of
highly
of
this
concentratel
strain
may
have
been
partitioned into the underlying felsic schist. Texturzs in
the
felsic
schist
originally
are
well-layered
probably
tuff
due
a combination
to
that
has
been
of
an
subjected
t-,
later layer-parallel shear. Although mesoscopic textures
are
mylonitic,
thin
sections
typically
reveal
granobla,stic
quartz and feldspar textures. This may have been due to
dynamic
recrystallization
during
shearing,
as
is
commo2
in
many shear zones. The Ortega Group apparently moved
southward
over
the
felsic
schist
at
Pilar
during
some,
or
much, of the ductile deformation history. Quantitative
estimates
shear
zone
marker
The
felsic
of
beds
displacement
are
or
presence
not
possible,
piercing
of
schistat Pilar
along
this discontinuous
the
due
to
the
duckile
absence
of
offset
points.
Mn-rich
suggests
that
layer
at
little
if
the
any
top
of
of
the
the
184
uppermost
part
of
the
schist
section
has
been
cut
out
by
shearing. In a relatively undisturbed section of the same
contact
the
in
Ortega
the
Tusas Range,
this
Quartzite
the
at
Mn-horizon
top
a transitional
of
schist sequence (Williams,1987).
ever
existed
in
been
sheared
out.
The
nowhere
the
felsic
in
schist
contact
structural
and
Pilar
in
Pilar
the
they
and
Picuris
stratigraphic
just
below
quartzite-
If transitional rocks
cliffs,
at
sits
have
the
Range,
subsequently
Vadito
and
relationships
Group
are
therefore
between
the
two
remain unknown.
Eastern
Block.
General. The eastern block
of the Picuris
separated
from
the
western
block
by
Range
the
is
north-trending,
high-angle Picuris-Pecos fault. The western block is
dominated
grained,
by
two
crumbly
north-south
granitic
elongate
rock
exposures
informally
of
fin(.--
called
G::anithe
te
of Alamo Canyon. In the southern end
of the western
granitic
block,
granitic
rocks
intrude
felsic
schists
that
Montgomery (1963) called Rio Pueblo Schist. Although these
schists
are
cliffs,
they
Pilar
similar
are
to
more
felsic
schists
muscovitic
and
exposed
P!.larin
less
resistant
the
than
rocks. A Mn-rich horizon has been mapped adjacent to
a pure, cross-bedded quartzite. Although exposures are poor
the
185
and
the
contact
zone
is
nowhere
visible,
this
quartzite
appears to be equivalent to the Ortega Quartzite. The
manner
block
in
is
which
these
rocks
to
thosewestern
in the
relate
unclear.
Deformational
Deformational
fabrics
fabrics
in
are
the
obscure
Granite
of
in
Granite
the
Alamo
Canyon.
of
Alamo
Canyon. In the southern areas, the strong
SI foliatio’l in
Rio
Schist
is continuous
Pueblo
Farther
north,
orientation,
this
into
granitic
rocks.
is
somewhat
variable
foliation
perhaps
due
to
reorientation
in
by
alon”~
drag
the
Picuris-Pecos fault. Locally, an earlier foliation is
visible in some outcrops. This foliation is defined
a
: a
vague
layering
represent
an
of
white
original
versus
flow
pinkish
bands,
and
may
foliation.
Several fine-grained, lensoid, well-foliated
amphibolite
bodies
lie
within
the
northern
half
of
the
western block of the Granite of Alamo Canyon. Granitic
rocks
around
the
amphibolites
are
commonly
coarse-grained
and laced with epidote mineralization. The age and origin
of
these
amphibolites unknown.
is
Deformational fabrics in Rio Pueblo Schist. Structures
within these rocks
are
identical
to those
in
the
Pilar
cliffs area. Rare kinematic indicators suggest that
quartzites
moved
southward
over
Rio
Pueblo
Schist.
186
Deformational fabrics in Ortesa Ouartzite.
On ths
southern
highly
endof the
fractured,
granitic
and
block,
bedding
grey
and
quartzites
cross-bedding
are
orientations
are inconsistent within small areas. The contact with Rio
Pueblo Schist
is
nowhere
exposed.
Summary
Rocks
in
each
of
the
three
major
domains
have
experienced polyphase strain histories. Rocks in the Ortega
Group
provide
evidence
for
early
folding
and
ductile
during Dl, major
shearing
and
faulting
faulting
during
DL, and
intense cleavage formation and faulting(?) during D3. Rocks
in
the
Vadito
Group
locally
contain
evidence
for
an
identical deformational history. Differences include fewer
map-scale
folds
in
Vadito
rocks,
and
less
transection
F2
of
folds by D3 cleavage. In the Vadito Group, the potent,ial
exists for abundant bedding-parallel faults. Rocks in the
felsic
schist
at
Pilar
are
dominated
by
structures
characteristic of bedding-parallel shear shear strain. In
the
of
Picuris
all
Range,
felsic
this
schist
may
occur
exposures
to
because
the
of
the
mechanically
proximity
stiff,
overlying Ortega Quartzite during shearing. Pristine quartz
ribbon
mylonites
along
the
basal
Ortega
Quartzite
contact
probably formed late in the metamorphic history. The Ortega
187
Quartzite
whereas
has
folded
thinner,
primarily
by
pFimarily
less
competent
passive
folding
Although a single
sufficient
to
by
lithologies
all
buckling,
have
folded
mechanisms.
polyphase
explain
large-scale
deformational
structures
in
history
all
is
three
lithostratigraphic groups, structural correlations are
difficult
due
differently
to
to
the
strain,
fact
that
each
group
and
each
preserves
a unique
has
responde3
deformational style. These structural variations are
summarized
in
Figure
7.18, and a synoptic
projection
is
given
Porphyroblast
profiling
in
Figure
7.19.
microstructures
suggest
that
stereographic
this
and
P-T
deformational
microprobe
history
was
progressive under prograde metamorphic conditions. This
progressive
history
porphyroblast
The
Ortega
growth
ductile
Group
support
the
was
in
punctuated
rate
shear
the
and/or
suggestion
strain
structures
northern
that
by
and
changes
rate.
recognized
southern
neither
in
of
below
Picuris
these
the
Rang(=
boundaries
represents a primary depositional contact. If these
contacts
may
be
are
major
juxtaposed
shear
across
zones,
them.
than
different
rock
packages
188
ORTEGA GFOOP
WIT0 GrnMDP
FELSIC 5 M S T AT P I U R
F i g u r e 7.18.
Summary and
comparison
of
s t r u c t u r a lf a b r i c s
i n t h e Ortega Group, Vadito Group,
and
f e l s i cs c h i s at t
Pilar.
I t is unknow$ whether S 2 i n t hV
e adito
Group i s
e q u i v a l e n t t o S2 or S2 i n t h e Ortega Group.
189
O R T E G A GROUP
\
maximum
L1 maximum
/
Figure 7.19. Synoptic equal-area stereographic projection
of D1-D2-D3 fabric elements in Ortega Group rocks, Picuris
Range.
190
Contacts
Between
Southern
At
Domains
Contact
Copper
inverted
Lithostratigraphic
Hill,
Vadito
right-side-up
Group
rocks
are
Ortega
Group
rocks
and
juxtaposed a along
near
bedding-parallel, ductile shear zone. There was some
component
of
dip-slip
motion
along
this
zone
D_.
i and
between
D3 time. Kinematic indicators in the Marquenas Quartzite
suggest
this
that
fault
at
zone
least
one
consisted
ductile
of
component
of motion
north-trending
along
shear
(Fig.
7.20).
To the east, along the same contact, fault sliv".rs of
upper
Ortega
Group
are
caught
between
mylonitic
Ortega
and
Vadito rocks. Near the Picuris-Pecos fault, where Vadito
and
Orteqa
mylonites
These
occur
data
boundary,
rocks
appear
just
all
perhaps
most
below
the
pointa highly
to
with
stratigraphically
continuous,
Orteqa
Quartzite7.21).
(Fig.
tectonized
Ortega-Vadito
extended
or
repeated
ductile
is
shearing. The amountof slip along this zone unkno~m,
but
must
inverted
The
considerable,
stratigraphic
direction
northward
the
be
or
northerly
of
sections
motion
southward,
vergence
judging
along
but
of
the
all
in
from
the
this
juxtaposition
Copper
zone
could
southerly
major
dip,
folds
suggests some (late Dl?) component
of transport
Hill
in
to
be
of
area.
eithe?
along
the
with
range
the
north. However, it should be noted that in the inner parts
191
F i g u r e 7.2 0.
Photograph
of
asymmetric
clast
in
metaconglomeratic Marquenas Quartzite at the Ortega-Vadito
contact.
Asymmetry suggests dextral shear of Vadito rocks
northward over Ortega rocks. Mylonitic foliation dips 45O
to thesouth (left in photo).
192
a.
b.
Figure 7.21. Photomicrographs of mylonitic rocks from the
Ortega-Vadito contact in the southern Picuris Range.
a. Marquenas quartzite in the western part of the range.
m
. b. Vadito schist from the eastern
Field of view is 14 m
m.
part of the range. Field of view is 16 m
193
of
orogenic
later
folding
the
rather
steepness
than
to
of
faults
original
may
be
orientation
due
to
(Coward,
Therefore, the pre-D2 dip of this shearmly
zone
1983).
have
belts,
been
similar
boundaries
suggesting
later
value
to either the north or south. 0thl.r
any
that
folding,
Northern
The
northern
unless
some
all
New
such
transport
Mexico
zones
along
dip
have
this
to
the
been
zone
s-mth,
revers?-d
was
by
northward.
Contact
contact
overlying
in
between
right-side-up
the
felsic
Ortega
schist
Quartzite
is
at
an13 Pilar
abrupt
and
well-
exposed in the Pilar cliffs. The quartzite contains small
shears
near
the
contact,
and
thicker,
schistose
shear
zones
within the basal quartzite. The felsic schist at Pilar
contains
mylonitic
Fig. 7.17).
textures
and
asymmetric
quartz-eyes
(see
Kinematic indicators suggest that Ortega rocks
moved southward over the felsic schist. Because early shear
features
rocks,
appear
this
to
be
southward
especially
shear
may
well
preserved
represent
the
in
these
earliest
observed component of strain. The northward transport
deduced
along
the
southern
boundary
may
therefore
a later
be
component ofDl motion. Alternatively, the geometry of the
structures
Perhaps
but
involved
movements
occurred
may
along
in
a complex
be
more
these
complicated
shear
zones
thrustor back-thrust
than
were
imagined.
synchronous,
settin?.
194
Fine-grained
Quartzite-felsic
quartz
ribbon
schist
at
mylonites
Pilar
at
contact
show
post-kinematic recrystallization (Fig. 7 . 2 2 ) .
probably
has
represent
experienced
It
is
unknown
how
or
repeated
the
strain
Orteg=i
no
of sign
These r x k s
synto post-D3 mylonites
prolonged
the
ina zone
ductile
was
that
simple
s’lear.
distributed
amon”J
the
contact, the schists, and the quartzite.
A geochemical
marker
horizon
that
characterizes
the 100
upper
m of t’le
felsic schist at Pilar (and equivalent rocks regionally) is
present
the
in
these
uppermost
xemoved
by
outcrops,
part
third
different
the
felsic
that
schist
little
section
if
has
any
of
bel..-n
shearing.
Picuris-Pecos
A
of
suggesting
Fault
important
Precambrian
shear
rock
boundary
that
juxtaposes
typestheisnorth-trending,
high-angle Picuris-Pecos fault. This fault separates Ortega
Group
on
the
west
from
the
Alamo Canyon
Granite
of
on
the
east. Ortega Group metasediments are bent southward near
the fault (Plate1).
associated
occurred
A
breccia
along
Because no apparent fracturing is
with
this
drag
during
the
Proterozoic
zone
most
of
of
the
represents a later,
mixed
Ortega
fault
more
folding,
zone
brittle
dextral
ductile
Group
in
the
movement
deformation
lithology,
map
component
of
probably
history.
which
area,
occurs
probably
motion.
195
a.
Figure 7.22.
a. Photograph of contact between gray OrLega
Quartzite above and pink felsic schist below. Mylonitic
quartz ribbon quartzites occur within the contact zone. b.
Photograph of mylonitic quartzite from within the contact
zone.
196
Discussion
All
have
contact
been
zones
highly
of
Ortega
tectonized
Group
along
with
underlying
bedding-parallel
rocks
ductile
shear zones. In the southwestern Picuris Range, where
displacement
must
relationships
be
large,
primary
between
Ortega
and
stratigraphic
Vadito
are
probably
n'3t
preserved. In the north, where displacement need not be
large, a primary
and
felsic
depositional
schist
is
stratigraphy
probably
more
between
closely
Ortl?ga
preserved.
However, in the north, the transitional quartzite-schist
sequence
seen
elsewhere
in
The
fact
that
exact
unknown
precludes
the
the
northern
natures
possibility
of
New
of
Mexico
these
is
absext.
zones
remain
developing
a unique
model for the evolution of the Picuris Range. Nevertheless,
a listing
minimize
of
the
Structural
important
number
of
potential
constaints
can
help
models.
Synthesis
Stratigraphic
1)
geological
and
Structural
Constraints
There are three supracrustal rock packages present
in the Picuris Range: a)
a heterogeneous sequence of
bimodal volcanic, volcaniclastic, and clastic sedimentary
197
rocks (Vadito Group); b) altered, metamorphosed felsic
volcanic
rocks
deposited
around
1700 Ma(?) (felsic schist at
Pilar); and c)a transgressive sequenceof sediments
deposited (on the felsic schist at Pilar?)
a shallov
in
marine, continental shelf environment (Ortega Group).
It is unknown how the Vadito and felsic schist
2)
units
relate
The
Vadito
stratigraphically
to
boundary
Group
high-angle
between
rocks
to
shear
the
zone
the
the
Ortega
Ortega
south a is
near
that
might
Group.
Group
to
the
north
and
bedding-parallel,
have
been
active
over
much
of the deformation history. Basal Ortega Quartzite ovl.-rlies
the
felsic
schist
at
Pilar aalong
zone of
high
shear
strain. Although the amount of bedding-parallel
displacement
zone
is
occupies
with
unknown,
an
respect
3)
that
has
occurred
possibly,
approximately
to
the
along
the
felsic
correct
Ortega
this
low-angle
schist
stratigraphic
at
shear
Pilar
position
Group.
The Vadito Group and felsic schist at Pilar are not
juxtaposed anywhere. Both lie structurally below the basal
Ortega
4)
but
Quartzite.
Granitic plutons intrude some Vadito Group rocks,
nowhere
intrude
Ortega
or
felsic
schist
at
Pilar
units.
198
Older granitic plutons are highly tectonized,
5)
whereas the youngest pluton is only weakly foliated. These
older
plutons
are
at
least
syn-D~,and
may
be
as
early
as
pre- or syn-D1. The youngest pluton is weakly foliatel, and
is
probably
to
late-syn-D3.
All rocks appear to have experienced lower to
6)
middle
amphibolite
metamorphic
with
syn-
or
facies
conditions
slightly
metamorphism,
of
with
peak
about
4 kb and 5OO0C, coinciding
post-dating
S2*
cleavage
development.
7 ) Supracrustal rocks have undergone
a common
progressive
formation
of
Dl is
shear
deformational
three
D2
rather
involved
moderate
major
characterized
strain,
folding
history
of
localized
zones
by
than
the
resulted
generations
by
macroscopic
in
that
large
folding
Vadito
fold
in
the
structures.
of
high
simple
structures.
in
the
Ortega
Group,
Group,
and
minor
folding
in
the felsic schist at Pilar. Extensive, near-beddingparallel
faulting
D3
was
probably
characterized
accompanied
by
D2.
formation
a strong
of
cleavage
(S2*) that slightly transects Ortega Group F2 folds, and is
now
the
dominant
cleavage
in
most
schistose
rocks
of
Ortega and Vadito groups.S2* reactivated and cannibalized
preexisting SI and S2 foliations. The effectsof D3 on the
felsic
schist
at
Pilar
were
minor.
the
199
All
under
three
of
conditions
these
of
fabric
forming
approximately
events
coaxial
develope3
principal
strain
axes.
The major structure in the Picuris Range is t\e
8)
gently
west-plunging,
F2 Hondo
syncline
The
Copper
parasitic
along
in
the
Hill
fold
on
to
isoclinal,
Ortega
Group.
anticline
the
Hondo
is
northward
veyging
thoughta mino-,
to be
syncline
that
has
been
uplifted
high-angleD2/D3 reverse faults.
In the north-central Picuris Range, the Ortega
9)
Quartzite
or
tight
is
doubled
earlyD 2 fault
thickness
by
what
may ahave
Dl
been
imbrication.
Kinematic indicators such as asymmetric
Itquartz-
10)
eyesgt
in
in
movement
the
of
felsic
Ortega
schist
at
Quartzite
Pilara component
suggest
southward
over
the
of
felsic
schist at Pilar. This is opposite to the northward vergence
sense
indicated
Vadito
contact
by
kinematic
and
D2/D3 folds
indicators
in
the
along
Ortega
and
the
Vadito
groups. The south-directed movement may represent
a later
component
of
motion
than
the
northern
Ortega-
movement.
200
Possible
The
Models
above
constraints
kinematic/stratigraphic
of
the
three
major
limit
models
the
for
the
number
of
evolution
lithostratigraphic
possible
and
terranes
joining
in
the
Picuris Range. Models presented in the following two
sections
are
founded
1) Different
positions
rock
beneath
on
the
following
sequences
Ortega
Group
occupy
on
the
two
major
points:
identical
strwtural
northern
(felsic
schist at Pilar) and southern (Vadito Group) limbs of the
Hondo
syncline.
2) There
nature
of
felsic
schist
exist
the
five
general
relationship
at
Pilar
possibilities
between
and
the
for
the
Ortega
Vadito
Group:
the
Group,
the
a) all three groups are related in an original, primary
stratigraphic manner. Within this model, there
are three possible stratigraphic relationships:
b) Ortega and felsic schist are related in an original,
primary
stratigraphic
manner:
c) Ortega and Vadito are related in an original,
primaxy
stratigraphic
manner;
a) Vadito and felsic schist are related
an original,
in
primary
stratigraphic
manner;
e) none of the three are related in an original,
primary
Models
invoking
stratigraphic
these
five
manner.
general
possibilities
are
2 01
presented
in
the
following
section,
and
then
kinematic/stratigraphic
critically
examined
in
the
mod2ls
discussio?
that
follows. All models involvea complex interaction of
faulting, shearing, and folding.
Well-constrained
ranges
may
aid
in
stratigraphic
relationships
limiting
number
the
of
in
n'zarby
variables
in
these
models. In the Tusas Range, in less tectonized regioniz, the
Ortega
Group
appears
to grade
stratigraphically
downward
into a felsic metavolcanic-metasedimentary sequence
characterized
by
(Williams, 1987).
schist
rich
at
quartz-muscovite
llquartz-eyelv
rock
schists
Portions of this unit resemble the felsic
Pilar
marker
of
of
the
horizon
is
northern
present
Picuris
in
Range,Mn-and
the
bothat places
the topof
the uppermost felsic sequence. If this represents
a
regionally-developed
then
all
stratigraphically
models
that
do
stratigraphically
below
the
Two of
general
the
five
not
continuous
place
Ortega
the
Group
possibilities
package,
felsic
can
can
schist
be
at
Pila
eliminated.
thus
be
eliminated: c) and e). Additionally, one of the three
subdivisions within possibility a) can be eliminated.
It is
important to note
have
Ortega
at Pilar,
in
that
although
Group
resting
the
northern
the
correct
stratigraphically
Picuris
Range
stratigraphy
on
the
felsic
contact
may
schist
between
the two is sharp and has been tectonized.
It is notknown
whether
than
in
this
transitional
area
as
the
it
original
is
in
the
contact
Tusas
is
abrupt
Range,
or
rather
wbetter
202
some
portion
of
the
upper
portion
of
the
felsic
schist
at
Pilar has been faulted out.
It should also be noted t’lat
other
portions
resemble
of
the
Vadito Groupn1
lithologies
in
the
in
Vadito
the
Tusas
Rang3
Group southem
of the
Picuris Range(M.L. Williams, personal communication,
1987).
One
these
further
two
primary
The
point
respect
lithostratigraphic
stratigraphic
progression
Pilar)
with
to
from
groups
sequence
felsic
transitional
to
likelihood
representing
concerns
volcanism
rocks
the
(absent
an
tectonic
(felsic
in
the
of
original
settings.
schist
at
northern
Picuris
Range) to stable shelf sedimentation (Ortega Group)
is
common in modern extensional tectonic settings. On the
other
hand,
graywacke
Range,
the
progression
deposition
exclusive
sedimentation
of
(Vadito
the
represents
from
Group
Marquenas
an
mafic
of
volcanism
southern
Quartzite)
uncommon
Picuris
to
evolution
and
stable
of
shelf
tectonic
settings. Thus, modern examples of tectonic progressions
are
consistent
development
of
with
the
an
interpretation
felsic
schist
at
that
favors
Pilar/Ortega
Group
stratigraphy.
Each
of
the
Dl sub-horizontal
remaining
ductile
models
shear
includes
strain,
some
which
component
was
concentrated below the Ortega Quartzite. On the scale of
the
Picuris
resulted
in
Range,
this
significant
shearing
may
or
may
not
have
disruptionthe
ofstratigraphic
section by low-angle ductile faulting. Although the figures
of
203
depicting
possible
structures,
all
kinematic
models
histories
include
the
maior
rock
do
not
show
possibility
of
these
such
faulting.
Model
a)
All
three
qrouDs
are
StratisraDhicallv related. In this case, there are tw2
possible
stratigraphic
1) Ortega
underlying
is
in
scenarios:
primary
stratigraphic
schist
at Pilar,
felsic
which
contact
is
in
with
primary
stratigraphic contact with underlying Vadito.
Dl subhorizontal
shearing
concentrated
Quartzite is followed byD2 folding
reverse
faulting
along
syncline (Fig. 7.23).
Group
rocks
the
beneath
the
Ortega
and
syn-
or
post-D2
southern
limb
of
the
Hondo
This ductile fault brings Vadita
against
Ortega
Group
in
the
southern
Picuris
Range. Locally, in the Copper Hill area, southward-yoimging
Ortega
Group
Copper
Hill
Group
rocks
on
anticline
the
are
southern
limb
juxtaposed
of
the
against
parasitic
inverted
Vadito
rocks.
Advantages:
a) consistent with the observed geology;
b)
permits
the
possibility
of
complex
faulting
in
the Vadito Group separating Marquenas
Formation, schist, and amphibolite;
c)
permits
than
the
the
Vadito
felsic
Group
schist
to
at
be
any
Pilar,
age
and
older
permits
204
S
N
ORTEGA
F E L S I C SCHIST
Ez.rly Dl
Figure 7.23. Stratigraphic/kinematic model al.
for discussion.
See text
205
their
contact
to
be
either
conformable
or
unconformable.
Problems:
a)
requires
that
no
granitic
through the Vadito
Group
intrusions
into
the
penetrated
overlying
felsic schist. Although this is no pro3lem
with
respect
plutons,
it
to
is
the or
presyn-D1 1680 Ma
more a of
problem
with
r'zspect
to the syn-D31450 Ma pluton. During
intrusion of the 1450 Ma
rocks
were
intrusions
felsic
same
2) Ortega
is
could
schist,
level
in
already
of
primary
folded,
have
and
pluton,
countrr
and
therefor12
penetrated
even
Ortega
Vadito,
rocks
assuming
emplacement.
stratigraphic
contact
with
an
underlying, stratigraphically complex Vadito/felsic schist
metavolcanic terrane. This model is similar to the above
model,
but
lithologic
differs
variations
in
that
it
calls
for
of
a stratigraphically
an
felsic
original
facies
metavolcanic
metasedimentary,
variation
section
mafic-felsic
in
the
to
It assumes
existeda thick
from
north
a mixed
to
metavolcanic
section
to
south. The felsic schist and Vadito are stratigraphic
equivalents,
perhaps
south
equivalent
felsic schist at Pilar/Vadito Group (Fig.
7.24).
that
north
analogous ato
variation
from
the
~
206
N
I
SCHIST
FELSIC
ORTEQA
Early
VADITO
~
D2
FELSICSCHIST
Figure 7.24. Stratigraphic/kinematic model a2.
for discussion.
D3
See text
Dl
207
homogeneous
This
cauldron
sequence
Folding
of
was
the
fill
to
heterogeneous
blanketed
section
by
would
Ortega
result
outflow
Group
in
the
shezts.
sediments.
observed
ap?arent
lithologic asymmetry. Reverse faulting along the sout’lern
limb
of
the
right-side-up
Hondo
Ortega
syncline
juxtaposes
inverted
Vadito
and
rocks.
Advantages:
1) consistent
with
the
observed
geology.
Problems:
1) requires a special
corresponds
case
with
the
where
the
facies
fold
hinge
change.
Model b) Ortesa and felsic schist are in orisinal
straticmaphic contact. A major fault is present between the
Ortega/felsic
7.25).
Group
schist
package
and
the
Vadito
Group
(Fig.
This south-dipping fault transported the Vadito
northward
over
the
Ortega
Group
in
the
southern
Picuris Range. The timing of this fault can range anywhere
from earliestDl to latestD2.
must
have
been
Displacement along the fault
large.
Advantages:
1) consistent
2) the
with
the
observed
geology:
Vadito Group can be older, younger, or
identical
in
age
to
the
Ortega/felsic
sequence:
3)
the
Vadito
Group
can
itselfa composite
be
schist
~
~
2 08
N
S
ORTEGA
-7
SHEAR7
-7
Early D l
FELSIC SCHIST
VADITO
Late D l
I
D2-D3
Figure 7.25.
discussion.
Stratigraphic/kinematic model b.
See
text for
2 09
terrane
of
Marquenas
Vadito
schist,
and
4) granitic
plutons
have
only,
because
schist
the
groups
Quartzite
Vadito
Formatio’l,
amphibolite;
intruded
the
Vadito
Group
Vadito
and Ortega/felsic
were
not
adjacent
during
granitic emplacement.
Problems:
1) no
Node1
d)
problems identified.
Felsic
schist
and
Vadito
are
in
Drimarv
stratiqrauhic contact. The manner in which the felsic
schist
and
Vadito
Group
are
related
is
not
specified
this model. A major thrust fault must exist between the
Ortega
Group
7.26).
In this model the Ortega Group was thrust
(southward?)
and
over
the
felsic
the
schist/Vadito
metavolcanic
felsic
package
(Fig.
schist/Vadito
terrane.
Advantages:
1) consistent
with
observed
geology.
Problems:
1) granitic
intrude
rocks
both
sequences.
might
the
be
expected
felsic
schist
to
and
have
Vadito
for
210
S
ORTEGA
A?
OR- 7
Dl
VADITO
SCHIST
FELSIC
D2
D3
Figure 7.26.
discussion.
Stratigraphic/kinematic model d.
See text for
211
Discussion
Although
each
the
observed
geologic
one
most
serious
Group
easily
of
been
schist
package
During
faulting,
primarily
models
all
This
is model
displaced
along
data
b),
type
Vadito
the
which
over
of
and
the
the
ductile
may
explain
RanJe,
displays
no
Vadito
Ortega/felsic
shear
have
structural
to
Picuris
sets,
in
Group
from
a deeper
adequate
in
northward
some
the
vertically
is
relationships
accomodates
drawbacks.
has
these
zone.
either
moved
level,
OT
moved mainly horizontally along
a flat shear zone. Both
scenarios
intrude
provide
Vadito
an
rocks
explanation
for
and
intrude
do
not
the
fact
that
plutons
Ortega/felsic
schist
rocks. Ortega/felsic schist rocks were spatially removed
from or above the zone of granitic emplacement. Similar
models
Range
of
evolution
of
Precambrian
have
recently
been
published
rocks
in
the
Picuris
by (1987)
Bauer and
Holcombe et al.(1985). As a variation on this model, as
the
Vadito
Ortega
Group
Group,
the
moved
northward
Ortega
Group
over
was
the
already
folded
correspondingly
moving
northward over the felsic schist. This idea is borrowed
from a kinematic
similar
rocks
Realizing
structural
model
in
that
elements
developed
the
all
is
Tusas
of
by
Williams
(1987) for
Range.
these
probably
models
more
contain
important
common
than
attempting to rigidly defend one particular model. All
2 12
possible
models
involve
deformational
histories
containing
ductile shearing, folding, and faulting. Early shearing was
low-angle and near bedding-parallel. Some imbrication may
have occurred. Shearing apparently continued well int-, (and
after)the phase
perhaps
of
deformation
dominated
by
u7right
folding. Coincident with the folding, reverse faultins3
disrupted
stratigraphy
lithotectonic
The
and
may
have
juxtaposed
units.
major
probably
locally
juxtaposition
occurred
early
of
in
the
lithostratigraphic
history,
when
grolps
shearing
and
horizontal transport dominated. Most stratigraphicstructural
groups
complications
were
related
to
that
occurred
folding
and
within
the major
ductile
rock
reverse(?)
faulting.
During
shearing,
high
strains
were concentrated
shear
below the Ortega Quartzite and within schists. All models
can
accomodate
progressive
reactivation
mylonitization
of
along
mylonitic
the
zones
basal
or
Ortega
Quartzite
throughout the deformation history. During crustal
shortening,
Ortega
Quartzite
forming
More
thick,
(and
large-scale
thinly
responded
mechanically
the
buckle
interbedded
ina more
Marquenas
folds
layers
ductile
stiff
layers assuch
the
Quartzite?)
and
with
manner
by
local
responded
imbrications.
contrasting
competencies
complex,
noncylindrical, passive folding. Williams (1987) concluded
that
strain
development
was
profoundly
by
dependent
on
213
proximity to the Ortega Quartzite in the Tusas Range. This
is
probably
This
be
setting
Alpine
shortening
and
of similar
the
Picuris
history
at
as
well.
depths
of approximatley 10 to 15
of
many
in
the
Himilayan
Range
occurreda Proteroz3ic,
in
interactionof shearing,
characteristic
Crustal
in
deformational
mid-crustal
A complex
true
folding,
such
fault/fold/shear
has
faulting
may
orogenic ofevents
any age.
northern
belts
and
km.
Appalachian
resulted
in
belt,
the
a?d
the
developnents
structures.
Williams (1987) and Grambling et al. (in prep.) have
identified
very
similar
lithostratigraphic
strain
terranes
in
histories
the
in
Tusas,
correlativl?
Taos,
Rio
Mo-a,
and Truchas areas. Both lithologic associations and
subsequent
laterally
deformational
continuous
some of early
environments
over
Proterozoic
may
much
of northern
time.
have
New
been
Mexico
during
CHAPTER 8.
DEPOSITIONAL HISTORY AND TECTONIC SETTING
Introduction
The
be
three
treated
major
rock
separately
packages
for
several
in
the
Picuris
important
Rangs
will
reasons.
Dominant lithologies (and presumably protoliths) diffe?
among the three groups. The Vadito Group contains
metavolcanic, metavolcaniclastic, and clastic
metasedimentary
of felsic
rocks:
the
metavolcanic
felsic
rocks:
schist
and
the
at
Pilar
Ortega
consists
Group
is
composed of clastic metasedimentary rocks. Structural
styles
also
differ
among
the
three
sequences,
and all
major
contacts are ductile shear zones. Lastly, the Vadito and
felsic
schist
Picuris
The
Pilar
units
are
nowhere
in
contact
in
the
Range.
Vadito
that it may
separated
at
Group
contain
by
bedding
presents
several
an
additional
different
parallel
shear
rock
zones,
complexity
in
packages
or
unconformities. However, crucial contacts between
metavolcanic
and
exposed in the
to be
clastic
southern
metasedimentary
Picuris
Range,
packages
and
much
are
work
poorly
remains
done.
Deductions
terranes
concerning
containing
possible
igneous
rocks
tectonic
are
settings
facilitated
by
for
215
comparative analysis of rock chemistry.
To date, no
geochemistry
on
supracrustal
rocks
in
the
Picuris
Rang?,
has
been published. In lieu of such geochemistry, evidenc.? for
tectonic
settings
consideration
of
of
major
rock
individual
packages
lithologic
must
come
a
assemblages
fron
and
their
depositional environments.
Depositional
Environment6
Vadito
In
three
Group
the
southwestern
major
Marquenas
Vadito
part
Group
Quartzite,
the
of
units,
Vadito
the
Picuris
Range,
from
north
to
south,
schist,
and
the
Vadito
the
are
amphibolite. In the southeastern part
of the range,a major
component
of
are
complexely
more
The
felsic
Marquenas
metaconglomerate
Soegaard
of
and
trough
schist
is
interlayered
Quartzite
and
present,
than
to
the
consists
texturally
cross
beds
in
the
all
lithologies
west.
of
immature
Eriksson
(1986) suggested
and
polymictic
quartzite.
that
Marquenas
the
orientation
Quartzite aindicates
provenance to the south of the Picuris Range. Shearing and
folding
may
have
invalidated
a straightforward
interpretation of transport direction in this unit. Clasts
in
metaconglomerates
consist
of
approximately
66 percert
the
216
quartzite, 34 percent felsic schist, and small amounts of
vein quartz, mafic schist, and calc-silicate rock. Th?
quartzite
clasts
characteristically
silicate
minerals,
and
were
are
therefore
devoid
not
of
aluminum
derived
from
the
Ortega Quartzite. The geographic source for all of th?
material in the Marquenas Quartzite unknown.
is
These rocks
were
of
deposited
on
an
alluvial
plain
a number
by
braided-
fluvial processes (Soegaard and Eriksson,
1986).
The
Vadito
schist
fine-grained
phyllitic
interlayered
with
unit
includes
a large
schists
lesser
and
amounts
varietyof
micaceous
of
felsic
quartzites,
schist,
amphibolite, and quartzite. Rare cross beds are presext in
the quartzites. The Vadito schist has been interpreted to
represent a sequence
of
fine-
to
medium-grained
graywackes,
micaceous quartz sandstones, pelitic shales, minor basalts
flows,
and
intruded
1983).
volcaniclastic
by
minor
and
mafic
rocks
sills
that
and
has
dikes
been
(McCarty,
A likely depositional setting for these rocks is in
a continental
the
felsic
sedimentary
shelf
accumulation
basin
of
or
intracratonic
graywacke
material
is
basin
in
punctuated
which
by
influx of quartz sands, pelitic muds, volcaniclastic
sediments,
The
Range
to the
and
minor
Vadito
consists
south
volcanic
amphibolite
of
of
two
the
main
Vadito
rocks.
of
the
southwestern
amphibolite
bodies;
a large
schist,
a smaller
and
body
Picuris
body
to
the
southeast. At least five lithologic units can be mapped out
217
in the Vadito amphibolite (McCarty,
1983).
Based on pillow
structures, pillow breccias, and relict amygdules, Lon3
(1974) interpreted
these amphibolites to represent
metamorphosed mafic volcanic rocks. McCarty(1983)
suggested the Vadito
amphibolite
sequence
flows
of
mafic
unit
and
a
was
volcanic
volcaniclastic
sedimentary
rocks
intruded by felsic dikes and sills. The pillow struct-xes
reported by Long(1974) indicate
mafic
flows
The
that
erupted ainsubmarine
contact
at
least
some
of
and
the
these
setting.
between
the Marquenas
Quartzite
Vadito schist is nowhere well-exposed. If the boundacr
represents a primary
atop
to
graywacke,
the
depositional
then
disparity
of
the
surface
of conglomerate
contact
must
depositional
be
unconformable
environments
due
between
In the uppermost
Marquenas Quartzite and Vadito schist.
Vadito schista calc-silicate horizon is present. If the
Marquenas
then
the
this
top
Quartzite
and
horizon
could
of
The
the
Vadito
are
represent
a weathering
eroded
contact
schist
Vadito
between
schist
Vadito
unconformable,
surface
on
section.
amphibolite
and
Vadito
schist is generally poorly exposed.
In some areas the two
are separated bya bedding-parallel fault zone. Based on
the
it
presence
has
been
or
absence
suggested
of
cross-cutting
that
an
amphibolite and schist units(D.A.
discontinuity
could
just
as
granitic
unconformity
Bell, 1985).
easily
plutons,
separates
the
This
represent
an
early
shear
218
zone. To date, no structures indicative of early, locslized
shearing
have
and
plutons
the
been
Alternatively,
identified
that
this
between
intrude
contact
the
may
the
Vadito
Vadito
amphibolite
schist.
represent
a continuous
depositional contact. A progression from accumulation
of
mafic
volcanics
deposition
of
and
volcaniclastics
graywackes
with
(Vadito
minor
amphibolit,?)
quartzose
sediments
to
and
intrusions (Vadito schist) represents
a typical evolution of
depositional settings. Further work is clearly needed to
resolve
stratigraphic
and
structural
relationships
within
the Vadito Group. For the following tectonic discussion,
the
Group
( 2 the
Vadito
considered
asa single
Felsic
In
schist
the
texturally
Pilar
Marquenas
Quartzite)
lithotectonic
at
be
package.
Pilar
cliffs,
monotonous
will
the
sequence
felsic
of
schist
feldspathic,
at a Pilar
is
quartz-eye,
quartz-muscovite schist. The only appreciable variation in
mineralogy
the
is
due
to
Mn
and
rare
earth
element
enrichment
uppermost30 m of the section (Codding et al.,
1983).
Textures
felsic
volcanic
Although
rocks
throughout
no
with
the
protolith
geochemistry
similar
sequence
such
has
texture,
as
been
are
consistent
a
rhyolitic
reported
mineralogy,
and
with
ash-flow
for
these
tuff.
rocks,
stratigrapric
in
219
position
from
the
Tusas
chemistries
similar
continental
rifts
to
or
Mountains
modern
exhibit
felsic
continental
trace-element
volcanic
margin
rocks
back-arc
in
basin-;
in
or near continental crust (J.M. Robertson, personal
communication, 1987).
Ortega
Of
Range,
Group
the
three
the
supracrustal
sedimentology
of
packages
in
the
Picuris
Ortega isGroup
the moat
the
studied, and its depositional setting is best documentt..-d.
The
Ortega
is
overlain
black
Group
by
consistsa thick
of
interlayered
graphitic
phyllite,
basal
pelitic
and
quartzite
schists
laminated
and
pelitic
which
quartzites,
phyllites
and schists. Pervasive primary sedimentary structures
confirm a sedimentary
origin
for
these
rocks,
and
provide
reliable stratigraphic younging information. The Ortega
Group
was
deposited aon
broad,
shelf
during
an
overall
continental,
transgression
shallow
(Soegaard
Eriksson, 1985).
Soegaard and Eriksson (1985, 1986)
suggested
the
that
shelf
sloped
gently
to the
south
marine
and
and
southeast, and received
a continuous influx of sediment from
the
north
and
Sedimentologic
northeast
evidence
Group
rocks
throughout
shelf
break
must
lie
during
for
prolonged
shallow
northern
to
the
New
subsidence.
gradients
Mexico
southeast
of
in
Ortega
suggests
the
that
Truchas-Rio
the
220
Mora area (Soegaard and Eriksson, 1985). Soegaard and
Eriksson (1986) concluded
Ortega
Group
that
the
basal
accumulated ainstable
quartzite
of the
tectonic
environm?nt.
Discussion
Constraints
Relative
Range
the
no
The
and
only
three
on
absolute
loosely
major
supracrustal
possible
tectonic
settings
ages
constrain
rock
in
exception
is
rocks
possible
groups.
rocks
of
With
the
the
Picuris
setting-,
of
possible
Group
Cerro
the
tectonic
one
Vadito
in
Alto
exception,
have
been
Metadacite
dated.
(ca.
1673 Ma, D.A. Bell,
1985), a shallow to extrusive body which
appears to intrude the Vadito amphibolite. U-Pb zircox ages
of about1685 Ma (D.A. Bell, 1985) for the Rana and
Puntiagudo
minimum
plutons,
age
for
which
deposition
intrude
of
Vadito
at
least
schist, a yield
part
of
the
Vadito
Group. Grambling and Williams (1985b) noted that an
unpublished
for
U-Pb
feldspathic
Mountains
zircon
age
of about 1700 Ma by L.T. Silver
quartz-muscovite
couldbe correlable
with
schist
the
in
the
felsic
Tusas
schist
at
Pilar. This date probably represents the crystallization
age of the felsic volcanic protolith. In the Picuris Range,
no radiometric
dates
have
been
reported
for
deposition
of
Ortega Group rocks.If the Ortega Group did rest in primary
221
stratigraphic
to shearing
probably
contact
along
the
accumulated
Deformational
three
major
individual
in
been
contact
styles
rock
zone,
not
settings,
then
at
the
Pilar
Ortega
histories
help
because
prior
Group
constrain
of
the
possible
structures
recopized
compatible
during
a single
schist
structural
do
are
felsic
after
1700 Ma.
and
groups
groups
deformed
the
shortly
tectonic
eachof the
with
with
progressive
all hwing
groups
deformational
event. It is also true, however, that mylonitic ductile
shear
zones
Group
and
that
any
felsic
or
different
which
places
and
schist
all
of the
Preliminary
Group
separate
at
at
three
Group
Ortega
Pilar
allow
sequences
different
metamorphic
Ortega
the
for
evolved
from
the
in
the
Vadito
possibility
widely
times.
studies
have
Group
suggest
experienced
that
the
similar
Vadito
most
recent
P-T histories. If these groups have been juxtaposed
tectonically,
different
then
it
metamorphic
is
possible
histories
have
that
been
both
groups
overprinted
a
with
by
common post-assembly metamorphic peak. Additional work on
P-T paths
within
rock
groups
will
aid
in
resolving
this
question.
Possible
tectonic
settings
Vadito Group. The tectonic setting
of the Vadito Group
is
the
most
poorly
constrained
of
the
three
major
rock
222
sequences in the Picuris Range. With the possible exclusion
of the
Marquenas
lithologically
metavolcanic
Moppin
Quartzite,
similar
sequences
Metavolcanic
to
the
Vadito
several
that
Series
scattered,
have
of
Group
been
the
is
mafic
described
Tusas
Mountains
1958; Wobus
and Manley,1982; Williams, 1987), the Pecx
greenstone
belt
of
the
southern
(Robertson and Moench, 1979), and
Sangre
an
de
exposure
of
sequences
range
in
age
from 1765
about
Ma
the
(Barker,
Cristo
in the central Taos Range (Reed,
1984) (Fig. 8.1).
mafic
as
Mountains
mafic
rocks
Th?se
for
th?-
Taos
Range (S.A. Bowring, personal communication in Williams,
1987) to
about1720 Ma for felsic metavolcanic rocks in the
Pecos greenstone belt (Bowring and Condie,
1982).
age
for
the
Moppin
Metavolcanic
Series
A minimum
of
1755about
M,1
(L.T. Silver, personal communication in Williams,
1987)
results
from
an
age
of
the
Maquinita
Granodiorite,
which
intrude the Moppin rocks. These mafic sequences gene-ally
contain
bimodal
volcanic
suites,
and
are
in
contact
with
pre- to syn-tectonic, generally calc-alkalic plutons. Each
of
these
rocks
occurrences
that
probably
is
dominated
formed
in
in
by
arc
mafic
or
metavolcanic
back-arc
setting
(Bingler, 1974; Robertson and Moench,1979; Klich, 1983;
Soegaard and Eriksson,
1986; Grambling and Ward,
1987;
Williams, 1987).
of a similar
If the Vadito Group represents the remains
lithotectonic
package,
then
these
rocks
are
probably the oldest terrane in the Picuris Range. Although
may
223
Granitic rocks
UndividedPrecambrian
0V a d h Group
Picurie
(eoutnern
Range only)
224
no
geochemistry
Range,
it
is
exists
for
possible
Vadito
that
Group
this
rocks
sequence
qf
in
the
rocks
Picuris
represents
the typeI1 supracrustal assemblage of Condie
(1982). This
assemblage
consists
bimodal
volcanics-quartzite-arkxe
of
assemblages, and represents "lithosphere-activated
continental
riftsor aborted
mantle-activated
rifts"
(Condie, 1982, p. 341).
It is
not
orthoquartzites
clear
of
how
the
the
Marquenas
Vadito
Group
Quartzite
fit
and
similar
into
tect'mic
this
picture. Neither the Moppin nor the Pecos mafic sequences
contains
appreciable
thicknesses
of
orthoquartzite
and
metaconglomerate. Therefore, if the Marquenas unit is an
original
is
then
partof the
tectonically
the
variety
equivalent
Vadito
of
Vadito
Group
mafic
Group,
to
of
and
if
the
Vadito
the or
Pecos
Moppin
the
arc
or back-arc
southern
association
terrane,
Picuris a Range
that
previously been described in the southwestern
U.S.
Soegaard
section
that
is
and
Eriksson
(1986) proposed
compriseda distinct
in
fault
that
the
lithostratigraphic
contact
with
Ortega
G.-roup
has
is
not
However,
Marquenas
sequence
rocks
to
the
north
Vadito rocks to the south. This idea is plausible.
Alternatively,
the
Marquenas
Quartzite
might
rest
unconformably above an older mafic Vadito section. In
either of these
Marquenas
two
Quartzite
cases,
is
the
unknown.
tectonic
setting
of
the
and
225
Felsic schists at Pilar. Textures in the felsic schist
at
Pilar
suggest
a phenocrystic
Similar 1700 Ma
have
felsic
geochemical
volcanic
rocks
metavolcanic
signatures
in
felsic
volcanic
rocks
similar
continental
in
to
rifts
protolith.
the
modern
or
Tusas
Range
felsic
continental
mar”~in
back-arcs in or near continental crust (J.M. Robertson.
personal communication,1987).
felsic
volcanic
contain
trace
Ifcontinental
rocks
element
rift
of
On a more regional scale,
this
age
chemistries
systems
and
Ma
probably
represents
volcanic-sedimentary
U.S.
southwestern
characteristic
basins
of
in
modern
or
nelr
The felsic schist at
part
terrane
the
back-arc
continents” (Condie,1986, p. 857).
Pilar
in
of
extensive
1680 tl3 1700
the
that
is
found
in
New
Ml..-xico
and Arizona (Conway and Silver,
1986; Condie, 1986).
The
It is
base
of
the
notknown what
felsic
type
of
schist
at
continental
Pilar
is
not
crust
was
exp3sed.
rifzed,
and what the felsic schist at Pilar was deposited on. One
possibility
is
that
basement
to
the
felsic
schist
at
P’.lar
is Vadito Group or equivalent rock. The absence of clastic
sediments
structures
and
such
Pilar
suggest
active
phases
Rocks
mafic
as
that
of
similar
stratigraphically
volcanics,
growth
this
and
faults
unit
the
in
lack
the
of
identifiable
felsic
accumulated
after
schist
the
at
most
rifting.
to
the
beneath
felsic
the
schist
Ortega
at
Group
Pilar
in
most
lie
of
major Precambrian-cored uplifts of northern New Mexico. It
the
226
is
possible
represent
crust
that
the
prior
provided
thickness
final
to
one
these
voluminous
stage
Ortega
of
quartz
Group
sand
volcanic
stabilization
deposition,
component
of source
of
felsic
material
that
of
rocks
continental
and hmay
w e also
for
the
enornous
blanketed
the
region
at
about
1700 Ma.
The Ortesa Group. The Ortega Group, which is
regionally
central
exposed
New
probably
the
supracrustal
in
northern
New
Mexico
and
perhaps
in
1985), is
Mexico well
as (Bauer and Williams,
youngest
regionally
lithostratigraphic
exposed
package
Precambrian
in
northern
Nev
Mexico. The Ortega Group belongs to the 1650 toMa 1750
quartzite-pelite
1986),
and
continental
terraneof the southwesternU.S. (Conllie,
developed
shelf
subsequent
to
during
a first-order
rifting
on a stable
sea
level
rise
(Soegaard and Eriksson, 1985).On a regional scale,
detrital
zircons
from
the
Ortega
Quartzite
range
in
age
from
1976; Aleinikoff et al., 1985;
about 1850 to 1700 Ma (Maxon,
S.A.
Bowring, personal communication in Williams, 1987).
This
range
of
ages
Ortega
Quartzite
rocks,
and
could
(1982).
be
was
another
(Williams, 1987).
implies
from
that
the
component
one
source
underlying
was
derived
component
felsic
from
volcanic
elsewhere
It is possible that the Ortega Group
assigned
to supracrustal
assemblageI of
of
Condie
Such as assemblage consists of quartzite-carbonate-
the
227
shale,
is
characterized
by
extensive
cross-bedded
quartzite
with lesser amounts of carbonate, shale, and siltstone, and
reflects
either
"stable
continental
interiors" (Condie, 1982, p.
Ortega
rocks
Group
were
Possibly,
is
not
carbonates
above
or
stable
craton
Because the top of the
348).
exposed,
it is
deposited
margins
unknown
what,
if
the
Piedra
Lumbre
accumulated
within
the
any,
Formation.
uppermost
Ortega
Group.
Precambrian
The
based
with
on
Picuris
Range
available
adjacent
outline
Tectonic
of
Arolution
geological
data
terranes
Precambrian
from
in
the
summary
that
Picuris
Range,
northern
history
of
New
the
follows
is
similarities
Mexico,
a general
and
southwestern
U.S. by
Silver (1987).
>1700 Ma
1) The
Vadito Group accumulates in an arc or back-
arc setting (on oceanic lithosphere?). The Vadito
Group
is
tectonically
metavolcanic
assemblages
equivalent
in
the
to
mafic
Moppin
Metavolcanic Series, Taos volcanics, or Pecos
greenstone
2) The
North
arc
belt.
system
American
matures
craton.
and
is
accreted
to
the
228
1700 Ma
3)
The
primitive
Successor
4) The
continental
basins
felsic
form
during
schist
a northeast-trending
crust
is
rifted.
rifting.
Pilar
is deposited
at
chain
of
felsic
within
volcanic-
plutonic centers on or near the continental
crust. These igneous rocks interfinger with
subsequent
<1700 Ma
5) The
the
transgressive
thick
felsic
Ortega
schist
proximal
Group
at
basin
sediments
Pilar
in a stable
sediments.
accumulate
on
continental
shelf setting. Basin subsidence matches sediment
influx during a first-order sea level rise.
1680 Ma
6) Extensive
bodies
1650-1450 Ma
in
plutonism
the
with
Vadito
emplacement
of
granitic
Group.
7) Extensive, prolonged tectonism with
approximately
north-south
Possibly
to
the
due
crustal
shortening.
accretiona microcontinen?
of
southwesternU.S. at
with
about1640 Ma (Cond,ie,
South-verging simple shear strain was
1987).
concentrated
below
the
mechanically
stiff
Orfega
Quartzite. With progressive shortening, n0Y-hverging
ductile
shear
zones
and
large
folds
develop. The Vadito Group moves northward over
the Ortega Group along
a ductile shear zone. It
is possible that this setting is analogous
to
that
described
in
the
Pecos-Rio
Grambling and Ward(1987), in
which
Mora
area
the
Pecos
by
229
greenstone
younger
belt
Ortega
has
been
Group
and
thrust
northward
underlying
over
felsic
rocks
along a south-dipping ductile fault. Deformation
has
waned
emplaced
<1450 Ma
8)
in
by
the
the
time
Vadito
anorogenic
Group
plutons
are
at 1450
about
Ma.
Horizontal isothermal and isobaric surfac?s are
superimposed on structures. At some later time,
rocks
levels
are
and
uplifted
to their
exhumed.
present
structural
CHAPTER
1)
CONCLUSIONS
9.
There exist three major supracrustal rock packages
in the Picuris Range: a) The Vadito Group, senso stricto,
a heterogeneous
sequence
of
metamorphosed
bimodal
volcanic,
volcaniclastic, and clastic sedimentary rocks; b) the
felsic
schist
homogeneous
at
Pilar,
feldspathic
scattered
exposures
quartz-muscovite
of
schists
relativnly
containing
distinctiveofquartzandfeldspar,thatprobably
represent
metamorphosed,
altered
phenocrystic
felsic
volcanic rocks: and c) the Ortega Group, an overall
transgressive, thick, stratigraphically continuous,
metamorphosed sequence of basal quartz arenite, interlayered
quartzites
laminated
shallow
the
pelitic
phyllites
marine
The
of
and
and
schists,
black
phyllites,
and
schists
that
accumulated
a
in
setting.
felsic
schist
at
Pilar
"Vadito
Group"
rocks
that
is
similar
to
one
compsment
Williams (1985)
et al.
described in the Tusas, Taos, and Rio Mora areas. Par"-s
of
the
more
are
similar
in
the
Fe
Range
2)
mafic
Tusas
to
Vadito
the
Range
(Pecos
Group
older
(Moppin
greenstone
in
mafic
the
southern
metavolcanic
metavolcanic
belt),
and
Picuris
sequences
series),
the
Taos
the
Range
found
Santa
Range.
It is unknown how these three lithostratigraphic
231
units relate to one another. The south-dipping boundary
between
Ortega
Picuris
Range
and
Vadito
group
rocks
is
a near-bedding-parallel,
in
the
ductile
southern
reverse
fault. Mylonitic structures indicative of simple shear
occur
all
In
Pilar,
at
along
the
zone.
northwestern
the
Pilar
this
basal
Picuris
Ortega
Range,
Quartzite
along
a low-angle
zone
of
in
the
cliffs
near
overlies
the
felsic
schist
high
ductile
shear
strain. The amount of bedding-parallel displacement that
has
occurred
3)
schist
along
this
shear
zone
is
unknown.
The Vadito Group to the south, and the felsic
at
Pilar
to
the
north
occupy
identical
structural
positions with respect to the overlying Ortega Group. Both
lie
structurally
4)
below
the
basal
Ortega
Quartzite
Form3tion.
The stratigraphic position of the Piedra Lumbye
Formation
is
found
to
be
above
the
Pilar
Phyllite,
and
is
therefore the youngest unit of the Ortega Group. The
section of Piedra
Lumbre
in
Picuris
the
trapped
southern
between
the
Formation
Range,
Pilar
is
Phyllite
that
is
the
type-section
actually
a fault
sliver
and
Marquenas
the
Quartzite.
5)
mainly
There exists some preliminary evidence, based
on
intrusive
relationships,
to
suggesta major
that
232
fault
or
unconformity
separates
the
Vadito
amphibolite
from
the Vadito schist.A disparity in probable depositional
settings
indicates
Quartzite
are
may
separated
the
Vadito
schist
by
either
a fault
or
and
Marquenac
an
It is therefore suggested that the Vadito
unconfomity.
Group
that
be
a composite
of
different
rock
groups
juxtaposed
along bedding-parallel faults and/or unconformities. If
such
faults
exist,
they
must
predate
D3,
as
the
contacts
are
folded by D3 folds.
6)
Although Ortega Group rocks locally contain
concordant, simple pegmatite bodies, no intrusions int3
Ortega
7)
rocks
have
been
found
in
the
Picuris
Range.
All rocks appear
to have experienced lower to
middle amphibolite facies metamorphism. Kyanite,
andalusite, and sillimanite coexist in the Ortega Grou,? in
the Hondo syncline area. Porphyroblasts of biotite, g,srnet,
and
staurolite
during
All
the
three
have
ductile
of
these
experienced
strain
multiple
history
minerals
grew
of
both
stages
Ortega
of
Group
pre-D3
and
grovth
rocks.
syn-
post-D3. Kyanite, andalusite, biotite, staurolite, and
sillimanite(?) all grew early in the strain history, pyior
to D3.
to
233
Supracrustal rocks have undergone
a progressive
8)
deformational
three
major
Dl is
shear
history
in
the
of
structures.
characterized
by
localized
zones
by
fold
rather
second
than
generation
inthe Ortega
folding
resulted
generations
strain,
The
that
large
of
Group,
formation
of
of
high
simple
structures.
structures,
D2, involved major
and
moderate
folding
in
the
Vadito Group. The Hondo syncline and the Copper Hill
anticline are both
F2 folds.
D2 involved considerable near-
bedding-parallel faulting.
In the
has
been
northcentral
tectonically
Picuris
doubled
Range
by
the
what
Ortega
may
Quartzite
have
a Dl, been
D2, or D3 imbrication. Similar shearing occurred benelth
the
Ortega
Quartzite
in
the
northern
Picuris
Range.
The third generation of structures,
D3, is
characterized by an intense, pervasive, east-trending
schistosity
or
crenulation
cleavage
(S2* in
the
Ortega
Group)
thatis the
rocks.
S2* has rotated and reactivated the earlier
dominant
cleavage
in
most
schistose
cleavages. Based on relict crenulation cleavages preserved
in
porphyroblasts,
passed
through
Minor F2* folds
All
the
S2* schistosity
an
earlier
are
recognized
threeof these
under
conditions
axes,
and
of
probably
crenulated
phases
in
of
approximately
with
has
the
at
least
stage
of
Ortega
deformation
coaxial
considerable
locally
development.
Group.
developed
principal
temporal
strain
overlap.
234
The major structural feature in the Picuris Range
9)
is
the
gently
west-plunging,
tight
to
isoclinal,
northward
verging F2 Hondo syncline in the Ortega Group. The Copper
Hill
anticline
the Hondo
thought
to
a minor,
be
parasitic
fold
on
syncline.
Kinematic shear indicators in the felsic schist at
10)
Pilar
is
suggest
that
the
Ortega
felsic schist sometime during
Dl.
vergence
sense
recorded
by
Quartzite
moved
southward
over
This is opposite to the
kinematic
indicators
along
the
Vadito-Ortega boundary and the attitudes of D2/D3 fold:.
This
south-directed
shear
may
have
been
the
earliest
component of movement in the Picuris Range during Dl. It
may
have
also
represent
been
reported
Pristine
schist
at
shearing
quartz
Pilar
regional
from
the
ribbon
contact
occurred,
transport,
at
Tusas
and
mylonites
indicate
least
as
locally,
Taos
at
that
similar
the
move-nents
ranges.
Ortega-felsic
south-directed
late
in
the
metam'xphic
history.
11)
plunging
The Copper Hill anticline ais
gently westF2
fold
that
dies or
out,
is
faulted
out,
to
the
east. This fold is anomalous, and may abe
parasitic fold
on
the
Hondo
south-dipping
syncline
D2/D3
that
reverse
has
faults.
been
uplifted
along
steeply
235
The older Rana and Funtiagudo plutons (ca.
1680
12)
Ma)
in
the
southern
may
be
as
early
The
probably
as
pre-
l'anorogenicft
1450 Ma
syn-
The
Range
Picuris
to
or
are
certainly
pre-D3,
and
syn-Dl.
Penasco
Quartz
Monzonite
is
late-syn-D3.
Granite
is
Range
of
probably
Picuris
related
Peak
to
in
the
the
1680ca.
Ma
southeastern
plutons
to
Picuris
the
west.
The
Granite
the eastern
correlated
13)
of
Tusas
with
Alamo
Range
the
that
Tres
resembles
have
Piedras
been
granitic
ro-ks
in
tentatively
Granite.
Stratigraphic and structural observations
an2
interpretations
listed
kinematic/stratigraphic
and
Canyon
accretion
of
the
above
models
three
limit
the
possible
major
number
for
the
of
evolution
lithostratigraphic
te-ranes
in the Picuris Range. All models involve
a complex
interaction of faulting, shearing, and folding in the midcrustal environment. In the best model, early southdirected
shear
mechanically
is
stiff
concentrated
Ortega
between
Quartzite
and
the
overriding,
the
relatively
incompetent felsic schist at Pilar. With progressive
crustal shortening, north-directed(?) ductile faulting
(thrusting?) transports Vadito Group rocks northward o-rer
the Ortega
Group-felsic
schist
at
Pilar
lithostratigraphic
package. As shortening continues, the deformational style
236
evolves
bulk
from
heterogeneous
Dl shearing to more
shortening
by
nucleation
and
homogeneous
growth
of
major
fold9
in
the Ortega and Vadito groups. The Ortega Quartzite-felsic
schist
at
Pilar
contact
shear
remains
a zone
of
intense
bedding-
much
theof
deformation
parallel
simple
throughout
history.
As D2 folding evolves (and locks up?)
an intense,
slightly
obliqueS3 cleavage
is
superimposed
on
earlier
structures in Ortega Group rocks. Near bedding-parallel,
steeply
south-dipping
structures,
faulting
accompanied
which
may
have
reactivated
Dl
the
D2 and D3 phases of deformation.
The felsic schist at Pilar is lithologically and
14)
geochemically
similar
to
felsic
schists
in
the
Tusas,
Rio
Mora, and Taos ranges. Geochemistries and textures of these
rocks
are
consistent
with
modern
continental
extension31
systems. The felsic schist at Pilar and related rocks may
have
accumulated
exposed
in
the
on
rifted,
Picuris
pre-1700
Ma
basement
that
is
not
Range.
The Vadito Group a
aswhole is dissimilar
to other
15)
pre-Ortega sequences in northern New Mexico. The
southernmost
Range
New
mafic
is
similar
Mexico
that
portion
to
are
mafic
of
the
Vadito
metavolcanic
geochemically
in
the
terranes
similar
to
Picuris
in
modern
northern
arc
back-arc settings. The northern metaconglomeratic-felsic
schist
portion
is
most
similar
to the
felsic
package
that
or
237
underlies the Ortega Group in the Tusas Range. Therefore,
if
the
Vadito
Groupa tectonic
is
composite
lithostratigraphic
sequences,
it
related
rocks
young
basement
felsic
a shallow
contain
old
rift-related
mafic
arc-
rocks.
schist
marine
at
Pilar
a stable
on
environment
continental
during
a major
shelf
sea-level
in
rise.
One possible model for the tectonic evolutio? of
17)
rocks
different
The Ortega Group accumulated depositionally above
16)
the
and
may
of
in
the
continental
Picuris
crust
as
Range
arcs
begins
or
with
back-arcs
accretion
prior
of
mafic(?)
to
1700about
Ma. At around 1700 Ma, rifting of this crust results in
production
crustal
mature
These
of
voluminous
stabilization
shallow
rocks
subjected
to
felsic
sufficient
marine
sediments
were
buried
to
an
extended
volcanism,
followed
to
accumulatio?
permit
along
moderate
orogenic
the
continent
crustal
event,
by
of
margin.
levels
and
due t
perhaps
l3
convergence and collision from the south. This extendI2d
orogeny
resulted
structures
three
which
in
formation
are
of
developed
lithostratigraphic
three
to
major
various
groups
of rocks
now
generations
degrees
exposed
in
in
of
the
the
Picuris Range. The extreme heterogeneity of strain on all
scales
Range
that
and
characterizes
adjacent
supracrustal
and
uplifts
plutonic
Proterozoic
is
rocks
the
at
rocks
effect
in
of
mid-crustal
the
this
Picuris
orogeny
levels.
on
Appendix 1.
Samples
Descriptions of geochronology samples
ranging
from
75 to 210 lbs
were
collected
from
outcrop exposures. Samples were crushed, pulverized,
and
separated
at
Washington
and
laboratories
University,
Mineral
Resources
in
and
at
the
the
New
Department
New
Mexico
Mexico
of
Geology
Bureau
Institute
of
of
at
Mines
Minin-j
and
Technology. Magnetic and heavy liquid separations
wer3 done
at Washington
University
and
the
Department
of
Geology
at
the University of New Mexico. Zircons were hand-picke-i by
M. Williams, P. Bauer, and students at Washington
University. U-Pb chemistry and mass spectrometry were
performed by Dr.S. A. Bowring at Washington Universit:?.
Plutonic
Rocks
Granite of Alamo Canyon. Montgomery(1953) corxelated
these
rocks
with
the
granitic
rocks
of
the
southern
Picuris
Range (the Embudo Granite). Textures, intrusive
relationships,
Granite
of
and
Alamo
deformational
Canyon
and
fabrics
the
four
differ
plutons
between
in
the
the
southwestern Picuris Range. Williams (1987) tentatively
correlated a similar
pluton
in
the
eastern
Tusas
Range
with
239
the 1654 Ma (Maxon,1976) Tres Piedra Granite
of the
northern Tusas Range. Therefore, the age of the Granite of
Alamo
Canyon
is
important
in a both
local
and
regional
perspective.
Granite of Picuris Peak. The Granite
of Picuris Peak
is a highly
tectonized,
complex
intrusion
that
is
intimately
interlayered with Vadito Group lithologies. This granite is
very
in
similarto the c.a. 1680 Ma
the
southwestern
Picuris
Rana
and
Puntiagudo
plutons
Range.
Cerro Alto Metadacite. Mappers in the Harding
Pegmatite
Mine
relationships
Metadacite
area
support
is
the
have
the
oldest
generally
agreed
that
interpretation
that
of
granitic
the
four
the
field
Cerrm3
Alto
plutons
emplaced in Vadito Group rocks (Long,
1976; R.J. Holcombe,
personal communication,1984; McCarty, 1983).
Bell (1985) reported a preliminary U-Pb zircon
1630 Ma
correct,
and
However, D.A.
age
of
about
for the Cerro Alto Metadacite. If this age is
then
Puntiagudo
examination.
the
Cerro
plutons,
Alto
and
body
field
is
younger
relations
need
than
re-
the
Rana
240
Metavolcanic
Rocks
Vadito GrOUD felsic cruartz-eve schist, southern Picuris
Range.
the
Original primary stratigraphic relationships between
Vadito
Group
and the
mafic
Taos
areas,
metavolcanic
and
Group
and
the
felsic
syhist
The Vadito Group is unlike both the'
unknown.
at Pilar are
older
Ortega
the
sequences
younger
in
felsic
the
Tusas,
Pecos,
metavolcanic
and
sequenzes
in
the Tusas, Rio Mora, and Taos areas.
An age of
crystallization
for
Group
southern
in
the
felsic
metavolcanic
Picuris
Range
rocks
may
in
help
the
Vadito
constrain
correlations. One possible complicationis that the Vadito
Group
might
tectonic
actually abecomposite
slices
of
various
package
rock
groups
containing
of
various
ages.
Rio Pueblo Schist, Comales CamDcrround. These rocys
were
tentatively
correlated
with
Pilar cliffs by Montgomery
(1962).
problematic
sheared
rocks
the
felsic
schist
in
the
Textures in these
permit
either
granitic
or
volcanic
protoliths. Zircons from the Comales rocks yielded
a
1665 Ma (S.A. Bowring, personal
preliminary age of about
communication, 1986).
with
Range,
ages
and
of
This age is relatively consistent
granitic
inconsistent
rocks
with
in
the
the
southwestern
proposed
Picuris
age/correlations
of felsic schist in the Pilar cliffs. Felsic schists at
241
Comales
phase
campground
of
the
could
be
southwestern
sheared
granite6
a volcanic
or
Picuris
Range
plutons.
Felsic schist, Pilar cliffs.
It is unknown how the
felsic schist relates to the Vadito Group. Rocks similar to
the
1700 Ma
(Williams, 1987).
primary
Pilar
of
schist
at Pilar
felsic
stratigraphic
in
the
felsic
deposition
rounded
conglomeratic
may
have
Range
are
arouyd
with
the
Range,
then
an
age
of
constrains
the
maximum
auartzite
cobble.
protolith
Group
felsic
schist
at
crystallization
age
for
sediments.
Rocks
Marmenas Quartzite
Large,
Tusas
contact
ofthe Ortega
Metasedimentary
the
If the Ortega Group does lie in
Picuris
schist
in
Formation,
quartzite
member
already
of
been
clasts
the
in
the
southern
Marquenas
quartzite
Quartzite
when
Formation
incorporated
into
the
sandy Marquenas protolith. This unit is thought to be older
than the Ortega Quartzite. Additionally, Marquenas
quartzite
Ortega
clasts
are
compositionally
Quartzite,so Ortega
rocks
were
different
not
a source
from
foz
th(3
the
Marquenas. Detrital zircons in the Ortega Quartzite in the
Picuris
Range
No rocks
of
have
this
U-Pb
age
ages
are
of 1830
about
Ma
recognized
in
(Maxon,1976).
the
southwestern
U.S.
242
It is
possible
originated
that
the
detrital
the
same
source
from
zircons
as
the
in
Ortega
quartzite
Quartzite
clasts
in
the Marquenas Quartzite. Even if this is not the case,
detrital
search
zircons
in
fora source
controversy
Marquenas
the
terrain,
concerning
Quartzite
Marquenas
the
and
clasts
add
some
stratigraphic
may
aid
in
perspective
position
of
Formation.
R6 schist member, southern Picuris Ranqe. The R6
member
that
of
the
probably
Rinconada
Formation
isotopically
contains
equilibrated
grains
??]tile of
during
peak
metamorphic conditions (M.L. Williams, personal
communication, 1986).
yield
an
absolute
U-Pb chronology of rutile should
age
for
peak
metamorphism.
th?
to
the
the
APPENDIX 2
Garnet-biotite geothermometry calculations for Picuris
samples.
microprobe
.
Equations are from Ferry and Spear (1978)
data are courtesy of M.L. Williams.
Range
All
244
Microprobe data for sample HC-64
GARNET
HCZSC AND 6.I E TR EACH
25?~1ov-86 123 10 An
Garnet Dara Sac: 4
Trav Lsn;-.h:
256
Step Lengsar
9.9
r l e l g n r r*rc.*::
Fa0
1
27.13
36.91
,X:30
1.39
3.03
2.97
2.31
1.:z
1.:1
3
4
zi.7:
1.40
1.94
97.11
1.99
E
2.21
1.a7
2.96
2.3:
I.:a
1.1:
6
37.11
1.99
1.92
2.74
2.75
2.66
2.60
2.48
2.38
2.54
1.19
1.12
2
-,
8
9
10
11
12
1
;
14
15
15
17
18
19
20
21
22
25
24
29
25
27
26.99
17.72
37.27
37.14
37.c4
97.60
37.39
57.94
37.95
3.99
38.25
:e.%
38.16
39.49
1.9;
1.94
1.97
1.9E
1.98
2.04
2.07
2.02
2.31
2.05
2.09
2.11
1.95
1.05
1.IC
1.14
1.08
l.Ll
1.09
2.14
1.12
1.06
2.09
2.07
1.99
1.82
1.63
3
8
.
:
'
39.79
2.08
2.10
1.52
1.40
2.00
1.32
S9.80
39.05
x.-?
37.01
1.29
2.0:
1.25
2.00
1.a:
0
.
"
0.AO
1.15
~~
1.:2
1.10
0.52
0.00
13-3
-7.30
20.86
20.53
20.92
21.09
20.59
20.95
21.15
21.10
37.46
57.15
37.07
37.21
37.26
~~
20.79
0.06
0.04
20.93
1.55
1.05
1.11
1.06
0.06
0.03
0.03
0.03
0.03
0.02
0.04
0.00
0.00
20.71
21.13
21.00
20.90
20.75
20.90
1.14
0.79
0.55
0.3
1.5:
0.00
i
0.06
0.07
0.05
0.04
0.03
0.04
0.04
11.11
1.0b
1.01
""
, 1Y
0.0-
20.92
17.50
-7.47
-7.17
20.74
20.79
21.05
BIOTITE
i i c iis
.
0.07
20.96
0.04
20.79
0.02
0.04
0.05
0.03
0.05
0.21
20.59
20.85
16.2
34.20
57.15
3.25
-7.45
37.09
36.76
36.86
3.94
76.51
-7.17
37.5
27.15
~
~~
36.98
3.93
57.24
36.99
37.09
Total
Bean
101.E1
101.21
100.51
102.07
101.41
100.9:
1oo.a5
20.04
20.04
20.04
20.04
20.04
20.03
20.04
20.09
101.72
101.17
100.91
101.40
101.62
100.73
100.71
100.81
101.i0
101.07
101.41
101.58
101.40
100.81
100.96
29.00
100.89
100.76
100.99
75.43
45.24
0 l . z
20.05
20.0z
20.0Z
20.09
20.04
20.0§
20.04
20.0:
30.09
20.04
20.04
20.04
20.01
20.020.03
20.04
20.04
20.104
20.04
245
Microprobe data f o r sample HC-377
GARNET
K177/4.15 OWNET
G a r n e t Data Set2 2
2:-Fah-87
0 1 4 4 PM
T r d v Lenprnr
232
3 t a ~Length:
8.0
Welpnr Percent,
1
FeO
MgO
MnO
CaO
TI02
A1203
5102
Total
Beem
59.19
2.04
2.04
2.11
2.03
2.05
2.09
2.06
1.06
1.07
1.08
1.07
0.71
0.71
0.64
0.65
0.60
0.59
0.00
0.EZ
0.01
0.02
0.02
0.00
0.02
21.04
20.88
36.7
36.55
3.74
36.50
36.60
Zb.34
3.60
100.42
100.83
100.75
100.01
100.46
100.40
99.95
100.D0
100.56
100.86
101.61
101.3
100.98
100.88
101.09
100.77
20.01
20.02
20.02
20.02
20.02
20.02
20.02
20.01
20.01
20.01
19.98
19.96
19.98
20.01
20.02
20.01
20.00
20.00
19.98
19.99
19.97
19.97
19.96
19.97
19.97
19.97
19.97
19.96
19.97
20.00
2
39.58
3 39.21
Z9.67
4
5 39.27
6 Z9.56
7 38.85
B Z9.54
~
2.09
2.CO8
10
11
12
z9.77
40.34
40.01
14 40.09
40.13
15
16 Z9.91
17 3 . 5 0
18 Z9.96
19 39.66
20 ;9.40
21 40.04
22 39.95
23 39.92
40.30
24
~~
26
=7
28
29
30
39.85
39.72
l.:8
0.12
0.00
2.09
2.10
2.11
2.05
2.02
2.08
2.02
2.~7
2.03
2.06
1.98
1.99
1.93
1.89
1.86
0.82
0.40
1.80
1.64
1.44
0.00
0.00
0.00
0.98
0.98
0.97
0.97
0.93
0.91
0.95
0.90
0.91
0.90
0.88
0.88
0.80
0.83
0.83
0.81
0.85
0.82
0.80
0.85
0.85
0.86
0.00
0.00
0.00
0.Z5
0.51
0.54
10.48
0.46
0.50
0.50
0.0.47
0.14
0.47
0.48
0.42
0.45
0.40
0.40
0.47
0.60
0.74
0.00
0.00
0.00
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.00
20.95
20.89
20.94
20.84
20.91
20.96
21.09
36.X
X.7Z
21.00
21.00
21.14
20.94
3.73
20.92
20.88
0.02 20.50
0.01
20.81
0.01
20.83
0.02
20.76
20.80
0.00
0.01
20.79
0 . w 21.59
20.92
0.02
0.02
20.92
0.02 20.65
0.01 20.30
0.27
0.00
0.14
0.00
0.00
0.00
36.61
Z6.79
36.80
26.9
36.41
36.58
36.61
35.59
36.79
26.46
36.67
57.03
36.78
37.80
?6.83
36.81
36.56
35.76
0.81
1.93
0.04
BIOTITE
F o i n t i 18 HC 377/455 BIOTITE
23-iab-87
BAS, Fila I
3 x13.625 Y =
49.104
F E i 80: nHF TI4 ALA 5IG X10 HAE CAA
Rel's:
s:?7 pn
99.1
100.90
100.X
100.06
101.16
100.66
102.42
101.15
100.84
100.17
98.83
2.46
2.19
0.04
246
Microprobe data for sample
HC-385
GARNET
H K S 5 ?AND 64 E Ti? EACH
24-N0~-86 1 0 : 3 PM
Garner. 3ara 3er: 2
Trav Lengtn:
194
Step Lengrh:
E.4
Weight F w c e n z l
Fei
1
2
=
4
5
5
7
a
9
10
11
12
79.79
w.07
Y9.79
40.25
40.10
40. 12
-9.61
30.0~
3.?2
40.65
40.3a
40.19
M5il
MnO
CaO
Ti02
A1202
Si02
Toral
Beam
1.ai
0.46
I,:
45
0. a=
0.03
0.05
20.95
21.08
-7.57
57.46
101.94
102.04
20.05
20. 0z
0.37
0. -6
0.
x
1.11
1.07
1.10
1.09
1. 10
1.06
0.99
20.31
-6.92
0.32
0.34
0.31
0. :2
0. 34
0.35
0.99
1.06
0.03
0.0:
0.01
0.01
21.21
21.12
20.96
27.41
-7.z4
0.96
0.95
0.95
0.91
0.74
0.0s
0.04
0.03
0.03
21.09
20.90
20.94
21.02
0.01
21.0~
5.s
0.78
U.0Z
0.00
0.04
20.99
21.12
21.12
20.96
21.05
21.01
27.X
57.61
37.32
56.97
3.12
102.6~
101.29
101.94
101.3
101.43
100.91
101.51
21.Z7
37.51
9.78
101.92
71.91
46.21
82.6:
1.85
1.89
1.35
1.az
1.8?
1.39
1.9z
1.75
1.97
1.97
1.96
2. 04
2.00
2.09
2.09
2.07
2.02
24
1.33
BIOTITE
.
0. "5
0.3F'
0.37
0.14
0.42
0.96
2.64
u. 4s
0. 66
0.62
0.62
2.05
1.78
1.36
0.20
0.51
0. 5 3
0.55
0.62
0. E5
0.57
0.54
1.05
0.01
0.46
0.06
0.00
20.05
20. uc
0.02
0.00
0.00
0.01
0.01
0.19
5.17
z.61
Z7.09
7.34
37.25
100.60
101.97
101.75
1Ol.X
3.29
102.21
101.54
101.59
-7.21
101.4:
37.Z8
20.05
20.05
20.06
20.05
20. 05
20. 05
20.06
20.06
20.06
20.06
20.05
20. 05
20.05
20.05
20.05
20. 05
20.05
20.05
20.06
20.06
247
Microprobe data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
37.x
56.79
37.06
-7.13
1.58
1.6:
1.63
1.60
Z6.62
1.62
T6.92
Z7.3
37.19
37.2:
27.29
3.76
37.29
37.49
37.5;
m.54
16
7.50
17
~7.87
19
20
3.x
37.31
38.02
Z7.98
~7.87
3e.:9
78.02
1.64
1.70
1.63
1.67
1.72
1.65
1.70
1.72
1.74
1.75
1.76
1.76
1.74
1.75
1.77
~~
l a za.15
21
22
25
24
25
26
27
28
29
30
-
1:
-2
7
~~
1.80
1.75
1..79
1.71
1.84
37.50
1.86
37.97
37.91
t.a?
1.81
1.94
x.41
38.84
19.32
1.91
1.93
1.57
1.14
78.5:
14.20
for sample HC-454
Z . 39
32
5.26
3.15
3.17
3.12
0.94
0.02
20.84
0.03 z0.75
0.91
0.90
0.02 2U.66
0 . ~ 9 0.06
20.88
0.81
0.02 2 0 . 6 9
0.91
0.0
25
0.84
36.71
36.68
76.63
-6.99
36.76
26.79
0.12
0.84
0.04
20.84
35.3
:.
-
5.08
2. 09
2.97
z. 98
1.91
2. a 7
2. as
2.85
2.79
2.75
2.66
2.65
2 . sa
2.53
1. cr,
2.45
2.34
2.28
2. z5
2.13
2. ua
2.01
1.75
1.17
0.81
0.00
0.94
0.04
20.79
37.09
0.85
0.01
20.94
0.89
0.85
0.02
20.76
20.72
zu.76
0.88
0.84
0.81
0.81
0.84
0.82
0.79
0.82
GU.77
0.78
0.04
0.77
0.77
0.77
0.74
0.72
0.66
0.61
0.56
0.55
0.58
0.Cb
0.00
*
~3.x
%ea.
20.~3
29 HC 377/455 BIOTITE
22-Frh-87
Rli8, F l l r
3 X=
22.310 Y =
16.070
Ref'$:
FE3 n6G HNF TI4 A L A SI0 610 NAE CAA
*
ni
100.11
100.16
100.7~
99.70
100.27
100.21
100.96
100.79
100.90
10u.34
100.~7
-6.98
7.25
0.u4
36.76
0.02
27.01
0.03 21.08 37.17
101.40
0.03
2U.60 36.99
100.55
0 . 0 ~ 20.79
~ 6 . ~100.~5
9
0.04
20.93
Z7.12
100.98
0.04
20.98
57.21
101.45
0.00 20.68
26.05
100.07
0.02 20.62 36.53
99.73
0.05 20.79
37.u4
100.31
20.80
~ 6 . 7 2 100.69
0.U3
20.57
36.73
100.33
0.02 20.91 3 . 1 9 101.00
0.01
20.67
36.80
100.69
0.01 20.92
36.95
100.76
0.00 10.63 57.17 100.1:
0.0u 20.94
36.69
100.33
0.01
20.73
3.11
100.26
0.C'U
20.77
57.27
1ou.96
0.03 20.52 26.97
100.97
0.01 20.90
37.14 101.25
0.00
22.06 38.Z8
101.91
0.38
26.66
95.76
BIOTITE
Point
100.84
::4F
FM
20.19
20.14
20.13
20.12
20.11
20.11
20.10
20.11
20.10
20.11
20.10
20.09
20.09
20.0E
19.97
20.00
20.10
20.09
20.10
20.10
20.09
20.09
20.09
c0.09
2U.09
20.09
10.10
10.10
20.10
20.10
20.01
20.03
20.12
248
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This dissertation is accepted on behalf of the facclty
of the Institute by the following committee: