Archean geology of the Spanish Peaks area, southwestern Montana
by Kenneth Julian Salt
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Kenneth Julian Salt (1987)
Abstract:
Archean exposures of the Spanish Peaks area of southwest Montana can be divided into at least two
distinct high-grade metamorphic terranes which are characterized by differences in lithology,
metamorphic grade, and structural style. The Gallatin Peak Terrane (GPT) consists of tonalitic
paragneisses, kyanite-bearing metapelites and intercalated amphibolites. This supracrustal package was
intruded by gabbroic dikes and sills and by a previously unrecognized suite of granitoids. The gabbroic
intrusions form two distinct series: one series recrystallized into nematoblastic amphibolites, while the
other series recrystallized into transitional granulites with complex corona textures. The granitic suite
consists of older hornblende monzodiorite and tonalite, biotite quartz diorite and tonalite, porphyritic
granodiorite, and younger trondhjemite and granodiorite to granite.
The Jerome Rock Lakes Terrane (JRLT) consists of K-feldspar paragneisses with locally extensive
development of anatectic migmatite, sillimanite-bearing metapelites, and intercalated transitional
granulites. The JRLT does not share the early granitic intrusive history of the GPT, but was intruded by
the youngest granitoids and by the two series of gabbroic dikes and sills.
The two terranes are juxtaposed along a previously unrecognized ductile shear zone which is parallel to
the regional foliation, which strikes northeast and dips steeply to the southeast. Field and textural
evidence indicates that juxtaposition occurred during or just prior to high-grade metamorphism and
injection of the youngest granitoids and the two series of gabbros. Textural evidence further suggests
that rapid uplift along the shear zone followed juxtaposition, perhaps facilitated by the presence of
anatectic and intrusive melt phases within the system.
The plutonic, metamorphic and structural styles in the Spanish Peaks are strikingly similar to the
Phanerozoic Cordilleran configuration of southeastern Alaska and northern British Columbia. The
emerging pattern in the Archean basement of southwest Montana of juxtaposition of discrete crustal
blocks in a Cordilleran-type setting may reflect 'a period of rapid growth of the Archean continent
through the accretion of possibly genetically unrelated terranes. ARCHEAN GEOLOGY OF THE SPANISH PEAKS AREA,
SOUTHWESTERN MONTANA.
by
Kenneth Julian Salt
A thesis submitted in partial fulfillment
of the requirements for the degree
Master of Science
in
Earth Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
March, 1987
I
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iv
ACKNOWLEDGEMENTS
The
author
Lageson,
and
encouragement
would
John
and
like to thank
Childs
of
invaluable
Professors
the
thesis
David
Mogk,
David
for
their
committee
advice throughout
the
course
of
the
project.
Partial funding for the research was provided by a grant from NASA
through Dr.
this
Mogk.
study.
Dr. Mogk also supplied microprobe analyses used in
from
the
Research Creativity fund of Montana State University which enabled
the
author
to
Travel
present
grants
the
were
results
provided to the
of
this
author
research
at
professional
meetings.
Able field assistance was provided by Paul Anderson, who tolerated
everything
course
from
mosquitoes to ridge-top lightning storms
of the research.
during
the
Will Gavin provided the use of his llamas to
pack samples out of the study area.
Reggie Clark and Mike Clow of the
U . S. Forest Service provided assistance when base camp facilities were
discovered to be missing after a long day in the field.
also of the U .
S.
Susan
Marsh,
Forest Service, found the missing equipment several
weeks later.
Finally,
financial
father
to
meetings,
the author indebted to his wife,
suppdrt
Vickie,
for the family of the author,
acted
who
provided
as
surrogate
his children while the author was away in.the field and
and
provided
compilation of this report.
moral
support
to
the
author
during
at
the
V
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS...............................................
LIST OF TABLES...............................
iv
vii
LIST OF FIGURES................................................ viii
,LIST OF PLATES. .............
ABSTRACT...........................................
x
xi
INTRODUCTION...................................................
I
GALLATIN PEAK TERRANE... ........................................
6
Tonalitic Paragneisses......................................
6
Hetergeneous Metasupracrustal Suite......... ..........••••
H
Granitoids...............................
Biotite Tonalite Gneiss......................
15
Hornblende Granitoid Gneisses,........................
16
•Porphyritic Granodiorite......
18
Granite...............
19
Pegmatites.....................
22
Summary..........................
22
Ultramafic and Mafic Rocks.................................
23
Ultramafites........................
23
Amphibolites..........
26
Transitional Granulites..............................
27
JEROME ROCK LAKES TERRANE......................................
Quartzofeldspathic Gneisses................
Granitic Paragneisses (KQFG).........................
Leucogneisses............. ........i..................
Metapelites and Quartzites....................
Transitional Granulites......... ....................... •••
Intercalated Granulites
Leucogranulite................................
Amphibolites and Ultramafites...........................
Granitoids............................. ..........-•......
Summary.... ..............'..... ................,..........
35
35
35
37
40
42
42
43
45
45
47
vi
TABLE OF CONTENTS— Continued
Page
DUCTILE SHEAR ZONE........................
49
PHYSICAL CONDITIONS OF METAMORPHISM............................
53
Petrogenetic Associations........
Geothermobarometry...................... ,............ .
Garnet-Biotite.......................................
Garnet-Clinopyroxene.................................
Geobarometry.........................................
Summary..............
53
55
55
57
58
60
STRUCTURE......................................................
63
CONCLUSION..........
69
Tectonic Evolution of the Spanish Peaks...................
Discussion................................................
REFERENCES CITED
69
73
76
vii
LIST OF TABLES
Table
Page
1.
Mineral assemblages of the Gallatin Peak Terrane.......
8
2.
Modal mineralogy of the granitoids.....................
14
3.
Summary of mineral assemblages in the JRLT.............
36
4.
Comparison of lithologies, metamorphic and plutonic
histories of GPT and theJRLT...........................
48
5.
Summary of P-T calculations............................
56
6.
Proposed sequence of geologic events for the
GPT and the JRLT.......................................
71
viii
LIST OF FIGURES
Figure
1.
Page
Archean exposures of the Spanish Peaks and other ranges
in southwestern Montana......... ;.....................
2
2.
Index map of the Spanish Peaks area....................
2
3.
Schematic cross-section through Spanish Peaks
area.....................................
4
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Modal quartz (QTZ)5 plagioclase (PLAG)5 K-feldspar
(KSP) ratios of paragneisses, central
Spanish Peaks area.... ................................
.7
Modal proportions of quartz (QTZ)5 plagioclase (PLAG)5
K-feldspar (KSP) of granitoids.........................
13
Cross-cutting relationships in granitoids near
Gallatin Peak...........................................
17
Magmatic epidote (E) cored by allanite (A)5 surrounded
by biotite in porphyriticgranodiorite..................
20
Transitional granulite corona texture in metabasite
near Deer Lake.....................................
29
Detail of corona texture from sample DC-6..............
29
Intermediate stages of development of corona textures
in cross-cutting transitional granulite metagabbro
MG-GP near Mirror Lake...
...... ................
30
Preservation of igneous exsolution in pyroxenes
from transitional granulite metagabbro DC-9
near Deer Lake.............. ..........................
32
Relict pigeonite (Pi) mantled by subcalcic augite (SA)
in cross-cutting transitional granulite
metagabbro DC-9............. .... .....................
32
Incipient development of garnet (G) coronas around
opaque oxide (0) and cpx (C) in metagabbro DC-9..... .
33
Ksp-bearing leucogneisses near the Spanish Lakes
38
ix
LIST OF FIGURES— Continued
Figure
Page
15.
Leucogneiss along trail to Mirror Lake....... ;.........
38
( 16.
Fibrolite (S) and biotite (B) embayments into garnet (G)
in metapelite sample CM-I7 from JRLT near ductile
shear zone...........................................
41
Successive retrograde symplectite coronas in
leucogranulite near DSZ.................
44
Youngest granite injected along mylonitized
amphibolite.........................
46
19.
Petrogenetic grids for the GPT and the JRLT............
54
20.
Graphic summary of P-T calculations................ ..
61
21.
Orientations of structures in the GPT and the. DSZ......
65
17.
18.
LIST OF PLATES
:
Plate
I.
Page
Geologic map of the central Spanish Peaks.......
(in pocket)
xi
ABSTRACT
Archean exposures of the Spanish Peaks area of southwest Montana
can be divided into at least two distinct high-grade metamorphic
terranes
which are characterized by differences
in
lithology,
metamorphic grade, and structural style. The Gallatin Peak Terrane
(GPT) consists of tonalitic paragneisses, kyanite-bearing metapelites
and intercalated amphibolites.
This supracrustal package was intruded
by gabbroic dikes and sills and by a previously unrecognized suite of
granitoids.
The gabbroic intrusions form.two distinct series: one
series recrystallized into nematoblastic amphibolites, while the other
series recrystallized into transitional granulites with complex corona
textures. The granitic suite consists of older hornblende monzodiorite
and tonalite,
biotite quartz diorite and tonalite,
porphyritic
granodiorite, and younger trondhjemite and granodiorite to granite.
The Jerome Rock Lakes Terrane (JRLT) consists of K-feldspar
paragneisses with locally extensive development of anatectic migmatite,
sillimanite-bearing
metapelites,
and
intercalated
transitional
granulites.
The JRLT does not share the early granitic intrusive
history of the GPT, but was intruded by the youngest granitoids and by
the two series of gabbroic dikes and sills.
The two terranes are juxtaposed along a previously unrecognized
ductile shear zone which is parallel to the regional foliation, which
strikes northeast and dips steeply to the southeast.
Field and
textural evidence indicates that juxtaposition occurred during or just
prior
to high-grade metamorphism and injection of the youngest
granitoids and the two series of gabbros.
Textural evidence further
suggests that rapid uplift along the shear zone followed juxtaposition,
perhaps facilitated by the presence of anatectic and intrusive melt
phases within the system.
The plutonic, metamorphic and structural styles in the Spanish
Peaks
are
strikingly
similar to the
Phanerozoic
Cordilleran
configuration of southeastern Alaska and northern British Columbia.
The emerging pattern in the Archean basement of southwest Montana of
juxtaposition of discrete crustal blocks in a Cordilleran-type setting
may reflect 'a period of rapid growth of the Archean continent through
the accretion of possibly genetically unrelated terranes.
I
INTRODUCTION
Archean
exposures
Madison Range,
in
the
southwestern Montana,
two distinct Archean terranes.
(Fig.
at
Spanish Peaks area
of
the
northern
occupy a transition zone between
To the east,
the
Beartooth Mountains
I) are comprised predominantly of granitoids which were emplaced
approximately
2.7
to
2.8
Ga
and
contain
inclusions
of
metasupracrustal assemblages as old as 3.4 Ga (Warner and others, 1982;
Wooden and others,
1985;
Richmond
1982; Mueller and others, 1982; Mueller and others,
and Mogk,
Archean
lithologies
(Peale,
1896;
1985).
West of the
Beartooth
Mountains,
are dominated by several metasupracrustal
Tansley and others,
1933;
Reid,
1957;
suites
Hfeinrich
and
Rabbitt, 1960; Garihan, 1979; Vitaliano and others, 1979; Erslev, 1983;
Clark
and
Mogk,
1985).
Reconnaissance geochronological studies
these
rocks yield a composite Rb-Sr model age of 2.7 Ga
resetting occurring at 1.9 and 1.6 Ga (Giletti,
1966,
with
of
thermal
1971; James and
Hedge, 1980).
Archean
by
Spencer
exposures in the Spanish Peaks were originally
and
Kozak
(1975) as
a
single
described
metasupracrustal
suite,
dominated by tonalitic and granitic paragneisses with minor metabasite,
metapelite, quartzite, marble, and ultramafite.
the
overall
structural
trends
and
Their study emphasized
attempted
to
correlate
the
deformational features to the initial geochronologic studies of Giletti
(1966,
does
1971).
not
allow
The generalized nature of the previous study, however,
adequate
constraints
to
be
placed on the tectonic
2
TOBACCO
ROOT
TN-S--^q 1
BOZEMAN
BEARTOOTH
\
MTNS
RUBY X
RANGE/:
;\ S OUT H
;\ r x MADI SON
X V ;:\ RANGE
BLACK-
TAIL
RANGE
SITE
Figure I. Archean exposures of the Spanish Peaks and other ranges in
southwestern Montana.
I R 4 E
R 2 E • R 3 E
Diamond
Lake
J E R O M E \ L, K.
/
R OCK
^Soli tude /
LAKES
I »
^
."
x
GALLATIN
PEAK
S p a n l s h c^ x
TSS
TGS
Wilson Pk
Figure 2.
Index map of Spanish Peaks area. Labelled are locations of
sites discussed in text, with pack trail access into the study area
shown as dashed lines. Outline is area of Plate I. Heavy waved line is
mapped extent of ductile shear zone.
3
history
of the Spanish Peaks,
placed in a regional context.
nor have these pivotal
Therefore,
exposures
been
this paper presents detailed
lithologic and petrographic descriptions in order to place more precise
constraints
should
on the timing and conditions of metamorphism.
provide
a
more
complete basis
for
the
This
study
development
of
an
integrated tectonic model for Archean crustal evolution in southwestern
Montana.
The results of this investigation suggest a much higher degree
complexity
central
than
Spanish
was
previously recognized.
suite
metamorphic grade,
The Gallatin Peak Terrane (GPT; Fig. 2) is a
of
predominantly
kyanite-bearing
rocks
Peaks area can be divided into two distinct
based on differences in lithology,
style.
Archean
tonalitic
paragneisses
metapelites and amphibolites,
and
of
of
the
terranes
structural
metasupracrustal
intercalated
with
which is intruded by
previously unrecognized suite of concordant granitoids (Plate I).
Jerome
Rock Lakes Terrane (JRLT;
suite,
but
is
intercalated
transitional
GPT
describes
with
sillimanite-kyanite-bearing
granulites (Plate I).
structurally
the
2) is also a
paragneisses
metapelites
The two terranes
are
and
juxtaposed
southeast-dipping ductile shear zone, with
overlying
lithologic
The
metasupracrustal
composed primarily of K-feldspar-bearing
along a northeast-striking,
the
Fig.
a
and
the
JRLT
petrologic
(Figure
3).
characteristics
This
paper
of
these
terranes and serves to illustrate their different geologic histories.
Previous models of the tectonic evolution of the Archean rocks
this
region
easterly
have
proposed
the collapse of a basin
continental source (Spencer and Kozak,
1975;
marginal
to
Vitaliano
of
an
and
S C H E M A T IC C RO SS SECTION, GALLATIN PEAK AREA
kilometers
Figure 3. Schematic cross section through central Spanish Peaks area. The GPT structurally
overlies the JRLT along the northeast-striking, steeply southeast-dipping ductile shear zone
(DSZ). Line of cross section shown on Plate I.
Abbreviations of lithologic units as for
Plate I.
5
others,
Archean
1979).
basement
genetically
have
However,
of
the results of this study suggest
southwestern
unrelated terranes,
Montana
may
be
a
and that accretionary
been an important process in the Archean history of
Montana.
that the
collage
of
tectonics
may
southwestern
6
GALLATIN PEAK TERRANE
Tonalitic Paragneisses
Grey,
well-foliated
quartzofeldspathic gneisses with an
overall
tonalitic composition occur between the ductile shear zone and Gallatin
Peak
(Plate
I).
In
contrast to the paragneisses of
feldspar is absent or present in trace amounts only
gneisses
have
centimeter-scale
biotite-rich
the
(Fig.
and
JRLT,
4).
These
hornblende-rich
compositional layering and are interspersed with centimeter- to
scale amphibolite layers and boudins.
K-
meter-
Although these gneisses do not
contain intercalated rock types of more clearly sedimentary origin, the
even,
small-scale
suggest
compositional layering and lack of igneous features
a supracrustal origin for these
gneisses.
gneisses are referred to as tonalitic paragneisses,
further
Therefore,
these
while noting
study is necessary to confirm a supracrustal origin for
that
these
gneisses.
The
order
main
of
mineral constituents of the tonalitic
decreasing
olive-green biotite,
minerals
abundance are plagioclase
(An
paragneisses
20-30),
and green (Z) hornblende (Table I).
include sphene,
apatite,
in
quartz,
Accessory
opaque oxides, and zircon.
Garnet
occurs rarely in the more mafic compositional layers. Secondary epidote
and
chlorite
occur
in some samples.
ranging from 0.1 to 3.0 millimeters.
form
a
Grain
size
is
heterogenous,
Plagioclase and quartz generally
mosaic ,texture with straight grain boundaries, but xenoblastic
7
QTZ
T ONA L I T E
GRANITE
GRANODIORITE
FLAG
A
-
PAR A GN E IS SE S,
GPT
PA R A GN E I S S E S , J R L T
■ — K s p - r i ch g n e i s s I KQF GI
# — L e u c o g n e i ss
Figure 4.
Modal quartz (QTZ), plagioclase (FLAG) and K-feldspar (KSP)
ratios of paragneisses, central Spanish Peaks area. Ternary fields
after Streckeisen (1976).
8
MINERAL ASSEMBLAGES OF THE GPT
Paragneisses
plag-qtz-biot-hbld-sph-ksp-apat-op-zir-chl-epid
plag-qtz-biot-hbld-gt-sph-apat-op-zir-epid
plag-qtz-biot-gt-cunun-apat-op-zir
Quartzites
qtz-epid-mag-hbId
qtz-musc-gt-mag
qtz-biot-zir
qtz-biot-ky-st-zir
Metapelites
plag-biot-qtz-ky-gt-zir-apat-rut
plag-biot-qtz-ky-musc-apat-op-zir
plag-biot-qtz-ged-musc-apat-op-zir
plag-biot-qtz-ged-gnt-(st-chl)-apat-op-zir
plag-biot-qtz-ged-gnt-ky-(st)
ged-ky-biot-op-zir
biot-ky-qtz-musc-sill
Amphibolites
hbld-plag-qtz-op-chl-epid-all
hbld-plag-qtz-biot-gnt-op-epid
hbld-plag-qtz-cpx-op
hbld-oa-plag
Ultramafites
mg hb-ol-opx-phl-chl-op-tc
mg hb-opx-sp-phl-op-chl
mg hb-anth-plag-apat-op-zir
mg hb-anth-mal
mg hb-cumm-plag-phl-op
mg hb-op
Transitional Granulites
plag-cpx-gt-hbld-op-qtz-scap
plag-cpx-gt-hbld-biot-op-qtz
plag-igneous cpx-igneous opx-gt-hbId-op
hbld-plag-cpx-sph-op
Table
I.
Mineral
assemblages of the Gallatin
Peak
Terrane.
Abbreviations are as follows, to be used throughout the study: plag
(plagioclase), biot (biotite), muse (muscovite), qtz (quartz), ky
(kyanite), sill (sillimanite), hbld (hornblende), ged (gedrite), anth
(anthophyllite), mg hb (magnesian hornblende), cumm (cummingtonite), gt
(garnet), zir (zircon), epid (epidote), chi (chlorite), apat (apatite),
op (opaque oxides), mag (magnetite), ol (olivine), cpx (clinopyroxene),
scap (scapolite), sph (sphene), mal (malachite), all (allanite), rut
(rutile),
ksp (K-feldspar), opx (orthopyroxene), oa (orthoamphibole).
Minerals in parentheses are relict inclusions.
9
textures are not uncommon. Biotite is aligned parallel to compositional
layering and hornblende commonly forms nematoblastic aggregates.
■>
The centimeter- to meter-scale amphibolite layers are composed
of
green
of
(Z)
quartz,
hornblende,
plagioclase (An 35-40), and lesser amounts
olive-green biotite,
apatite,
opaque oxides,
Plagioclase and
quartz
and sphene.
and zircon.
form
mosaic
Accessory minerals include
Diopside occurs in one
textures
with
sample.
straight
grain
boundaries', and hornblende exhibits nematoblastic textures. While there
is
no
evidence
amphibolites,
some
shear zone.
by
plagioclase
up
show signs of retrogression,
(Z)
amphibole and locally
any
of
the
especially near
the
green hornblende is mantled
replaced
by
epidote,
distinct
migmatite
Semi-concordant
styles
occur
interlayers
within
of black
the
30
meters.
amphibolite
Interlayering of the two rock types
centimeter- to meter-scale.
which
regional
cross-cut
foliation.
the
occurs
on
a
Within these sequences, small trondhjemite
the amphibolite have been flattened
The composition and textures of
of granulite-forming reactions.
amphibolites.
and
thickness
into
amphibolite
these packages are very similar to the layers described above,
evidence
and
tonalitic
trondhjemite occur as distinct packages that range in
to
dikes
in
exhibits locally extensive sericitization.
paragneisses.
white
relict higher grade assemblages
In retrograded amphibolites,
blue-green
Two
of
in
with no
No mafic selvages occur
Trondhjemitic layers consist of optically
the
in
unzoned
plagioclase with composition varying from An 20-40 in different layers,
quartz,
and
present
in
minor hornblende and biotite.
accessory
amounts.
Interstitial microcline is
Plagioclase and
quartz
form
mosaic
10
textures
and
foliation.
biotite
The
relationships
(Yardley,
lack
is generally aligned
of selvages and the
parallel
presence
to
of
surrounding
cross-cutting
suggest that these migmatites are of the injection type
1978).
The deformational features and microtextures suggest
that injection occurred prior to or during peak erogenic activity.
Migmatites
with
a
layered structure,
or
stromatic
migmatites
(Johannes and Gupta, 1982), are characterized by leucosome layers which
vary
in
Leucosome
thickness
layers
concentrated
hinges
grade
into
millimeters
several
centimeters.
which
is
Bordering
Johannes
and
compositions
melanosomes
Gupta,
1982)
The leucosome
and
layers
as seen in other migmatite
are not
concentrations
melt-forming reaction.
rocks
of
have
well-developed
terranes
in
these
garnet and
indicate that biotite has broken down
in
a
This relationship has been described in Archean
the Superior Province (Harris and
specifically
fold
hypidiomorphic-granular
migmatites, but in tonalitic gneiss adjacent to leucospmes,
magnetite
commonly
in the pressure shadows of mafic boudins and in the
granodioritic
textures.
to
granitic pegmatite
of both isoclinal and open folds.
overall
(e.g.
from a few
Goodwin,
1976)
and
was
related to the generation of a melt phase by the reaction
biotite = garnet + magnetite + (quartz + H^O + K+)melt.
Therefore,
the
while some of the leucosomes may be related to injection of
youngest granitoids,
at least some of the stromatic layers may be
the result of in situ partial melting of the tonalitic paragneisses.
11
Heterogeneous Metasupracrustal Suite
Tonalitic
Bear
Basin
schists
paragneisses
area
(Fig.
on Gallatin Peak and southward
2) have numerous
intercalations
of
boudins,
These rocks
therefore mapped as a heterogeneous metasupracrustal suite
I).
East
of
Wilson Peak and in the Gallatin River
the
pelitic
and quartzites in addition to amphibolite layers and
in.contrast to the tonalitic paragneisses described above.
are
into
Canyon
(Plate
(Fig.
2),
pelitic assemblages again become rare to absent.
Metapelites
occurrence
of
of
this
kyanite
suite
and
are
characterized
gedrite.
Garnet
constituent of many of the pelitic rocks (Table I).
developed
Basin
in
metapelites
contains
mats
can
be
common
also
a
common
A contact
aureole
is
intruded by a large granitic sill
in
with
used
In the same vicinity, an important limiting
to place tight brackets on
1985) (see below).
inclusions of staurolite and chlorite,
superimposed
Bear
gedrite
of gedrite-garnet-kyanite-biotite-plagioclase occurs
(Hudson and Harte,
is
the
of coarse grained gedrite-kyanite
crystals up to 15 cm long.
assemblage
by
the
conditions
Gedrite and garnet both contain
indicating that this assemblage
on lower-grade assemblages.
with the southern Madison Range,
metamorphic
which
This finding
contrasts
where gedrite has been interpreted to
be retrograde after granulite-facies assemblages (Erslev, 1983).
Three
green
types
micaceous
Quartzite
with
granoblastic
zone
of quartzite occur in the GPT.
quartzite with traces of garnet
millimeter-scale
epidote
foliation
The most common
and
defined
opaque
by
occurs in Bear Basin and proximal to
near the Chilled Lakes.
Trace amounts
of garnet and
is
oxides.
layers
the
of
shear
magnetite
12
are
visible in hand sample in both
quartzite with tiny (.05mm)
occurrences.
Blue,
kyanite-rich
anhedral staurolite and scattered
garnets
occurs west of Wilson Peak.
. The
above
tonalitic paragneisses, metapelites, and quartzites described
comprise
distinct
from
a
metasupracrustal
the
suite
K-feldspar bearing
which
suite
of
is
compositionally
the
JRLT. Mineral
assemblages and textures in these rocks indicate that peak metamorphism
reached
upper amphibolite facies conditions.
There is no textural or
mineralogical evidence of any earlier, high-grade
GPT,
in
contrast
metamorphism
to previous interpretations involving two
in the
or
high-grade events in the Spanish Peaks area (Spencer and Kozak,
Instead,
the
relict
that
staurolite
and chlorite inclusions
any .earlier
metamorphism
1975).
j
pelitic
in
occurred
more
schists
indicate
at
lower
grades.
These relicts may be remnants of a separate, lower-grade event
or may represent the early stages of a single, prograde event.
Granitoids
The
metasupracrustal .rocks
described above were intruded
previously unrecognized suite of largely concordant
■comprises
roughly 1/4
granitoids,
by
a
which
to 1/3 of the total volume of Archean exposures
in the Gallatin Peak area.
Modal plagioclase-quartz-K-feldspar ratios
are plotted in Figure 5 and a summary of the total modal mineralogy
is
presented in Table 2.
The oldest granitoids are hornblende monzodiorite
tonalite
quartz
granitoid
gneisses,
and
diorite granitoid gneiss.
and
porphyritic biotite
These granitoids are
hornblende
tonalite
to
well-foliated.
13
QTZ
GRANITE
GRANODIORITE
T ONALITE
QUARTZ
Q U A R T Z M O N Z O N I TE
QUARTZ
MONZODIORITE
MONZODI ORI TE
DIORITE
PLAG
♦ — GRANITE
• — PORPHYRITic g r a n o d i o r i t e
▲ — B I O T I T E TONALI TE
■ - H OR N B L E N D E GR A N I T OI D S
Figure 5. Modal proportions of quartz (QTZ), plagioclase (FLAG), and
K feldspar (KSP) of granitoids. Field names for the granitoids as
discussed in text. Ternary fields after Streckeisen (1976).
HORNBLENDE
GRANITOIDS
BBR-GGD SP-35 SP- 34
Flag
51
Qtz
—
Ksp
MLM
BIOTITE
TONALITE
BBR-14GR BBT BBE-15G
44
51
71
45
50
51
8
18
17
28
25
15
20
2
tr
tr
2
Biot
I
2
12
13
19
19
Hbld
32
23
17
18
—
Sph
tr
I
—
tr
Muse
—
—
--
—
Epid
tr
tr
tr
tr
—
—
All
tr
tr
tr
tr
—
—
OD
tr
tr
tr
tr
I
tr
Apat
tr
tr
tr
tr
tr
Rut
—
—
—
—
Zir
tr
tr
tr
Flag
77
62
Qtz
—
Ksp
23
PORPHYRITIC
GRANODIORITE
BBGD BBE-10G SP-23
42
43
45
MLG
50
YOUNGEST
GRANITE
BBNG CLRMM
42
49
GP-12
34
15
30
20
29
26
30
14
29
—
14
21
17
16
24
28
34
12
4
3
5
8
4
6
3
—
—
7
11
I
—
—
I
tr
—
—
2
tr
I
—
—
tr
tr
—
tr
—
—
5
2
2
—
—
—
tr
I
2
tr
tr
2
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
—
—
—
—
—
—
—
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
tr
71
75
61
66
83
49
51
49
54
44
54
35
11
26
25
39
32
17
35
24
32
28
31
15
30
27
3
tr
— —
— —
16
25
19
17
25
31
35
—
2
Table 2. Modal mineralogy of the granitoids. 1000 points/sample. Mineral abbreviations as in
Table I.
Recalculated modal proportions of quartz, plagioclase and K-feldspar, as summarized
in Figure 5.
15
and
the
hornblende granitoids have a moderate
hornblende
The
two types of hornblende granitoid are very similar in outcrop
are
virtually
indistinguishable in hand sample.
lineation.
Because samples
both types have been obtained along strike of the same body,
mapped
can
as
the relationships between the
tonalite-quartz
rock
they
a single unit of hornblende granitoids until further
resolve
diorite
paragneisses
two
and
types.
is distinguished from the
of
are
study
The
biotite
tonalitic
country
described above by higher mafic content and by
the
presence of relict plagioclase phenocrysts and xenoliths.
Porphyritic granodiorite contains xenoliths of both the hornblende
and
biotite
types.
The
granitoids and is therefore younger than these
two
porphyritic granodiorite is moderately to weakly
rock
foliated
and is locally deformed by open to isoclinal folding.
The
youngest granitoids range in composition from granodiorite to
quartz monzonite (Fig. 5), and are referred to collectively as granite.
The granite intrudes all other granitoids and supracrustal rocks.
number
and size of intrusions increases with proximity to the
The
ductile
shear zone. Individual intrusions may lack foliation or may have weakly
to moderately developed foliation.
Many of the granite intrusions are
highly deformed by open to isoclinal folding.
Biotite Tonalite Gneiss
Biotite tonalite granitoid gneiss occurs only in Bear Basin (Plate
I).
Biotite
plagioclase
xenoliths
imparts
a
color index in the range
of
10-20.
Relict
phenocrysts up to 2 cm long are found within the unit
of
plagioclase-orthoamphibole
rock
occur
rarely
margins.
\
at
and
the
16
The
mineralogy consists of optically homogenous
25-30), quartz,
oxides,
in
(An
biotite, and muscovite, with accessory apatite, opaque
zircon,
present
plagioclase
and
trace
rutile
(Table 2).
quantities
in
Interstitial microcline
some
samples.
Grain
size
is
is
heterogenous, ranging from 0.05mm to 2.0mm. Plagioclase generally forms
mosaic
textures
boundaries
parallel
biotite
with straight grain boundaries,
are
more curved and irregular.
alignment
and
is
of biotite.
rimmed by
while
quartz
Foliation is
defined
Euhedral muscovite often
opaque
oxide
material,
grain
by
cross-cuts
suggesting
that
muscovite formed after biotite and is not of igneous origin.
Hornblende Granitoid Gneisses
Mafic
hornblende
layers
in Bear Basin,
(Plate
I).
because
These
gneisses (Cl 25-40) occur in
on Gallatin Peak,
units
thick
concordant
and in the Mirror Lake
are interpreted to be
of
plutonic
area
origin
they
contain scattered xenoliths of amphibolite and diopsidic
hornblendite.
The hornblende granitoids have ^ell-developed foliation
defined
by
alignments
of
hornblende and biotite and
developed hornblende lineation.
are
flattened
into
the regional
Early,
have
a
well-
cross-cutting pegmatites that
foliation
(Fig.
6)
are
commonly
associated with the hornblende granitoids.
Based on petrographic analysis, these granitoid
divided
into hornblende mbnzodiorite and hornblende
However,
their
as
noted above,
can
be
tonalite
gneiss.
they are mapped as a single unit because of
similar outcrop appearance.
monzodiorite
gneisses
The mineralogy of
consists of plagioclase,
microcline,
hornblende (2Vx = 60-70) with variable amounts
of
the
hornblende
and blue-green (Z)
quartz
and
olive-
17
Figure 6. Cross-cutting relationships in granitoids near Gallatin Peak.
Granitoids, from oldest to youngest, are labelled as follows: HG =
Hornblende monzodiorite; Pl = early pegmatite; G = granite; P2 =
younger pegmatites.
green
biotite (Table 2).
sphene,
epidote,
apatite,
oxides.
Plagioclase
slightly
more
microcline.
calcic rims.
Plagioclase,
Hornblende
occurs
nematoblastic
textures,
allanite,
suggesting
recrystallization.
chlorite,
Minor myrmekite occurs in
microcline
is
that
with polygonal
the hornblende has
25,
with
contact
with
grain
undergone
32%
and
boundary
metamorphic
Biotite is cross-cutting with respect to hornblende
within biotite grains,
coeval
and opaque
ranges in modal abundance from 23% to
aggregates,
of
and quartz generally form mosaic
and exhibits minor replacement by chlorite.
occurs
zircon,
has an average core composition of An
textures.
in
Minor and accessory mineralogy consists
Euhedral epidote
commonly
but it is not clear whether the epidote
with biotite or secondary.
Some
epidote
has
symplectic
18
intergrowths
formation
of
of
epidote
actinolitic
is
hornblende,
quartz and in rare instances is associated with the
associated
it
Hammarstrom,
is
rims on
with
biotite
probably
1984).
Opaque
hornblende.
that
Since
the
postdates
not of magmatic origin
euhedral
metamorphic
(e.g.
oxides do not occur in the
Zen and
matrix,
but
tonalite gneiss consists primarily of plagioclase
(An
occur as thin rims around sphene or biotite.
Hornblende
25-30),
quartz,
trace amounts.
epidote,
more
and biotite.
Microcline occurs only
Minor and accessory minerals include sphene,
chlorite,
abundant
abundant
hornblende,
opaque oxides, and zircon.
than
(Table
The
apatite,
Quartz and.biotite are
in monzodiorite samples, and
2).
in
hornblende is
less
grain size is less heterogeneous than
biotite tonalite gneiss and averages 1.0mm.
forms mosaic textures with quartz.
the
Plagioclase is unzoned and
Hornblende aggregates such as those
found in the monzodiorite are lacking in the tonalite where
hornblende
occurrs more commonly as single nematoblastic grains. Biotite, epidote,
and chlorite form secondary textures as described in the monzodiorite.
Porphyritic Granodiorite
Moderately to weakly foliated porphyritic granodiorite gneiss with
relict K-feldspar phenocrysts occurs in Bear Basin and on Indian
(Fig.
2,
Plate I).
Indian Ridge
tonalite
body.
Ridge
Relict phenocrysts impart an augen texture to the
Inclusions
of monzodiorite
gneiss occur in the Bear Basin outcrops,
gneiss
and
biotite
demonstrating
that
the granodiorite is younger than both of these rock types.
The
mineralogy
plagioclase,
quartz,
of
the
porphyritic
microcline,
granodiorite
biotite,
and
consists
of
blue-green
19
ferrohastingsite-rich
hornblende C2Vx = 40-50).
mineralogy includes sphene,
and ' zircon (Table 2).
Textures in thin section are sub-granoblastic,
more
composition
calcic
rims
microcline.
but
in
(An
Relict
thin
section
granitoids,
separated
by
individual
which
• xenomorphic
texture.
ranging from 0.05mm to 3.0mm.
averages An 25,
but individual
30) and myrmekitic
texture
in
grains
have
contact
with
phenocrysts of potassic feldspar average 2-3
are
perthitic subgrains.
older
lobate,
grain size is heterogeneous,
Plagioclase
accessory
epidote, allanite, opaque.oxides, apatite,
with limited preservation of original
Groundmass
Minor and
found to
be
recrystallized
Hornblende is not lineated,
to
cm,
smaller,
in contrast to
the
but instead occurs in optically continuous segments
intervening
segments
quartz,
have
curved
is interpreted as a relict,
resorbed
hornblende
biotite,
sphene,
allanite
(Fig.
segments
and
7).
igneous mineral (e.
euhedral
plagioclase,
and
embayed
igneous
commonly
epidote
or
K-feldspar.
grain
resorption
have reacted
boundaries,
texture.
to
form
which is commonly
The
platy
cored
These textures suggest that epidote is a
g.
The
by
relict
Zen and Hammarstrom, 1984), in contrast to the
retrograde epidote of the older hornblende granitoids.
Granite
Unfoliated
ubiquitous
to
moderately
throughout
foliated granite
to
granodiorite
the GPT. Map-scale intrusions
subconcordant with the regional foliation;
(Plate
is
I)' are
smaller veins occur as both
dikes and sills and range in thickness from tens of centimeters to tens
of
meters.
greatly.
The
degree
of deformation of granite
intrusions
Many of the intrusions are isoclinally folded,
while
varies
other
20
Figure 7. Magmatic epidote (E) cored by allanite (A),
biotite (B) in porphyrinic granodiorite.
intrusions
are
apparently
undeformed.
toward the ductile
The
intrusions
increase
intrusions
of granite are moderately foliated,
are hypdiomorphic-granular,
igneous feature.
biotite
schist
shear
surrounded
number
zone.
and
size
Although
thin section
by
of
some
textures
indicating that the foliation is a primary
Forceful injection of granite along a contact between
and
monzodiorite
gneiss in
agmatitic
complex of cliff-scale proportions.
scattered
ultramafic xenoliths,
Bear
The
Basin
created
granite
contains
including diopsidic hornblendite
metaultramafite composed of Mg-homblende,
an
and
orthopyroxene, and olivine.
No other types of xenoliths were found.
The
Q-Or-Pl
gran odiorite
ratios
of
the granite lie close
boundary on the IUGS diagram (Fig.
5),
to
the
granite-
overlapping
the
21
field
of the older porphyritic granodidrite.
similar to the latter
The mineralogy is
very
but is finer grained (ave. grain size of 0.5 mm)
and has no relict K-feldspar
phenocrysts.
Euhedral grains of ilmenite
up to 2 cm in diameter commonly occur along the margins of larger sills
near Gallatin Peak.
with
a
Hornblende occurs in trace amounts and is blue (Z)
low 2Vx of 10-15,
(Griffen and Phillips,
same
indicating a high ferrohastingsite
1980).
Where present,
hornblende displays the
segmented habit as in the porphyritic granodiorite
embayment
and
associated
replacement by biotite and epidote.
with
content
with
Biotite
euhedral epidote where hornblende is
similar
is
absent
also
and
is
partially altered to chlorite.
The textural relationships between hornblende, biotite and epidote
in granitoids are similar to those described in Phanerozoic
of
the
epidote
North
has
resorption
American Cordillera,
where the formation
been interpreted as a late-stage
product
granitoids
of
of
euhedral
hornblende
at moderate to high pressures (Zen and Hammarstrom,
1984).
In both the porphyritic granodiorite and in the granite, the resorption
textures of the hornblende suggest that similar processes occurred
that
the
epidote is of magmatic origin.
hornblende granitoids,
hornblende
and,
actinolitic
Therefore,
probably
rims,
in
However,
in
the
and
foliated
the euhedral epidote is formed from metamorphic
rare instances,
and is
is associated with formation
probably of secondary,
retrograde
of
origin.
it is suggested that in the youngest granitoids, epidote is
of
magmatic
origin,
but
epidote in
probably metamorphic or secondary in origin.
older
granitoids
is
22
Pegmatites
At
the
least
GPT.
two generations of intrusive pegmatites are present
This
fact was noted by Spencer and Kozak
(1975),
but
descriptions of their occurrences or mineralogies have been made.
oldest
intrusive pegmatites are restricted in occurrence to
veins within the hornblende granitoid gneiesses.
hornblende-rich
mineralogy
flattened
These pegmatites
quartz
and
no
The
and have an average grain size of 0.5 to 1.0 cm.
consists primarily of microcline,
in
are
The
hornblende,
with accessory apatite and opaque oxides.
The
younger
pegmatites
are ubiquitous throughout
the
GPT
and
intrude foliated and unfoliated granite (Fig. 6). These pegmatites are,
in
general, coarser-grained than the older pegmatites, with an average
grain size of 2-3 cm.
microcline,
The mineralogy of these pegmatites consists
plagioclase,
and
quartz,
with
accessory
apatite
of
and
ilmenite.
Summary
The relative ages and timing of emplacement of the granitoids
be
roughly
degree
The
oldest
foliation
textures
from cross-cutting relationships
granitoids
are
monzodiorite
and
lineation
hornblende
granitoid
and
gneisses.
and the development
biotite
The
of
by
the
tonalite,
high
and
degree
of
metamorphic
mosaic
in these phases suggest their emplacement prior to or
during
early stages of high-grade tectonism of the GPT.
granodiorite
biotite
and
of development of metamorphic textures in the different phases.
hornblende
the
determined
can.
The porphyritic
contains xenoliths of the hornblende granitoids
tonalite,
demonstrating
that
it is younger than
and
the
these
two
23
granitoids.
that
of
The partial development of metamorphic textures
indicate
this phase was also emplaced prior to or during the early
orogenesis.
At least some of the granite,
however,
during the peak of tectonic activity in the GPT.
interpretation
comes
from
foliation and from contact
assemblages
as
the
development
aureoles
in
stages
was emplaced
The evidence for this
of
primary,
metapelites
metapelites away from intrusions.
with
The
igneous
the
same
formation
of
magmatic epidote in the granite further suggests that emplacement,
and
therefore
7-8
high-grade tectonism,
occurred at mimimum pressures of
kbars.
Ultramafic and Mafic Rocks
Mafic and ultramafic bondins, dikes, and sills comprise between 10
and 20 percent of total outcrops in the GPT,
outnumbering
facies
ultramafic
assemblages,
metagabbros
trending
with
diabase
occurrences. Most possess
with
metamorphism (Table I).
no
evidence
of
any
upper-amphibolite
higher-grade
relict
The only exception is a series of synkinematic
transitional
and
with mafic bodies greatly
granulite
assemblages.
basalt dikes are unmetamorphosed
Northwest­
and
are
not
considered to be part of the Archean suite.
Ultramafites
The
Plate I).
largest ultramafic body is located near Wilson Peak (Fig.
2;
The Wilson Peak ultramafite is about 100 meters thick and is
continuous
increasingly
ultramafite.
over
at least 2 kilometers *
The surrounding
schists
muscovite-rich in a 20 meter thick zone approaching
The
ultramafite is mantled by a thin outer margin
are
the
of
24
garnetiferous
amphibolite.
large,
3
up
to
Garnets
cm in diameter,
in this margin
with
are
typically
plagioclase-depleted
haloes.
Amphibolite rapidly grades into an inner ultramafic zone which consists
almost
entirely of nematoblastic anthophyllite and
which crude gneissic
two amphiboles.
malachite
Mg-homblende,
banding is defined by limited segregation of
A weak schistosity is imparted by anhedral,
grains
which
may be secondary
mineralization
in
the
flattened
along
pre­
existing partings.
ultramafic body at Summit Lake (Fig.
2, Plate I) is roughly 20
meters
thick and occurs near the contact between hornblende
granitoid
gneiss
and
an
outer
mantle
of
Wilson
Peak
weakly foliated granite.
garnetiferous
ultramafite.
The
hornblendite,
oxides.
core
with
amphibolite
This ultramafite has
similar
to
the
of the Summit Lake ultramafite is
essentially
minor mm-scale bands of cummingtonite
Plagioclase
and
opaque
comprises up to 5 modal percent in some samples.
In the central core of the body, patchy remnants of orthopyroxenite are
preserved.
These
orthbpyroxene
oxides.
grain
remnants
grains
consist of aggregates of large
that
exhibit
extensive
exsolution
(2.0
of
mm)
opaque
The large orthopyroxene grains have heavily corroded,embayed
margins,
recrystallizing to form a fine-grained mosaic
of
Mg-
hornblende, inclusion-free orthopyroxene, and green spinel.
A
zoned
ultramafite
about 100 meters long occurs in
above the Chilled Lakes (Fig.
2,
Plate I).
the
The ends of the body
ridge
are
tapered and folded, indicative of post-emplacement tectonic disruption.
The ultramafite is texturally and mineralogically zoned,
with an outer
amphibolite margin, an outer core of magnesian-hornblendite, and a core
25
of
megacryst-bearing
amphibolite
rock
of
harzburgite
composition.
The
consists of subequal amounts of blue-green (Z)
and plagioclase with minor sphene and opaque minerals.
hornblende
In contrast to
the amphibolite mantles of the ultramafites described above,
absent.
The
hom b lende
outer
and
minerals.
core
of
phlogopite,
with
minor
garnet is
pale
cummingtonite
green
and
Mg-
opaque
Foliation is defined by planar alignments of phlogopite. The
Mg-homblende
inclusions.
is
riddled
with crystallographically
oriented
opaque
Grain size of I mm is nearly uniform, but some hornblende
megacrysts are up to 4 mm long.
equigranular
with
the body consists of
outer
The inner core consists of a matrix of
Mg-hornblende, olivine, orthopyroxene, and opaque oxides,
megacrysts
of
orthopyroxene up to
one
centimeter
long.
The
orthopyroxene megacrysts have numerous inclusions of opaque oxides, Mghomblende,
and phlogopite which appear to have grown along
in the orthopyroxene.
fractures
Some fractures in the megacrysts are filled with
talc that surrounds a core of opaque minerals.
Ultramafites
and
also
in the granite.
hornblendite,
diopside,
oxides.
and
which
occur as xenoliths in the monzodiorite
The most common type of inclusion
consists
of
light
blue-green
biotite with accessory apatite,
In. late-stage
is
(Z)
allanite,
granite in the Spanish Lakes area
gneiss
diopsidic
hornblende,
and
opaque
(Fig.
2),
xenoliths with mineralogy and texture similar to the inner core of
the
Chilled Lakes ultramaflte were found.
The following points should be noted regarding the ultramafites of
the GPT.
upper
First, the assemblages described above are representative of
amphibolite-facies
metamorphic
conditions
(Evans,
19775
26
Desmarais,
1981).
superimposed
on
There
any
is
relict,
no evidence that these assemblages
higher-grade
metamorphic
are
assemblages.
Second, many of the textures described above are similar to those found
in
ultramafites of the nearby Ruby Range (Desmarais,
1981) which were
interpreted to be pre-tectonic emplacements that had been serpentinized
prior
to metamorphism.
ultramafites
surfaces.
Lake
An important difference in the Spanish
is that opaque oxide inclusions do not define
Furthermore,
ultramafite
ultramafite,
and
the
Peaks
relict
S-
presence of chill-margins in the Summit
the long-range continuity of
the
Wilson
Peak
together with the formation of L- and S-textures, suggest
that these may be syntectoriic intrusions.
Amphibolites
Amphibolitized
metabasites are ubiquitous throughout the GPT
form a sequence ranging from older, isolated meter-scale mafic pods
younger,
mafic
more
continuous metamorphosed dikes and sills.
layering
Foliation and lineation are quite variable in
degree of development and orientation within these bodies,
earlier
to
The isolated
pods are formed by boudinage or by disruption of mafic
by nappe-style folding.
and
studies (Spencer and Kozak,
1975).
as noted in
Coarse-grained
granitic
pegmatite is commonly concentrated at the margins of the isolated pods.
It
is
origin;
other
not
clear whether the isolated mafic
are
of
many may be part of the original supracrustal suite.
intrusive
However,
amphibolite boudins form more continuously aligned sets and
more clearly disrupted intrusions.
dikes
pods
are
A swarm of amphibolitized sills and
with low discordance angles on Gallatin Peak were interpreted in
a previous study to be undeformed (Spencer and Kozak,
1975).
However,
27
closer
inspection
disrupted
by
shows
that many of these
small-scale
nappe style folds.
intrusions generally are strongly lineated
axes.
The
are
Hornblendes
locally
in
fold
foliated
Summit Lake and exhibit diabasic textures in outcrop.
mineralogy
hornblende
these
roughly parallel with
youngest amphibolitized dikes intrude moderately
granite near
The
intrusions
(Z)
of the amphibolites consists primarily
and
plagioclase
(An 35-45) with
ofx green
lesser
quartz
and
variable amounts of sphene. Accessory minerals include apatite, opaque
oxides
and
zircon.
amphibolites.
Olive-green
biotite
occurs
in
many
Garnet is most commonly found in the isolated
although it occurs in some younger, cross-cutting bodies.
boudins
of
the
boudins,
The isolated
are commonly zoned with respect to garnet, with overall garnet
content decreasing from core to rim.
Textures
are primarily nematoblastic,
amphibolite-facies
higher-grade
assemblages.
amphibolites.
isoclinal
Examination
fold
nematoblastic
exhibit
assemblages
types
were
hornblende.
have
with no evidence that
been
superimposed
intergrowths
the
of
and
fold hinges shows that both
accompanied
The
of
relict
Only minor retrogression was found in
by
youngest
synkinematic
amphibolitized
static recrystallization of mafic minerals to
symplectic
over
the
quartz,
open
growth
of
metabasites
hornblende
and
retaining
its
with plagioclase
original lath-shaped igneous habit.
Transitional Granulites
The assemblage
occurs
only
Distinctive
in
garnet-clinopyroxene-plagioclase-hornblende-quartz
widely
corona
scattered
mafic
boudins
and
intrusions.
textures are developed in these bodies which
vary
28
systematically
incipient
from
total recrystallization in
recrystallization
textures
in
classified
of
otherwise
assemblage as a high-pressure
De
Waard
have more recently shown that this assemblage is
between
amphibolite
facies
(Turner,
to
igneous
(1965)
granulite,
workers
and granulite
bondins
well-preserved
more continuous mafic intrusions.
this
isolated
but
has
other
transitional
1981;
Percival,
1983)s an interpretation adopted in this study.
Outcrops
of boudins with well-developed corona textures are
reddish-brown,
Most
massive,
dark
and are generally several meters in diameter.
have no apparent tectonite fabric in outcrop,
but one boudin
the Deer Creek area exhibits millimeter-scale shearing.
in
The mineralogy,
consists primarily of plagioclase (An 40-60), blue-green (Z) hornblende
(2Vx = 70-80), diopside, garnet, and opaque oxides.
as
symplectic
ilmenite
intergrowths with hornblende.
and
- 0.1
mm)
plagioclase by coronas of garnet or hornblende (Fig.
8).
In some instances,
modes.
Domains of diopside
are separated from a mosaic of fine-grained (0.01
granoblastic
quartz
Quartz occurs only
hornblende coronas with symplectic intergrowths
are also mantled by a garnet corona.
Larger
(1-2
Diopside occurs
mm) grains occur that have myriad
inclusions and rare patches of sub-calcic augite and/or
in
opaque
of
two
oxide
orthopyroxene.
These larger diopside grains are recrystallized into mosaics of smaller
(0.05 mm),
inclusion-free grains.
with plagioclase,
poikiliblastic
Garnet is inclusion-free in contact
but proximal to cpx or hornblende,
garnet is highly
with . inclusions of both hornblende and diopside
(Fig.
9). Scapolite occurs within the mosaic of recrystallized plagioclase in
boudins of the Gallatin River Canyon area, but not in the Gallatin Peak
29
Figure 8.
Transitional granulite corona texture in metabasite near
Deer Lake. Coronas of garnet (G) and hornblende (H) around cpx (C).
Matrix consists of plagioclase (P) and minor scapolite (S). Mm-scale
shear zone (SZ) also contains the transitional granulite assemblage.
Figure 9.
Detail of corona texture from sample DC-6. Large cpx grain
(C) recrystallized to fine-grained mosaic of smaller, inclusion-free
cpx (c). Garnet (G) forms corona around both hornblende (H) and cpx.
30
area.
The transitional granulite-facies mineral assemblage is present
in mm-scale shears, but corona textures have been obliterated (Fig. 8).
An
intermediate
exhibited
in
stage
two metabasites
of development of the corona
near Mirror Lake.
continuous dike south of Mirror Lake (Fig.
monzodiorite
layers.
gneiss
The other,
and
texture
une of these
2) which clearly
tonalitic gneisses
with
is
is
a
postdates
amphibolitic mafic
east of Mirror Lake (Fig. 2), is disrupted into a
linear series of boudins. Both metagabbros have amphibolitized margins,
but in the core, diabasic texture is moderately well-preserved. In thin
section,
garnet
(Fig.
diabasic
textures are modified by coronas of hornblende
around all clinopyroxene and opaque oxides,
10).
The
clinopyroxene is diopside
and
as described above
with patchy
remnants
of
Figure 10. Intermediate development of corona textures in cross-cutting
transitional granulite metagabbro MG-GP near Mirror Lake. Plagioclase
(P) has undergone little recrystallization while coronas of garnet (G)
and hornblende (H) form around all clinopyroxene (C).
31
subcalcic augite that retains complex exsolution t e x t u r e s Plagioclase
(An
50)
(Fig.
is
occurs as relict laths with only
10).
not
Red
recrystallization
biotite is part of the equilibrium
present.
consists
moderate
The
primarily
mineralogy
of
the
assemblage,
but
amphibolitized
' margins
of hornblende and plagioclase with minor
diopside
and sphene. •Corona textures and garnet are absent from the margins.
The
most complete preservation of igneous features occurs.in
metabasite
near
discontinuous
truncate
grades
bodies
Lake
(Fig.
2).
This
metabasite
separated . by covered intervals but
granitic pegmatite.
A thin (I meter) margin
forms
appears
of
In thin
plagioclase and pyroxene form sub-ophitic textures which
slightly modified by incipient development of metamorphic
textures.
to
amphibolite
into an interior with well preserved diabasic texture.
section,
only
Deer
one
are
corona
Pyroxene exhibits complex exsolution features resulting from
the inversion of pigeonite, producing intergrowths of subcalcic augite,
orthopyroxene,
and exsolved opaque oxides
domains
a
mantle
have
strongly
(Fig.
11).
Orthopyroxene
pleochroic center (eulite) with
of nonpleochroic orthopyroxene.
In some
instances,
an
outer
subcalcic
augite contains a core of relict pigeonite (Fig. 12).
Igneous textures
are
development
only
slightly
modified
by
the
metamorphic
discontinuous rims of greenish-brown hornblende around
in
isolated
(Fig.
13).
cases,
garnet coronas around pyroxene and opaque
Plagioclase
recrystallization
pyroxene,
(An
55)
laths
have
undergone
to smaller ( < .01mm) granoblastic aggregates
of
and,
oxides
minor
along
grain boundaries.
Similar
corona
textures
with
transitional
granulite-facies
32
Figure 11. Preservation of igneous exsolution
transitional granulite metagabbro DC-9 near Deer
Intergrowths of subcalcic augite (SA) and opx (0).
in pyroxenes
Lake
(Fig.
from
2).
Figure 12.
Relict pigeonite (Pi) mantled by subcalcic augite (SA) in
cross-cutting transitional granulite metagabbro DC-9.
33
Figure 13. Incipient development of garnet (G) coronas around
oxide (0) and cpx (C) in metagabbro DC-9.
assemblages
basement
the
have
been described in metagabbros from
of Quebec (Barink.
1984).
the
opaque
Precambrian
Barink modelled the formation of
corona textures as a direct response to cooling of
synmetamorphic
gabbroic intrusions by the following reaction:
H^O + cpx' + hbld' + plag' + Fe-Ti oxides' = gnt + cpx" +
plag" + Fe-Ti oxides" + qtz.
Crosscutting
indicate
relationships
that
the
described
this
transitional granulites are not relicts
metamorphic
events.
Instead,
syntectonic
gabbroic
intrusions,
structurally
above support
disrupted
into
these
rocks
with the
isolated
form
oldest
boudins
a
model
of
older
continuum
intrusions
and
and
of
being
completely
34
recrystallized
intrusions
igneous
which
into
are
more
mineralogy
is
Lindsley,
metamorphic
continuous
and textures.
assemblages;
progressively
and preserve more
of
the
The presence of relict
unstable in slowly cooled intrusive
rocks
younger
original
pigeonite,
(Huebner,
1982;
1982), suggests that intrusion of this series of metagabbros
may have been accompanied by rapid uplift, resulting in the "quenching"
of this high-temperature pyroxene.
Of further interest is the formation of the transitional granulite
facies
rocks
assemblage
However,
this
rock,
series
added
it is possible that any water
associated
of metagabbros may have been driven off into the
promoting
assemblage.
assemblages.
mafic
model, HgO is consumed to form the corona texture assemblage.
the Spanish Peaks,
amphibolite facies
other
Barink's
heat
typical
while
In
in
formed
in the syntectonic metagabbros
the
formation of the transitional
granulite
with
country
facies
Water driven off into the country rock, combined with the
to
the system by the intrusion
of
the
high-temperature
metagabbros, may have been a significant factor in the formation of the
in situ partial melts described above (Mogk and Salt, 1986).
35
JEROME ROCK LAKES TERRAME
The
ductile
shear zone marks an abrupt change in composition
quartzofeldspathic
paragneisses from tonalitic in the GPT to
and granodioritic in the JRLT (Fig. 4).
in
the
JRLT
is indicated by
metapelites
and
lithology
of
the
of
sillimanite-bearing
intercalations
quartzofeldspathic
JRLT,
granitic
A higher grade of metamorphism
presence
centimeter-scale
granulite. K-feldspar-bearing
predominant
the
of
with
of
transitional
gneisses
subordinate
are
amounts
the
of
metapelite, metabasite, quartzite, and ultramafite. Granitoids occur in
minor, amounts near the shear zone.
Quartzofeldspathic Gneisses
Two
distinct' types
paragneisses
have
Lakes area (Fig.
(KQFG)
(Fig.
4)
quartzofeldspathic
In the
Jerome
Rock
2), paragneisses with an overall granitic composition
4).
(Ieucogneiss)
form
K-feldspar-bearing
been recognized in the JRLT.
predominate (Fig.
paragneisses
of
a
Near the shear zone,
with an
lighter
colored
overall granodioritic composition
roughly continuous unit from the Spanish
Lakes
to
Diamond Lake (Fig. 2, Plate I).
Granitic paragneisses (KQFG)
The
biotite
Lineations
KQFG
has well-developed foliation defined
and
by
of
centimeter-scale
hornblende
and
mafic
biotite
by
lepidoblastic
compositional
impart
a
distinct
layering.
streaky
36
appearance
to
amphibolite
layers
and
the gneisses.
transitional
Mafic layers consist of biotite
granulite.
Metapelitic
are also intercalated within the gneisses on
a
and
schist,
quartzite
centimeter- to
meter-scale, indicating a supracrustal origin for these gneisses.
MINERAL ASSEMBLAGES, JRLT
Leucogneiss
plag-ksp-qtz-biot-apat-epid-op-zir
plag-ksp-qtz-biot-hbld-apat-epid-op-zir
Granitic Gneiss
ksp-plag-qtz-biot-hbld-zir-apat-op
ksp-plag-qtz-biot-hbld-cpx-gt-zir-apat-op
Transitional Granulites
cpx-hbld-gt-op
plag-qtz-cpx-gt-hbld-op-apat
plag-qtz—cpx-gt—hbld-biot—epid—sph-op-apat
Metapelites
qtz—plag—biot—musc-sill-ky-gt-apat—rut—op
qtz-biot-sill-(ky-musc-gt)-apat-op
Table 3.
in Table
Summary of mineral assemblages in the JRLT.
I.
Abbreviations as
The principal mineralogy of these gneisses consists of K-feldspar,
quartz,
plagioclase, biotite, and blue-green (Z) ferrohastingsite-rich
hornblende
Accessory
(Table
3).
Garnet and/or diopside occur in some
minerals include apatite,
feldspar is mostly microcline,
80
to a low of 15-20,
allanite,
and
zircon.
samples.
The
K—
but in some samples, the 2V varies from
which falls into the range of sanidine (Stewart
37
and
Ribbe,
feldspar
1983).
grains
equilibrium.
Sanidine
having
A
forms straight grain boundaries with
higher
2V,
and appears to
be
in
K-
textural
possible explanation for this occurrence is that
the
sanidine formed under high-temperature metamorphic conditions, followed
by
rapid
post-metamorphic cooling,
sanidine
in
various stages of inversion
plagioclase,
and
boundaries.
Lepidoblastic
aggregates
these
to
"quenching"
microcline.
quartz form a polygonal mosaic with
with
is
straight
zircon.
typically large (up to 1.0mm) and
of hornblende, , quartz or apatite.
of
K-feldspar,
grain
biotite and nematoblastic hornblende
associated allanite and euhdral
clusters
inclusions
resulting in the
form
Zircon
often
in
contains
Zircon also occurs
as
large rounded grains in the quartzofeldspathic mosaic.
Leucogneisses
Leucogneisses
appearance
Outcrops
have
and exhibit
are
interspersed
composed
a highly deformed and
heterogeneous
many features indicative of
of
with irregular,
leucocratio
felsic
partial
layers
outcrop
melting.
(Cl
compositional layering that is disrupted by shearing and
style
displacements
mantled
by white,
14).
Disrupted
granitic pegmatite.
mafic layers
Foliation in the
layers is highly variable and gradational, passing from
centimeter-scale
defined
5),
centimeter- to meter-scale quartzite and
mafic
(Fig.
<
are
nappeoften
leucocratic
well-developed
mafic compositional layering into a "ghost" foliation
by mm-scale "wispy"
mafic compositional layers which
an extreme degree of deformation (Fig.
15).
locally grade into massive, unfoliated rock.
exhibit
These nebulitic gneisses
38
Figure 14.
Ksp-bearing leucogneisses of the JKLT near the Spanish
Lakes.
These gneisses generally exhibit a much higher degree of
deformation than those of the GPT.
The field of view is approximately
10 meters.
Figure 15.
Leucogneiss along trail to Mirror Lake.
The nebulitic
aspect of the leucogneiss is the result of extensive partial melting.
The formation of a melt phase probably facilitated the extreme degree
of deformation observed in the leucogneisses (as in Figure 14).
39
The
main
plagioclase,
mineral
mafic layers.
random
the
leucocratic
are
hornblende (2Vx = 25-30) is present in the
and
Accessory minerals include apatite,
opaque oxides.
distribution,
some
zircon,
samples exhibit
the leucogneiss are hypidiomorphic granular,
of microcline and plagioclase.
thin
allanite,
While felsic minerals generally exhibit
mm-scale
microcline- and
plagioclase-rich layering separated by mafic rich banding.
are
layers
quartz, microcline, and biotite (Table 3). Blue-green (Z)
ferrohastingsite-rich
epidote,
constituents of
Textures of
with complex intergrowths
Microcline embayments into plagioclase
often in optical continuity with microcline inclusions within
plagioclase.
Plagioclase
is
frequently myrmekitic in
contact
the
with
microcline.
Idiomorphic epidote cored by allanite occurs in the center
of
grains
biotite
granitoids.
in
a similar manner to epidote
in
the
youngest
Allanite also occurs as isolated, zoned grains up to I mm
long in the leucocratic matrix.
While
the
the textures described above are very similar to
granite of the GPT,
interlayering
intrusive
outcrop
1982)
the
and lack of cross-cutting relationships suggests a
Instead,
similarities to migmatite terranes (e.g.
sequence
quartzite
that
this
unit is
a
mafic
original sequence.
the
textural
Johannes and
heterogenous
that has undergone extensive in situ
and
in
millimeter- to meter-scale compositional
origin for the leucogneisses.
suggest
those
and
Gupta,
metasupracrustal
partial
layers representing refactory
non-
melting,
remnants
of
with
the
40
Metapelites and Quartzite
A
layers
sequence
of
quartzite
leucogneiss
quartz,
of reddish,
and
garnet-rich, pelitic schists, with
amphibolitic
and the shear zone.
Plagioclase
is
indicative
of
occurs
between
The pelitic schists are
plagioclase (An 25-30),
and sillimanite,
schist
biotite,
thin
the
composed
of
muscovite, garnet, kyanite,
with accessory apatite, rutile, and zircon (Table 3).
absent
higher
in some samples.
titanium
Biotite
content than
the
is
reddish
green
brown,
biotite
in
metapelites of the GPT.
Textures in one plagioclase-absent sample from this unit
that
sillimanite (fibrolite)
is
formed through a complex
indicate
series
of
replacement reactions involving garnet, muscovite, and biotite, as well
as
kyanite.
The
lepidoblastic
embayed
from
following replacement textures
were
observed:
I)
biotite replaces lepidoblastic muscovite;
2) garnet
is
by both biotite and fibrolite,
the
breakdown
sillimanite
(Fig.
fibrolite.
However,
of
garnet
16);
and
direct
with excess Fe and Ti released
forming
3)
opaque
kyanite
is
oxides
directly
within
the
replaced
by
replacement of kyanite does not occur
in
regions of the thin section where garnet is absent, suggesting that the
breakdown
of
Based
these replacement
on
garnet is a requisite step in the breakdown of
textures,
the
generalized
kyanite.
sillimanite-
forming reaction in these rocks is
ky + gar + mus = sill + biot + op ox +/- qtz,
which
(1977).
is similar to sillimanite-forming reactions proposed by
In
contrast to the GPT,
Yardley
staurolite does not appear to
have.
Al
been
part of the prograde metamorphic path in the metapelites
JRLT.
this sample is tightly
sericitic
of
the
The stability of coexisting muscovite and quartz indicates that
the reaction occurred below the second sillimanite isograd.
in
of
muscovite.
folded and
Fibrolite
is partially replaced by
late,
This suggests that deformation outlasted growth
sillimanite and occurred under retrogressive
conditions,
possibly
related to late movements along the shear zone.
Figure 16. Fibrolite (S) and biotite (B) embayments into garnet (G) in
metapelite sample CM-I7 from JRLT near ductile shear zone. Note
concentration of opaque minerals in fibrolite: Fe-Ti from breakdown of
garnet not absorbed by sillimanite formed separate Fe-Ti oxide phases.
Metapelites
in
the Jerome Rock Lakes area are similar
to
near the shear zone,
but have more plagioclase and are less
deformed.
Sillimanite—forming
reactions in these rocks are more ambiguous,
the
presence
of biotite-embayed garnet
suggests
similar
those
but
processes.
42
Replacement
of
aluminosilicates
by
sericitic
pronounced in the Jerome Rock Lakes area,
muscovite
is
less
perhaps related to increased
distance from the shear zone.
Quartzite lenses are intercalated in both the KQFG and leucogneiss
sequences
and
in the pelitic schists near the shear
cases,
quartzites
opaque
oxides.
are
composed of quartz,
zone.
In
emerald green
most
mica, and
Nd epidote-quartzites of the type occuring in the
GPT
were found north of the shear zone.
Transitional Granulites
Three distinct varieties of transitional (cpx-gnt) granulites were
found
in
the
JRLT.
centimeter-scale
thick
(50
m)
distinctive
First,
fine-grained
mafic
granulites
form
intercalations within the KQFG gneisses and within
mafic
unit of
zone
near
leucocratic,
the
DSZ
(Plate
coarse-grained
I).
Second,
a
a
plagioclase-garnet-
diopside rock (referred to as leucogranulite) which varies in thickness
from
2-3
meters
described
above.
development
scattered
terranes
were
of
occurs between the mafic zone
Finally,
some
isolated
and
mafic bodies
the corona textures described in the
throughout
the
JRLT,
indicating
the
that
with
GPT
The
are
although
were juxtaposed by the time the high-temperature
injected.
metapelites
total
widely
the
two
metagabbros
textural descriptions of these metagabbros
were
presented in the previous chapter and are not repeated in this section.
Intercalated granulites
Fine-grained
gneisses
are
transitional'
granulites intercalated with the
similar to those described from the
Ruby
Range
KQFG
(Dahl,
43
1979) and the Blacktail Range (Clark,
of plagioclase,
olive-green hornblende,
biotite (Table 3).
rutile,
includes
normal
garnet,
diopside,
and minor
opaque oxide,
Plagioclase generally forms a mosaic texture
relict
larger (I-2mm) grains that
compositional zonation,
compositions
The mineralogy consists
Accessory minerals include apatite,
and zircon.
some
1986).
of An 30.
exhibit
but
continuous
from core composition of An 45 to
Hornblende grains are in granoblastic
rim
contact
with each other but are replaced by garnet and diopside. Garnet engulfs
and
has
numerous
inclusions of hornblende.
Diopside
is
in
grain
boundary contact with garnet and commonly intervenes between hornblende
and
garnet.
These
textures
indicate that hornblende
is
either
a
reactant in the formation of the gfanulite facies assemblage or part of
the stable assemblage,
but that it
is not a product of
retrogression
from earlier metamorphic assemblages.
The
thin
bands
of
granulite in the mafic body near
zone are composed entirely of garnet,
diopside,
3), giving the bands an ultramafic appearance.
the
shear
and hornblende (Table
Grain boundaries form a
granoblastic mosaic texture. The borders of the ribbons are composed of
granoblastic
hornblende
and
diopside
with
minor
interstitial
plagioclase, which grade outward into diopside-bearing. amphibolite.
Leucogranulite
The
coarse-grained
leucogranulite
near the shear
zone
has
assemblage of plagioclase, quartz, diopside, and garnet,
with
coronas
of retrograde blue-green hornblende and epidote (Table
present in minor amounts.
oxides,
and calcite.
3).
an
Sphene is
Accessory minerals include apatite,
opaque
Garnets reach a maximum size of 5 cm in diameter
44
and
are intensely fractured.
maximum
size
garnet.
where
greenish-blue
hornblende
is
nearly totally altered to
by garnet.
Inclusions
than
sericite
Both diopside and garnet have
17).
between
diopside
and
a
the
except
coronas
is subsequently mantled by coronas of epidote with
of
The
similar
Epidote and hornblende also occur
fractures in diopside and garnet.
contacts
with
hornblende with symplectic intergrowths of quartz.
symplectic intergrowths (Fig.
along
smaller,
of about I cm and is generally less fractured
Plagioclase
mantled
Diopside tends to be
Sphene occurs
hornblende
and
mostly
within
of hornblende commonly occur within the sphene,
the
along
garnet.
indicating
that growth of sphene outlasted that of hornblende.
Figure 17. Successive retrograde coronas in leucogranulite near DSZ.
Cpx (C) is mantled by hornblende (H), which in turn is mantled by
epidote (E).
45
Amphibolites and Ultramafites
Amphibolites
occur
as centimeter- to meter-scale
intercalations
within gneisses, as isolated bondins, and as amphibolitized intrusions.
Intercalations
sphene,
and boudins consist
of
plagioclase,
hornblende,
and
with accessory apatite, allanite, and opaque oxides. Garnet or
diopside is present in some samples. Numerous amphibolitized intrusions
postdate
both gneissic units and are commonly rich in garnet.
Pods of coarse-grained pyroxenite occur near the shear zone within
the
white gneiss and in outcrop show no signs of metamorphic
such as found in the ultramafites of the GPT.
also
report
zonation
Spencer and Kozak (1975)
the occurrence of a large ultramafic body
north
of
the
shear zone consisting of augite, olivine, and plagioclase.
Granitoids
Clearly intrusive granitoids form only a minor portion of the JRLT
and
area
are largely associated with the shear zone.
(Fig.
commonly
2),
occur
composition
from
the
centimeter- to
along
is
planes
meter-scale
of
identical to the leucogneiss and
epidote.
mylonitization
intrusions
mylonitization
GPT and exhibits the same textural
"magmatic"
In the Spanish Lakes
This
spatial
of
(Fig.
18).
youngest
involving
suggests
was in part coeval with the injection of
The
granitoids
relationships
association
granite
the
that
youngest
An agmatitic complex of trondhjemite and amphibolite occurs on the
ridge
south
agmatite
of
Lake Solitude
(Fig.
complex exhibit shearing,
2).
Amphibolites
and injection of the
within
the
trondhjemite
46
appears to be partly controlled by the orientation of the shear planes.
The agmatitic fabric passes into concordant interlayers of trondhjemite
and
amphibolite,
similar to occurrences in the tonalitic country rock
gneisses of the GPT.
Pegmatites are also common in the JRLT1
but the early
pegmatites
associated
with the hornblende granitoids of the GPT were not found in
the JRLT.
The pegmatites of the JRLT are of granitic composition, and
often
contain
xenoliths
of
supracrustal
assemblages,
including
metapelite and marble.
Figure 18. Youngest granite injected into mylonitized amphibolite.
Sigmoidal fold is disrupted by mylonitization (M), with injection of
granite along shear plane.
47
Summary
The JRLT is a supracrustal terrane dominated by K-feldspar-bearing
paragneisses,
Mineral
are
some of which have undergone extensive partial
assemblages
indicative
in mafic compositional layers and in
of transitional
granulite
facies,
melting.
metapelites
sillimanite-grade
metamorphism.
The compositional,
above
mineralogical, and textural evidence presented
demonstrates that the JRLT records a different geologic
than the GPT (Table 4).
an
history
The K-feldspar-bearing paragneisses represent
entirely different source of supracrustal material than that of the
tonalitic
paragneisses
granulites
of
the
GPT.
The
intercalated
transitional
and sillimanite-bearing assemblages indicate that the
experienced
a
different,
higher-grade path
of
metamorphism.
JRLT
This
change in metamorphic grade from the GPT to the JRLT is not continuous,
with
recognizable
Finally,
the
suggests
that
epoch
of
terranes
terranes.
absence
is abrupt across
the
shear
the two terranes were not juxtaposed during
plutonism.
occurred
prior
the
as
but
zone.
of the older granitoids of the GPT in the
granitic
metagabbros,
sequences,
isograds,
to
However,
emplacement
youngest granite,
evidenced
juxtaposition
and the
of
the
the
of
corona
JRLT
early,
the
two
texture
trondhj emite-amphibolite
by the presence of these rock types in
both
COMPARISON OF THE GEOLOGIC HISTORIES OF THE GPT AND JRLT
GALLATIN PEAK TERRANE
Tonalitic paragneisses
Kyanite-gedrite metapelites,
with early staurolite
JEROME ROCK LAKES TERRANE
Granitic paragneisses
Sillimanite metapelites
with no evidence of
early staurolite; early
.kyanite + muscovite
Transitional granulites only in
corona-texture metagabbros
Transitional granulites
intercalated with gneisses
in addition to coronatexture metagabbros
Older foliated granitoids,
and youngest granite and
trondhj emite-amphib olit e
Youngest granite and
trondhj emite-amphibolite
only
Table 4. Comparison of lithologies, metamorphic and plutonic histories
of the GPT and the JRLT.
49
DUCTILE SHEAR ZONE
The
ductile shear zone (DSZ) is roughly 500 meters thick
and
is
coincident,- with the break in lithologies and metamorphic grade between
the
two
terranes described above.
This zone is continuous
Spanish Lakes into the Diamond Lake area (Fig.
and
parallel to the regional foliation.
bands
that are
2),
from
the
trending northeast
The DSZ is
anastomosing
mylonite
interleaved
macrolithons
of relatively less deformed gneiss and
characterized
with
by
meter-scale
amphibolite.
The
mylonite bands are developed predominantly within tonalitic gneisses of
the
GPT,
bearing
but
some mylonite bands also occur within the
metapelites
and .Ieucogneiss of the
JRLT.
sillimanite-
As
described
in
previous sections, injection of the youngest granite occurs along shear
planes
bordered
generally
they
form
bands
thickness,
bands
mylonite
(Fig.
18).
The
mylonite
parallel to the foliation of the surrounding
also
mylonite
by
have
planes
oblique
to
regional
from one centimeter to one
are
gneisses,
but
foliation.
are locally discontinuous and are highly
ranging
bands
Individual
variable
meter.
The
a dense, black, blastomylonitic fabric which
in
mylonite
varies
from
aphanitic to highly porphyroclastic.
Microstructures
progression
of
of samples from within and near the DSZ exhibit a
deformational
textures in a manner
described by Bell and Etheridge (1973),
in
gneisses
development
which
shows
little
similar
from incipient
deformation
in
of blastomylonitic fabric in the mylonite
to
that
mylonitization
hand
sample
bands.
to
Initial
50
deformation
in gneiss is characterized by development of subgrains and
deformation
bands within quartz grains and by incipient ductile
size reduction at quartz grain boundaries.
grains commonly are seriate.
undulose extinction.
Boundaries between
grain
quartz
Feldspars are bent and fractured and have
Biotite exhibits extensive kinking.
Intermediate deformational textures occur in augen gneisses and in
■isoclinally folded amphibolites.
are
Microstructures in the augen gneisses
characterized by the development of quartz ribbons and millimeter-
scale seams of fine-grained biotite.
present)
with
Plagioclase (and
becomes more rounded and forms asymmetrical augen
tails of mixed quartz and plagioclase (e.g.
1983).
garnet,
Quartz-feldspar
amphibolite,
hinges of
structures
Simpson and
also form
augen
Schmid,
textures.
In
hornblende and plagioclase are bent and broken around the
isoclinal
reduction.
microlithons
where
folds and show
signs
of
mechanical
grain-size
A new, weak planar fabric, defined by bands of fine-grained
material,
is
developed at an oblique angle to the axial plane of
the
isoclinal fold.
The
most
intense
submicroscopic,
In
some
bands
that
and
deformation
results in
biotite grains are rotated into the edge
where extreme reduction in grain size
the mafic matrix is biotite-rich.
microcline,
formation
of
a
mafic-rich matrix with no apparent internal foliation.
instances,
epidote
the
occur
sphene,
in all samples
garnet,
and
of
takes
place,
of
the
suggesting
Porphyroclasts of plagioclase
mylonite
apatite
are
bands.
variably
Hornblende,
present
as
porphyroc'lasts. Mylonite bands rich in hornblende porphyroclasts often
have
associated
garnet
porphyroclasts.
Rarely,
quartz-plagioclase-
51
garnet
microlj.thons
occur
hornblende porphyroclasts.
do
not
exhibit
in association with mafic
bands
rich
in
Porphyroclasts are subrounded to ovoid and
the asymetric augen
microstructures
found
in
less
deformed zones.
Most plagioclase grains exhibit mechanical grain size
reduction,
rarely,
but
recrystallization
planar
Quartz
forms
"teardrop"
quartz
grains
boundaries,
grains
textures at the margins
fabrics (S-C surfaces),
bands.
have
plagioclase
are
however,
which
exhibit
preserved.
dynamic
Composite
are common within the
ribbons one or two grains thick which
(Simpson,
within
the
1983) or
ribbons
fishhook
have
mafic
commonly
microstructures.
mosaic
or
The
seriate . grain
indicative of dynamic recrystallization and recovery (Bell
and Etheridge, 1973).
The
preservation
textures
indicates
of plagioclase with
dynamic
recrystallization
that early development of the shear zone
occurred
under at least amphibolite facies conditions (White and others,
However,
the dominance .of generally brittle textures in the
indicates
lower
that
grades.
the latest deformation in the shear zone
Mineral
assemblages
variable degrees of retrogression.
blue-green
(Z)
sericitization.
grained
rims
Tiny
and
(0.05)
within the
matrix of one mylonite sample.
brittly deformed
epidote-bearing granite,
occurred
zone
and
at
.exhibit
exhibits
locally
extensive
euhedral tourmaline occurs in the
The formation of
may be the result of
of late-stage fluids in the shear zone.
been
feldspars
Hornblende is generally mantled by
plagioclase
,which is absent in either terrane,
has
shear
1980).
fine­
tourmaline,
concentration
Most epidote in the mylqnites
may have been derived from the
late,
but it is also possible that some epidote may.
52
be
a
product
of retrogression.
Locally,
biotite
is
replaced
by
chlorite and in rare instances, actinolite forms homoaxial replacements
of hornblende.
' Therefore,
shearing
shear
while
there
is evidence for at least
stage
the
zone occurred under, lower-grade conditions than seen in the
two
injection
equivilant
suggests
However,
of
at
that
the
the
the latest deformation
of
in
terranes.
under high-grade conditions,
one
intimate association of shear bands with the
youngest
granite,
which
has
been
least in part to high-grade metamorphism
the
final
stages of
mylonitization
shown
in
the
occurred
progressively cooler stages of the same erogenic event,
being the product of a separate greenschist facies event.
to
be
GPT,
during
as opposed
to
53
PHYSICAL CONDITIONS OF METAMORPHISM
Petrogenetic Associations
Metapelites in the GPT containing relict staurolite provide useful
petrogenetic
information
for
conditions of this terrane.
of
staurolite
to
form
bracketing
the
minimum
metamorphic
Various reactions involving the breakdown
orthoamphibole
assemblages
(Fig.
19a) have
recently been modelled by Hudson and Harte (1985) for K O-poor
systems
in the FeO-MgO-Al2O3-SiO2-H2O (FMASH) field for Pr 2o = Ptotal and ideal
mineral
compositions.
The
reaction
of
staurolite
to
form
the
assemblage orthoamphibole-kyanite-garnet observed in the metapelites of
Bear' Basin indicates
metamorphism occurred at minimum
680-690 C and 7.5 kbars (Fig.
minimum
conditions
19a).
necessary
to
conditions
of
This estimate is consistent with
form
the
anatectic,
stromatic
migmatites in the tonalitic paragneisses (Fig.
19a;
Furthermore,
orthoamphibole-kyanite-
the
formation of coarse-grained
garnet assemblages in contact aureoles adjacent
granite indicates
under
that
granite
the same conditions.
to
Johannes,
intrusions
1985).
of the
emplacement occurred at least in part
This interpretation is consistent with the
presence of magmatic epidote in the granite, which has been modelled as
forming
at
minimum pressures of 7 to 8 kbars
(Zen
and
Hammarstrom,
1984).
Petrogenetic
precision
in
associations
in
the
JRLT do not
allow
bracketing metamorphic conditions as do those
the
from
same
the
Figure 19. Petrogenetic grids for the GPT and the JRLT. a) Petrogenetic grid showing minimum
peak metamorphic conditions in the GPT (stippled area). Modified from Hudson and Harte (1985).
Kyanite-sillimanite curve from Holdaway (1971). Minimum solidus curves from Johannes (1985).
b) Petrogenetic grid roughly bracketing minimum metamorphic conditions for the JRLT (area of
ruled lines). Aluminosilicate and minimum solidus curves as in (a).
Muscovite curves from
Storre and Karotke (1972) and Day (1973).
55
GPT.
Qualitatively, however, the occurrence of transitional granulite
assemblages
kyanite,
that
intercalated
within
the gneisses
and
the
reaction
of
garnet, and muscovite to form sillimanite and biotite suggest
the
JRLT experienced higher-grade
metamorphic
conditions.
The
stability of muscovite and quartz indicates that metamorphic conditions
were below those required to form partial melts in pelitic rocks
19b;
Storre
development
and
Karotke,
1972).
However,
the
locally
(Fig.
extensive
of anatectic migmatites in the leucogneiss units indicates
that conditions extended beyond the minimum-melting curve for rocks
of
granitic compositions (Fig. 19b; Johannes, 1985).
Geothermobarometry
Suitable assemblages for P-T studies in the Spanish Peaks area are
garnet-biotite
thermometry,
(GT-BT)
and
clinopyroxene-garnet
higher
(CPX-GT-FLAG)
peak temperatures than did the GPT,
more
records
kbars,
for
and
barometry
The results indicate that at one time, the JRLT experienced
but later shared a
thermal history with the GPT at lower temperatures.
are
for
and garnet-aluminosilicate-qtiartz-plagioclase (GASP)
clinopyroxene-garnet-plagioclase-quartz
(Table 5).
(CPX-GT)
ambiguous,
however,
the highest pressures;
whereas
common
Pressure estimates
and it is not yet clear which
terrane
the GPT has a minimum pressure of
mineral assemblages of the JRLT allow
somewhat
7.5
lower
pressures.
Garnet-biotite
Garnet-biotite temperature estimates were obtained for two samples
from
each terrane.
While several currently available models were used
56
CALCULATED TEMPERATURES AND PRESSURES FOR THE GPT AND THE JRLT
GPT
JRLT
TEMPERATURES (C)
I)
2)
3)
4)
BBE-7
683-728
665-707
587-630
638-690
BBE-32C
689-740
674-726
530-561
634-682
CM-17
770-776
760-764
754-759
647-652
SP-12
758-797
745-782
727-755
696-733
Peak
(average)
2)
690
699
762
770
Retrograde:
(range)
2) 583-617
585-605
647-715
644-706
Retrograde
(average)
2)
595
681
668
CPX-GT
DC-6
5) 675-711
6) 580-637
DL-33B
678-713
61o-673
CM-20
719-762
620-687
GT-BT
peak:
(range)
599
PRESSURES (kbars)
GASP
Peak:
BBE-7
A) 6.8-7.7
B) 4.2-5.3
BBE-MP
7.5-8.3
6.2-7.I
SP-12
6.0-8.3
3.7-5.4
Retrograde:
A) 4.5-5.3
4.8-6.0
5.6-7.3
CPX-GT-PLAG
DC-6
c) 5.4-7.2
DL-33B
5.2-7.0
Table 5. Summary of P-T calculations. The following models were used:
I) Hodges and Spear (1982); 2) Ferry and Spear (1978); 3) Ganguly and
Saxena (1984); 4) Indares and Martignole (1985); 5) Ellis and Green
(1979); 6) Dahl (1980); A) Newton and Haselton (1981); B) Ganguly and
Saxena (1984); C) Newton and Perkins (1982).
Samples BBE-7, BBE-32C.
and BBE-MP are metapelites from the GPT.
Samples CM-I7 and SP-12 are
metapelites
from
the JRLT.
Sample CM-20 is an
intercalated
transitional granulite from the JRLT.
Samples DC-6 and DL-33B are
corona-texture metagabbros from the GPT and the JRLT, respectively.
57
for temperature calculations (Ferry and Spear,
temperatures
Spear
1978; Hodges and Spear,
given in this study were calculated using the
(1978)
model
because of the uncertain effects of
Ferry
the
and
various
Margule's parameters used in the other models (Table 5).
Peak
sample
peak
temperatures for the GPT range from 665-726 C (Table 5).
BBE-7,
garnet inner rim and adjacent biotite
temperatures.
However,
in
sample
BBE-32C,
generally
garnet
In
yield
core
and
interior biotites give peak temperatures of 674-726 C; garnet inner rim
and both adjacent and matrix biotites give temperatures in the range of
604-670,
which
temperatures.
are
interpreted
Garnet
rim
to
be
compositions
partial
of
re-equilibration
samples
from
consistently yield a lower temperature range of 583-617 C;
the
this
GPT
range
is reported in Table 5 as retrograde temperatures.
Peak garnet-biotite temperatures for the JKLT range from 745-784 C
(Table
5).
temperatures
Garnet
in
inner
rim
compositions
both the JRLT samples.
samples
biotites
from
yield
the
highest
sample
CM-17,
while in sample
SP-12,
However,
matrix biotites give the highest temperatures,
adjacent
yield
the highest temperatures.
the JRLT yield a range of retrograde
in
Garnet
rims
temperatures
in
from
644-715 C, which fall within the range of peak temperatures of the GPT.
Garnet-clinopryoxene
Temperature estimates for the assemblage clinopryoxene-gamet were
obtained
The
using the models of Ellis and Green (1979) and
formulation of Dahl (1980),
Dahl
(1980).
which was empirically calibrated from
transitional granulites from the nearby Ruby Range, yields temperatures
58
about 100 C lower than those calculated from the Ellis and Green model,
and
are
problabIy
One
corona-texture
intercalated
temperature
too low to represent peak
metagabbro
from
north
each
terrane
and
transitional granulite from the JRLT were used to
estimates (Table 5).
of
transitional
the
shear
granulite
one
obtain
DC-6 is a metagabbro from south
the shear zone near Deer Lake (Fig.
just
conditions.
zone
of
2) and DL-33B is a metagabbro from
near
intercalated
Diamond
Lake.
within the
CM-20
mafic
is
gneisses
a
and
schists on the ridge above Chilled Lakes (Fig. 2; Plate I).
The
model of Ellis and Green (1979.) yields a temperature range of
675-713 C for metagabbros from both terranes (Table 5) using both
and
rim
core
compositions.
compositions
compositions
yield
For the intercalated transitional
yield
temperatures
temperatures
of
751—763
of 719-726 C
C,
(Table
5).
core
granulite,
while
rim
The
core
temperature estimate from the intercalated granulite is consistent with
peak garnet-biotite temperature estimates for the JRLT. The temperature
estimates from the metagabbros,
however, are the same for samples from
either terrane and correspond to peak temperatures of the GPT.
Geobarometry
Pressure
estimates for both.terranes were obtained using
aluminosilicate-quartz-plagioclase
(GASP)
and
clinopyroxene-garnet^
plagioclase-quartz (CPX-GT-PLAG) assemblages (Table 5).
Newton
garnet-
The models of
and Haselton (1981) and Ganguly and.Saxena (1984) were used for
the GASP geobarometer.
(1984)
does
kyanite
in
not
the
However, since the model of Ganguly and Saxena
yield pressures consistent
GPT,
the
model of Newton
with
and
the
stability
Haselton
(1981)
of
is
59
preferred in this study.
Pressure estimates for the CPX-GT-PLAG system
were calculated using the formulation of Newton and Perkins (1982).
For the GASP geobarometer,
from
the
JRLT,
were
used
two samples,
(Table 5).
Core
plagioclase
and
rim
garnet
compositions
yield
combined
pressures
6.8-8.3 kbars for the GPT using a peak
700 C, consistent with estimates using
core
inner
compositions
of
with
one from the GPT and one
petrogenetic
estimates for the JRLT are somewhat ambiguous,
temperature
grids.
of
Pressure
ranging from 6.0 to 8.3
kbars,
using a peak temperature of 770 C (Table 5). The large range of
values
for the sample from the JRLT is the result of variation in core
anorthite
mole
Pressures
calculated using the lower value range from
use of the
For
the
of
individual grains
from
0.28.
higher anorthite content gives pressures of 6.0-6.8
kbars.
both ranges
fall
within
the
estimates using the empirical calibration of Newton
and
field of sillimanite.
(1982) for the
assemblages
clinopyroxene-plagioclase-garnet-quartz
note
comparison
(GPC)
of the corona-texture metagabbros range from 5.0-7.2 kbars
for the two samples from both terranes (Table 5).
that
with
orthopyroxene,
low.
to
kbars;
Pressure
Perkins
0.22
7.5-8.3
assumed temperature of 770 C,
stability
Perkins
fraction
their
the
formulation
formulation
using
using
However,
this
Newton and
assemblage,
assemblages
in
containing
yields pressure estimates on the order of 1-2 kbar
too
However, other studies have noted that the development of corona
textures
suggest
disequilibrium conditions related to
uplift
(Dahl,
1979; Newton and Perkins, 1982) and the lower pressure estimates may be
more
realistic.
The
interpretation
that these
textures
represent
60
uplift
conditions
is
consistent
with
the
preservation
of
relict
pigeonite in the metagabbros, which is not likely to be preserved under
slow cooling conditions.
Summary
Petrogenetic
conditions
associations
of
the
GPT
constrain
minimum
of metamorphism of this te'irane to 680-690 C and 7.5
peak
kbars
and also constrain the timing of emplacement of the youngest granite to
be
coeval
with high-grade metamorphism of
conditions
the
GPT.
of the JRLT can only be roughly bracketed
However,
by
peak
petrogenetic
associations to be above minimum melting in quartzofeldspathic gneisses
of
granitic
composition,
but below melting reactions
in
muscovite­
bearing metapelites.
Geothermobarometry calculations, summarized in Figure 21, indicate
that the two terranes have different early metamorphic
histories,
indicate that they may have shared a later history (Fig.
experienced
temperatures
20).
peak metamorphism under amphibolite-facies
21).
The GPT
conditions
of 665—726 C and pressures of 6.8 8.3 kbars (Box I,
Temperature
estimates
from
intercalated
but
at
Fig.
metapelites
and
I
transitional
granulites
immediately north of the shear
zone
confirm
that the JRLT experienced early higher peak temperatures of 745-782
C,
either at lower pressures or at roughly the same pressures (Box 2, Fig.
20) .
Temperature and pressure estimates for the metagabbros which occur
in
both terranes,
however,
are the same for samples from either side
61
of
the
shear
temperature
finding
two
zone
(Box 3,
estimates
of
Fig.
20) and
the GPT from
coincide
gamet-biotite
with
the
pairs.
constrains the timing and conditions of juxtaposition
terranes
to prior to or during the injection
recrystallization
and
7 50
This
of
the
synmetamorphic
of the corona texture metagabbros at the
700
peak
prevailing
C
Figure 20.
Graphic summary of P-T calculations. I) Peak estimates for
GPT.
2) Peak estimates for JRLT.
3) P-T estimates using CPX-GT for
metagabbros of both terranes. 4) Estimated retrograde re-equilibration
of the JRLT (Dashed box). 5) Estimated retrograde re-equilibration of
GPT (Dashed box).
Dashed arrows are hypothetical post-peak paths for
both terranes, suggesting that the two terranes shared a common thermal
history during the synkinematic recrystallization of the corona-texture
metagabbros.
peak temperatures recorded in the GPT.
in
part
be
reflected
This shared thermal history may
by the lower temperature range
of
recorded in garnet rims from metapelite samples of the JRLT;
644-715
C
pressures
62
calculated at these lower temperatures range from 5.6 to 7.3 kbars (Box
4, Fig. 20), coincident with the temperature and pressure ranges of the
corona
texture, metagabbros.
Since the
corona
preserve relicts of high-temperature parageneses,
the
texture
metagabbros
it is suggested that
shared thermal history is associated with a pulse of rapid
of the two terranes.
uplift
The mineral assemblages of the GPT continued
to
re-equilibrate during this uplift phase, with final re-equilibration at
roughly 590-617 C and 4.5-6.0 kbars (Box
5,
Fig. 20).
63
STRUCTURE
While
a
comprehensive structural comparison between the GPT
the
JRLT is beyond'the scope of this study,
the
GPT
and
the
relationships
between
juxtaposition.
suggest
that
orogenic
DSZ were examined
The
deformation,
deformational
juxtaposition
event,
in
with
the structural aspects of
order
to
high-grade
establish
during
subsequent,
a
single,
continued
timing
metamorphism,
patterns of the GPT
occurred
and
and
and
the
DSZ
high-grade
deformation
under
progressively waning, post-peak conditions.
The
megascopic fabric of the Spanish Peaks area is
northeast-striking
dominated
foliation that is folded into kilometer-scale
by
open
to tight folds with shallow northeast plunge (Spencer and Kozak, 1975).
Within the study area,
poles
to
foliation define a diffuse great circle pattern on an
area projection (Fig.
(1975) indicate
Spanish
southeast-dipping foliation predominates,
21a).
but
equal
The structural data of Spencer and Kozak
that this pattern is only present in this part of
the
Peaks area, proximal to the shear zone, .which contains a high
percentage of injected and anatectic melts; other domains away from the
shear zone exhibit a clustering of poles to foliation.
Units
in
the
south
kilometer-scale synform.
end
of Bear
Basin
are
deformed
into
a
On the west ridge of Bear Basin this synform
is overturned with both limbs dipping to the southeast, but on the east
ridge,
the
the
synform is asymmetric with northwest-dipping foliation
southern limb (Plate I).
Lithologic contacts in Bear Basin
in
show
64
apparent
offset between the two ridges„
northwest-trending fault
suggesting the presence of
a
(Plate I).
On a mesoscopic scale, the rocks are deformed by isoclinal to open
fold
styles.
meter-scale
that
In
the GPT,
wavelengths
isoclinal folds occur
commonly occur as isolated,
folding
centimeter^ to
and generally possess axial-planar
is roughly parallel to the regional
meter-scale
in
of
foliation.
foliation
Isoclinal
folds
intrafolial, fishhook-shaped folds, and as
the
compositional
layering.
In
addition,
isoclinal folds with nappe-style attenuation and apparent offset in the
overturned
limb occur in both gneissic and amphibolitic layers.
nappe-style
folds often verge toward the crest of larger
Many
layers
of what appear to be mafic boudins are actually
which
folding.
been
disrupted
by
mesoscopic-scale
The folds in these mafic bodies are
however,
layers.
have
being
defined
only
by
often
millimeter-scale
open
These
folds.
amphibolite
nappe-style
difficult to see,
plagioclase-rich
It is suggested that nappe^style folding may be more pervasive
throughout the Spanish Peaks area than previously recognized.
Open-style
wavelengths.
folds,
but
folds
occur
on
centimeter- to
kilometer- scale
Open folding is superimposed coaxially on some isoclinal
in
some instances.
Class IA (Ramsey,
1967)
open
folds
develop into coeval Class 2 similar fold geometries within the cores of
the folds.
meter-scale
Open fold styles include gentle flexures,
non-coaxial dome-and-basin structures.
kink folds,
Kink
folds
often form parasitic structures on the limbs of larger open folds.
dome-and-basin
structures
are the result of non-coaxial
and
most
The
interference
between northeast- and southeast- to southwest-plunging open folds;
it
65
•
Figure 21. Orientations of structures in the GPT and DSZ.
Lower
hemisphere equal area projections.
a) 185 poles to foliation, GPT. Cl
= 2%/l% area.
Warping of foliation along the girdle shown may be due
to the high percentage of melt phase near the shear zone, b) SW to NE
trends of 30 open (spiked open circles) and 19 isoclinal
(solid
dots) fold axes, GPT. Similarity of open and isoclinal axial trends
suggests coeval development of the two fold styles, c) SW to NE trends
of 15 open and 26 isoclinal fold axes, DSZ.
Similar deformation
patterns of the GPT and the DSZ suggest a single, protracted erogenic
event related to the juxtaposition of the JRLT and the GPT.
66
has
not yet been determined whether the fold sets are
diachronous
or
coeval.
Spencer
fold
and
styles
However,
Kozak (1975) postulated that the isoclinal and
formed
several
during
separate,
high-grade
erogenic
lines of evidence suggest that the two
were formed during the same orogenic event.
As noted
some isoclinal folds are refolded by open folds,
into isoclinal fold geometries.
open
events.
fold
styles
above, although
many open folds grade
Also, the vergence of isoclinal nappe-
style folds toward the crests of larger open structures suggests coeval
development.
orientations
of
On
an
equal
area
projection
and
nematoblastic
indicative
hinges,
are
open
fold hinges
textures
of
there
overprinted
indicates
21b),
axial
of the two fold styles from the GPT show similar patterns
dispersal along a great circle.
isoclinal
(Fig.
with
Finally,
generally
upper
in thin
possess
amphibolite
synkinematic recrystallization.
section,
granoblastic
facies
any
earlier,
or
assemblages,
In the isoclinal
is no evidence that the amphibolite facies
on
both
higher-grade
fold
assemblages
assemblages.
formation of the two fold styles under the same
This
metamorphic
conditions.
Both
Hingeline
isoclinal
orientation
and
open
fold styles
occur
within
the
DSZ.
patterns of the two fold styles are similar
those of the GPT (Fig.
21c),
to
as are foliation trends, suggesting that
initial development of the ductile shear zone at the same time as highgrade metamorphism of the GPT.
some
This is supported by the
presence
of
plagioclase grains which exhibit dynamic recrystallization at the
margins,
indicative
of
minimum upper amphibolite
facies
conditions
67
(White
and
others,
1980),
and by the presence of
milllimeter-scale
shears within the corona-texture metagabbros which preserve
assemblages (Fig.
disrupt
8).
amphibolite
Within the DSZ1
however, some isoclinal folds
facies assemblages under
conditions,
with
plagioclase
muscovite.
This
indicates
conditions
in
the
DSZ
brittle,
retrogressive
being frequently altered
that isoclinal
outlasted
high-grade
open
folding
folding
to
sericitic
under
post-peak
under
high-grade
conditions in the GPT.
Recent
studies
support the premise that isoclinal and open
styles can occur during the same erogenic event (e.g.
Platt,
1983).
attributable
The
in
Jacobson,
fold
1983;
coeval development of the two fold styles may
part
to the large percentage of melt in
this
be
area,
which may have caused local differences in stress distributions arising
from contrasts in competency of the, rocks (McLellan,
1983).
The large
percentage of melt may also explain the warping of foliation (Fig. 21a)
which
is
not
present
in other domains of
the
Spanish
Peaks
area
(Spencer and Kozak, 1975), where large amounts of melt do not appear to
have
been present.
patterns
of
erogenic
event
It is therefore suggested that the
the GPT and DSZ developed
during
a
deformational
single,
protracted
which is related to juxtaposition of the GPT
and
the
JRLT along the shear zone.
It will be necessary to determine the structure of the JRLT as
relates to this event.
qualitative
GPT.
The
sense,
KQFG
It was noted in this study,
it
however, that in a
the structural style of the JRLT differs from
the
gneisses possess a lineation defined by alignments of
hornblende and biotite which is much more pronounced than in
tonalitic
68
gneisses
unit
and
of the GPT.
Also,
the style of folding in the
is much more chaotic than in gneisses of the GPT,
open
(Fig. 15).
refolding
of earlier folds much more
leucogneiss
with isoclinal
commonly
encountered
69
CONCLUSION
Tectonic Evolution of the Spanish Peaks
Spencer
and
Kozak
(1975) interpreted the Archean rocks
of
the
Spanish Peaks area to represent a single metasupracrustal sequence with
a common metamorphic and deformational history.
of
this
exposures
the results
study demonstrate that the present configuration
in
the Spanish
juxtaposition
histories.
the
However,
of
Peaks
terranes
area
with
is
the
fundamentally
result
of
of
different
Archean
tectonic
geologic
This conclusion requires the Archean tectonic evolution of
Spanish
Peaks
area
to be modelled in
terms
of
the
different
geologic histories of the individual terranes, as outlined in Table 6.
The supracrustal suite of the. GPT
paragneisses,
sequence
may
with minor metapelite, amphibolite, and quartzite.
be a
metavolcanic
flows
metasediments
shed
composition.
terrane,
consists primarily of tonalitic
volcanic
and/or
from
arc
suite,
consisting
of
metagreywacke
sediments,
or
pre-existing
sialic
crust
of
This
dacitic
may
be
tonalitic
As the simplest model for the tectonic evolution of this
relict staurolite assemblages represent the early stages of a
single prograde tectonic event, culminating in upper amphibolite facies
,metamorphism.
tonalite
The oldest granitoids .(hornblende
granitoids,
biotite
and porphyritic granodiorite).would have been injected during
the early stages of this event (Table 6).
Alternatively, the formation
of low-grade assemblages and injection of the oldest granitoids may have
70
occurred during an earlier event unrelated to upper amphibolite
metamorphism.
It
is
also
possible that the
emplaced prior to the onset of metamorphism.
older
facies
granitoids
were
In any case, later upper
amphibolite facies assemblages occurred at peak metamorphic
conditions
of 665-726 C and 7-o kbars. This suggests that the supracrustal package
of
the
presence
through
GPT
was buried to minumum depths
of
nappe-style
folding suggests
(Okuma,
1971;
Karasevich
tectonic
The
thickening
A similar mechanism has also been
and others,
1981).
Ruby
and
amphibolitized).
amphibolitized
metagabbros
may
injection
have
of
outlasted
the
was
(granite
packages) and metagabbros (both
The
Range
Peak metamorphism
in part by the emplacement of younger granitoids
trondhjemite/amphibolite
texture
that
kilometers.
for Archean supracrustal sequences in the nearby
accompanied
and
20-25
stacking of nappes was an important mechanism for transporting
this package to mid-crustal levels.
proposed
of
corona-
granite
peak
and
metamorphic
conditions.
In
contrast to the GPT, the metasupracrustal suite of the JRLT is
dominated by paragneisses of granitic composition,
representative of a
more
and
highly
Condie,
that
evolved
supracrustal setting (Engel
1982) than that of the GPT.
others,
1974;
At present, there is no evidence
the JRLT shared the early plutonic history of the GPT (Table
6).
The JRLT experienced a different path of metamorphism than did the GPT.
Kyanite—muscovite
of
the JRLT,
facies
8.3
assemblages formed early in the metamorphic
while peak metamorphism produced transitional
history
granulite
assemblages at temperatures and pressures of 745-784 C and 6.0-
kbars,
and
resulted
in the
locally
extensive
development
of
71
GALLATIN PEAK TERRANE
JEROME ROCK LAKES TERRANE
I. Accumulation of tonalitic
supracrustal sequence.
I . Accumulation of granitic
supracrustal sequence.
2. Metamorphism to staurolite
grade. Emplacement of hornblende
granitoids, biotite tonalite,
porphyritic granodiorite.
Possible early development of
' isoclinal folding.
2. Sillimanite-grade,
transitional granulite
. facies metamorphism.
Development of extensive
anatectic migmatites.
3. Metamorphism to kyanite-grade,
upper amphibolite facies.
Development of isoclinal folding,
with axial planar transposition
foliation.
lx .
(No data yet available
pertaining to structural
history of JRLT)
Juxtaposition and initial development of ductile shear zone,
under high-grade conditions, with coeval development of the
following:
Continued metamorphism
of GPT at kyanitegrade, upper
amphibolite facies, —
at peak conditions of
665-726 C, 7-8 kbars.
Continued isoclinal
folding, with coeval
development of open
folds.
Emplacement of granite,
trondhjemite/amphibolit e,
corona-texture metagabbro
into both terranes.
—
Magmatic epidote in granite
suggests initial emplacement
at pressures of 7-8 kbars.
Partial re­
equilibration
of JRLT to same
conditions as
GPT. -
5. Pulse of rapid uplift of both terranes,
accompanied by continued injection of
granite and amphibolitized metagabbros;
uplift lasting through progressively
waning stages of metamorphism.
Continued isoclinal and open folding,
largley restricted to shear zone.
Table 6. Proposed sequence of geologic events for the GPT and the JRLT.
The GPT and the JRLT have different, geologic histories prior to
juxtaposition and a shared history following juxtaposition.
72
anatectic migmatites. The JKLT shared the later platonic history of the
GPT,
as
zone
indicated by the presence of the youngest granitoids near the
and
Pressures
the
corona-texture
and
temperatures
metagabbros
metagabbros
calculated
throughout
from
the
the
JKLT.
corona-texture
indicate that re-equilibration occurred in the JKLT at the
roughly the same conditions as the peak conditions of the GPT.
Therefore,
the
terranes
can
youngest
granitoids
present
relative
timing
of
juxtaposition
of
the
two
of
the
which
are
be narrowed to prior to or during the injection
in both
and the corona
terranes (Table 6).
texture
metagabbros,
Furthermore,
since there is no
evidence for high-grade metamorphism after Archean time in southwestern
Montana (Giletti, 1966, 1971; James and Hedge, 1980), the
the
unique,
high-grade,
presence
corona-texture metagabbros in both
of
terranes
indicates that amalgamation occurred during Archean orogenesis.
The preservation of relict,
the
paragneisses
suggests
unstable,
of the JKLT and in the
high temperature phases in
corona
texture
metagabbros
that a pulse of rapid uplift of the amalgamated terranes
have occurred during or shortly after juxtaposition.
may
Rapid uplift may
have been facilitated in part by the presence of anatectic melts in the
JKLT
and by the injection of mafic and granitic magmas,
resulting
accelerated deformation along melt-lubricated shear planes,
similar
to
that
described from the Coast
Plutonic
Complex
of northern British Columbia (Hollister and
and
in
in a style
Metamorphic
Crawford,
1986).
The formation of retrogressive symplectite textures in granulites
near
the
zone
shear
zone
and retrogressive assemblages within the
shear
indicate that uplift continued through progressively cooler conditions.
73
possibly
accompanied
and facilitated by the
continued
injection
of
granite.
The
many
Archean
striking
processes.
tectonic evolution of the Spanish Peaks
area
similarities to Phanerozoic Cordilleran-style
First,
bears
tectonic
pressures and temperatures of metamorphism for the
Spanish Peaks area indicate a metamorphic gradient of 25-35 C/km, which
is
roughly
equivilant to that reported from
high-grade
terranes
of
southeast Alaska and northern British Columbia (Hollister and Crawford,
1982).
Second,
transported
the
to
structural
mid-crustal
stacking of nappes.
style suggests that these rocks
levels
by
tectonic
thickening
were
through
Finally, the compositional range of the granitoids
from early hornblende monzodiorite and tonalite to younger trondhj emite
and
granite,
formation
the
of
descriptions
melt-lubricated
the
(Barker and others,
Hollister and Crawford,
present
shears
are
intrusion,
nearly
and
the
identical
to
of the Coast Plutonic and Metamorphic Complex of northern
British Columbia,
1982;
generally concordant style of
1981;
Crawford and
Hollister,
1986). These similarities indicate that
configuration of Archean exposures of the
Spanish
Peaks
area is the result of Cordilleran-style collisional processes involving
the amalgamation of terranes with divergent geologic histories.
Discussion
Accretionary,
in
an important
mechanism
the Phanerozoic growth of the North American continent (e.g.
and others,
This
or microplate tectonics is
Coney
1980; Iverson and Smithson, 1982; Jones and others, 19.83) .
mechanism has also been successfully applied in
modelling
early
74
and middle Proterozoic growth of North America (Karlstrom and
1984).
However,
tectonic
while
processes
Houston,
it has been suggested that Phanerozoic
were operating in the Archean (Dewey and
plate
Windley,,
1981), and that the Archean continents may have been stabilized through
microplate accretion (Dickenson,
available
which
adequately
1981;
Condie, 1982), few studies are
document
the
occurrence
of
Archean
accretionary processes.
The
Archean
basement
blocks
known
results of this study form part of an emerging pattern in the
with
or
of southwestern Montana, in which
discrete
crustal
widely differing geologic histories are juxtaposed
postulated
Precambrian structural
Beartooth Mountains (Mogk,
1981,
southern Madison Range (Erslev,
1982;
1983). •
discontinuities
Thurston,
Use
1986),
along
in
the
and in the
of the word "terranes",
which is defined by Jones and others (1983) as
"fault-bound entities of regional extent, each characterized
by a geologic history that is different from the histories
of contiguous terranes,"
is
particularly
JRLT,
where I) the two terranes represent much different
settings,
the
applicable in the Spanish Peaks for the GPT
2)
JRLT,
and
supracrustal
the GPT records an early plutonic history not shared
and
3)
the
two
terranes
record
different
the
by
metamorphic
histories (Table 6).
Furthermore, Mueller and others (1985) have
'
■
. V
documented distinct geochemical and isotopic differences
between
metasupracrustal
suites of the Beartooth region and suggest that .they
may be genetically unrelated terranes.
terrane
,concept
to
the
Archean
Therefore,
basement
of
application of the
southwestern
-— ItTr^
Montana
75
suggests
that previous models which invoke a single depositional basin
proximal
to
1975;
a stabilized continentental source
(Spencer
and
Kozak,
Garihan, 1979; Vitaliano and others, 1979) may not be sufficient
to explain the lithologic,
metamorphic, and plutonic diversities which
occur in the Archean basement of southwestern Montana.
Instead,
different
the demonstrated tectonic juxtaposition of terranes with
geologic histories in the Spanish Peaks and other ranges
southwestern
Montana
strongly
suggests that growth
of
the
of
Archean
craton of the northern Wyoming Province occurred through the horizontal
accretion
of
unrelated,
.Archean.
discrete
during
This
metamorphic,
to
the
is
supported by the
Montana
may
close
Plutonic and Metamorphic Complex
is
a
major
"tectonic
be
genetically
the
late
similarities
welt"
of
northern
in
resulting
British
from
accretion of allochthonous terranes to the western
of North America (Monger and others, 1982).
should
which
a Cordilleran-style collisional event in
premise
which
Phanerozoic
blocks,
structural, and plutonic styles of the Spanish Peaks area
Coast
Columbia,
crustal
the
margin
Therefore, future studies
regard the various Archean lithotectonic suites of southwestern
as
"suspect terranes" (Coney and others,
1980)
until
their
genetic correlations can be demonstrated by detailed field, structural,
petrologic, geochemical, and geochronological studies.
76
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and
its
GENERALIZED
GEOLOGIC
MAP,
CENTRAL
SPANISH
PEARS
PLATE 1
LEGEND
A Ig
—
Leucogneiss, granodioritic composition. Intercalated mafic and quartzite layers.
Extensive migmatization.
Amg —
Mafic gneiss and schist. Intercalated cpx-gt granulite.
Ams —
Sillimanite-metapelite, quartzite.
DS Z —
Ductile shear zone. Semi-continuous cm- to m-scale mylonite bands.
Atg
—
Grey para (?) gneiss, tonalitic composition. Local development of stromatic migmatite.
Ata
—
Trondhjemite-amphibolite injective migmatite.
Contacts solid where known, dashed where inferred
Amk —
Heterogeneous metasupracrustal suite. Tonalitic paragneiss, kyanite-metapelite, quartzite.
Aum —
Ultramafite.
Granite.
Porphyritic granodiorite.
Biotite tonalite granitoid gneiss.
Hornblende granitoids: includes hornblende monzodiorite and hornblende tonalite gneisses.
Amphibolitized intrusions and boudins.
Corona-texture metagabbro intrusions and boudins.
Inferred Laramide fault.
Strike and dip of foliation.
Topography from the Spanish Peaks, MT quadrangle
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