A Thesis
Presented To
t h e Faculty of t h e Depatment cf Geoscience
New Mexico 1nstitu.te of klining m c i . !PecImol.ogy
. ...
"
In Partial Fulfillment
of t h e Bequlrements f o r the Dogsee
14axtar of Science
This
i
..............................................
..........................
Purpose
t h e I n v e s t i g a t i o n .......................
Location
A c c e s s i b i l i t y .........................
Methods of I n v e s t i g a t i o n ............................
PAGE
..
N3STXACT..
V-i
1
I N T R O D I T G T I O N . ~ ~ . ~ . ~ . ~ ~ ~ ~ * . ~ ~ ~ ~
1
ol"
1
and
..
5
...............................
...................................
..............................
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............................
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.................................
.
Mountain .
..
P r e v i o u s !(lork.O.O.OO
G
.Acl.o\~.led~ements
9
11
STRATIGRAPHY fiTD PP
.'RO.OGY
Prevolcanic Rocks
If
Tertiary Volcanic Rocks
12
Spears .Formation.
14
Lower Me!nber
14
Upper
16
ember...............................
Tuff of Nipple
e e e
*.
Upper Spears Epiclastic Rocks
a
..........
Upper Spears C r y s t a l R i c h L a t i t e T u f f s
.
..........................
A-L
Formation .............................
Lower Cooling Unit .........................
Interbedded .desites
......................
Cooling Unit ........................
Upper Cooling Unit .........................
Andesite of Landavaso Reservoir ................
T e r t i a r j i I n t r u s i v e Rocks ...........................
Mafic
............................ '........
Hells Hesa Pormation.
Pesk
Hiddle
Dikes
16
21
23
26
32
33
38
41
44
45
49
49
ii
Latite Intrusives
...............................
57
............................
53
Tres Nontosas Stock.....2 ................. 56
F a c i e s .....................................
58
P e t r o g e n e s i s ...............................
74
Post-Oligocene Rockfi ................................
78
Tertiary-Quaternary Gravels ....................
78
80
Tertiary-Quaternary Basalt .....................
Quaternary Deposits ............................
82
82
Talus cand Colluvium ........................
Eolian Sand ................................
82
Alluvium ...................................
83
P e t r o g e n e s i s o f Oligocene Igneous ROC!^^............ 83
............................................... 89
R e g i o n a l S ~ r u c t u r.................................
e
89
Local
92
Oligocene Tectouics ............................
94
.aulting ....................................
94
Folding .....................................
99
S t r u c t u r a l P a t t e r n s Around S t o c k s .......... 03
Basin and Range T e c t o n i c s ......................
L o n g i t u d i n a l Faults ........................
T r a n s v e r s e F a u l t s ..........................
Rhyolite Intrusives
.,eo.a
.
STRUCTUXE
. .
...
structure.,.,,,,.,...,..,......,....,.,.*..^^
1
105
105
106
109
116
122
122
129
iiri
ILLUSTRATIONS
PLATE
1.
PAGT
Geologic map a n ds e c t i o n s o f .theCouncil'Rock
d i s t r i c t , S o c o r r o County, Nen Nex.ico..ea.e...in
pocket
FIGURE:
1.
Index map o f Kew Mexico, shoving location of
t h e s i s area.............*.,..^^^.^................
3
2.
Panora3i.c vie17 o f t h . e s i s a r e a ,
4
3.
G e n e r a l i z e d s t r a t i g r a p h i c col.umn o f Oligocene
r o c k u n i t s i n the Council Rock district,,........
I3
4.
OutcropofthetuffofItippleMountain.
79
5.
I ~ h d f l o v sof t h e u p p e r S p e a r s
p r o p g l i t i zme adf i c
dike..
....................
..........
rr,e~nberc u t by
. *..
e
e
e e
22
B a s a l g r a y n a s s i v e rneaber of t h e A-L Peak
Formation near G a l l i n a s Springs..............,.,.
35
A n t i c l i n a l f o l d ' i n t h e c o n t o r t e d member. o f ' t h e
A-L Ped5 Formation...,...,..,....................
35
8.
Basaltic-andesite lava flow interbedded in the
A-L Peak FormBtion.,s.o.oc..,..,.,,....,
39
9.
Plunging syncl.ina1 flow fold
of Landava.so
...........,.......,.,.....riovrese9
47
6.
-7.
10.
i n t h e andesite
I n f e r r e d d i s t r i b u t i o n o f rock t y p e s i
n the
!?res Xontosas stoc~Ioo.cro.o......ooo..aol........
59
........................................
1I n .
Photomicrographofgranophyricgranodiorite
porphyry.
63
IIB.
Photoxicrograoh o f .granod.iorite porphyry
shoi7i-nc o r t h o c l a s e m a n t l i n g a high-temperature
plagioclasephenocryst..
..........................
63
Frequency diagram o f p l a g i o c l a s e c o m p o s i t i o n
f o r Sord.er f a c i e s r o c k s o f the Tres X m t o s a s
stoCk..........a.o....ar......s.or,r...~..~~.....~*
66
12.
.......
13e
Photoxicrogra2h of sanidine
g r m i t e porphyry
of the c o r e f a c i e s , %res Xontosas stock....
14.
Photomicrograohofheden'oergitesyenite
fro2
netason!atized border f a c i e s r o c k s i n t h e Tres
14on.tosa.s s t o c k . ;
70
.................................. 72
iV
Photo~~icro~yraph of spotted hornfels after
from t h e
amygdaloidal "turkey track" andesite
contactmetamorphicaureolearouad
Tres
hiontosas stoclr..............,..~..
..............
16.
Major s t r u c t u r a l f e a t u r e s i n t h e l:lagdalena
r e g i o n with r e s p e c t t o thesis area..
17.
Welded f a u l t c o n t a c t betvieen gray massive
member a n d c o n t o r t e d member o f t h e A-L Peak
For~ation,.........~.~.~.................
18.
Diagramitic cross sections illustrating
d e v e l o p m n to ft h eG a l l i n a sS p r i n g sF a u l t
and a n i n t r u s i v e monocline.
19.
75
93
......... 96
the
.......................
102
Vein b r e c c i a from the Council Rock mining
district........,,..................,.,..~~..~...
114
Appendix
1A.
f o r X'A(Ot0)
Rittnann zone method curves
versus composition of volcanic
and p l u t o n i c
plagioclase..c.o..o.c......,.............*....~..
121+
............................
2.
2V versus composition o f volcanic
and
.....
pltitonic plagioclase.
124
V a r i a t i o n i n 2Vx of alkali f e l d s p a r with
composition..,..............,,..,......,~~~~..~.~
128
.................128
clinopyroxenbs.
".. . '128
3A *
V a r i a t i o n i n 2V and ZAG i n Ca-i4g-Fe
clinopyroxenes &it11 composition.
3B
Iiomenclature of Ca-bXg-Fe
e e.
f
e
*.
V
TABLES
TBLE
1..
2.
PAGE
B e p r e s e n t a t i v e modal amalyses of .
p
r i n 1c i p a l r o c k t y p e s from t h e Tres Hontosas.
stoc~i..........,,...............,,,,,,,,~~.~.....
Approximateequilibriumcrystallization
temperatures o f s e l e c t e d a s h - f l o w t u f f s
i n d i c a t e d by the two feldspar geothermometer.
61
....
85
Appendix
f o r s t r u c t u r a l state
ExanFlesofdeterminations
o f p l a g i o c l a s e mad.e by the combinedRittmann
zone- 2 V 2 ~~iethod..............
o..I
........
...... I 2 6
..
vi
Al3STRACT
. .
Eedrocli exposures i n t h e C o u n c i l
o f t h e D a i A l volcanics.
chiefly Oligocene ash-flov tuffs
A n d e s i t el a v a
flows
Rock d i s t r i c t are
.
a n dv o l c a n i c l a s t i cs e d i m e n t so c c u ri n
sone c o o l i n g breaks i n t h e t u f f s .
No s i g n L f i c a n td i f f e r e n c e s
i n s t r a t i g r a p h i c r e l a t i o n s h i p s o r p e t r o l o g y were observed i n
comparison t o a n e q u i v a l e n t O l i g o c e n e s e c t i o n i n t h e S o u t h e r n
1972),
Bear Kountains (Broen,
A late Oligocene s t o c k p a r t i a l l y
exposed i n the s o u t h c e n t r a l p o r t i o n o f
the area c o n s i s t s o f
a g r a n i t e c o r e wiri%ha n d e s i t e t o q u a r t z m o n z o n i t e a r o u n d t h e
basalt f l o w s
b o r d e r .T e r t i a r y - Q u a t e r n a r yp e d i m n tg r a v e l sa n d
occur i n grabens forned by basin and range
Observationsoutsidethe
t h eb u r i e df l a n k
block' f a u l t i n g .
area c o n f i r m t h a t c o u n c i l BOCK i s on
o f a Lararnide u p l i f t e .
1kajor s t r u c t u r a l f e a t u r e s o f the Council Rock & i s t r i c t
are h i g h a n g l e f a u l t s
(NNU and P I E trends dorninant) and
a
partially exposed north-trending monoclinal fold which forms
t h e east f l a n k of the G a l l i n a s u p l i f t , B o t h
havebeen
f a u l t t.rends
activeepisodicallysincemid-Oligocenetine
and
i t is proposed that they mirror preexisting fault
zones i n
b a s e n e n tr o c k s .F o m a t i o no ft h em o n o c l i n a lf o l d
is a t t r i b u t e d
t o i n t r u s i o n of a 20-mi1.e-long
Oligocene t i n e .
pluton a t depth i n l a t e
I n t e r s e c t i o n s o f majorOligocenefaults.
s t r i k i n g a p p r o x i m a t e l y N 10" \!/ and iJ' 45" E were i m p o r t a n t
c o n t r o l s i n t h e emplacenent o f s e v e r a l s t o c k s w h i c h
are
i n f e r r e d t o be e x t e n s i o n s o f t h e e l o n g a t e p l u t o n .
Two' unexposed stocks
are i n f e r r e d i n t h e a r e a
on t h e
basis o f f e l s i c d . i h siv%rms, h g d r o t h e r n a l a l t e r a t i o n , v e i n i n g
I
.
.
vii
a n dl o c a ls t r u c t u r a lf e a t u r e s .E p i i t h e r m a lq u a r t z - c a r b o n a t e
veinsnearCouncil
f o r their silver content,
iiock,oncemined
oxe of t h e s e u n e x p o s e d . s t o c k s . . S t r a t i g r a p h i c
areanexpressionof
.
.
and s t r u c t u r a l r e l a t i o n s h i p s i n this a r e a sugiest t h e roofof
t h e s t o c k naybe
*:1ithin'2000
f e e t , of %he s u r f a c e ;
recommended as a f a v o r a b l e t a r g e t
.
The a r e a i s
f o r furtherevaluationwith
r e g a r d t'o p o r p h y r y - t y p e b a s e m e t a l s u l f i d e m i n e r a l i z a t i o n .
Extensive hgdrotherrxd. alteration coincident
.-
s i d e o f the Kulligan. Gulch graben
with t h e wst
is proba'blygenetically
t i e d t o the deep pluton vhich produced the monoclinal' fold.
A petrologic study
o f t h e Tres N o n t o s a s s t o c k s u g g e s t s
'vapor vas t h e most
t h a t upward d i f f u s i o n o f a l k a l i - r i c h w a t e r
significantprocessin
its d i f f e r e n t i a t i o n .
Narrow rims of
o r t h o c l a s e commonly mantle high-temperature plagioclase
..
phenocry8ts i n t h e b o r d e r f a c i e s r o c k s .
.~..,
T h i s featu-re i s
i n t e r p r e t e d as evidence o f a n a l k a l i d i f f u s i o n p r o c e s s which
has been
tlfrozenll i n t o t h e r o c k s
by r a p i d c r y s t a 1 l . i z a t i o n .
Compositional v a r i a t i o n of f e l d s p a r s i n t h e
Bornmtion i n d i c a t e ' i t i s an "upside
zonedash
flon sheet,
(Lipmanand
*
of t h e Bells Mesa and A-JA Peak
i s expandedupon.
that incorporation of
dom'l compos-itional3.g
R model proposed by B r o m (1972, p. 7 4 )
concerningthepetrogenesis
a s h - f l o ws h e e t s
Hells Hesa
The expanded model advocates
a l a r g e cmount of meteoric water
f i i e d a a n , 1974) i n t o a b a t h o l i t h i c s i z e d volume of
a l k a l i - a n d e s i t e m g l a folloilred domi.nant1y by upward d i f f u s i o n
of an alkalj.-rich aqueous vapor
in the developaent
form ash-flow tuffs.
phase i s t h e c r i t i c a l p r o c e s s
o f s i l i c i c aagmas which are e r u p t e d t o
-
Purpose of .t-h e
In~estlgation
The o b j e c t i v e s of t h i s t h e s i s a r e
t o .d.etermine t h e .
stratigrapbac r e l a t i o & h i p s , s t r u c t u r a l t r e n d s , p a t t e r n s
hydrothersnalalteration,and
r o c k s i n the Council
Nexico
the d i s t r i b u t i o n
Of
of
.
.
inimxive
Rock d i s t r i c t , SocorroCounty,
Hew
These r e l a t i o n s h i p s are i n p o r t a n t f o r t h e f o l l o w i n g
reasons :
1,
Structural relationships that controlled the
emplacenentofanesposed,non-mineralized,
hypabyssal Tertiary stock
(here named t h e Tres
Nontosas stock)' may be used t o l o c a t e o t h e r i n t r u s i v e
c e n t e r s as t a r g e t s o f possibleftporphyry-typett o r
"replace~aent-type" basa aetal"n i n e r a l i z a t i o n .
2.
The i n t e r - r e l a t i o n s h i p s o f i n t r u s i o n s t o hydrotherxal
alteration,epithernalveins,stratigraphicunits,
andstructuralfeatures
a t CouncilRock,provides
a basis on which t o e v a l u a t e t h e d i s t r i c t ' s e c o n o x i c
potential,
3.
Descriptions of variations in
the v o l c a n i c s t r a t i g r a p h y
betvieen t h e S o u t h e r n Bear Mountains (Drown, 1972)
and the Council
Rock d i s t r i c t may be of v a l u e i n
futuregeologicinvestigations
t o t h e west o f
Council I?oc!c.
Location
& Accessibi1.it.y
The C o u n c i l 2 o c k d i s t r i c t ,
as mapped, c o v e r s a n e a r l y
r e c t a n g u . l a r m e a of a b o u t t h i r t y s q u a r e m i l e s w i t h i n t h e
2
CibolalJationa1Forest.
14
m i l e s duewest
o f t h e t o m ofMagdalena,SocorroCounty,
New Mexico ( f i g . 1 )
a r e a i s approximately,bounded
The'map
by 34" 12' N l a t i t u d e , t o t h es o u t h
t ot h en o r t h
latitude, t o theeast
west by
The c e n t e r of t h e d i s t r i c t i s about
by 34"
by 107" 23' 7" W longitude and
N
t o the
107 a 28' 30" W l o n g i t u d e .
The- s t u d y a r e a i s 1ocat.ed i n t h e n o r t h e a s t e r n c o r n e r
(Fig. 1 )
t h eD a t i l - H o g o l l o nv o l c a n i cf i e l d
t h e name g i v e n t o t h e
pxominentoutcropof
t h en o r t h e a s tq u a r t e r
of t h e d i s t r i c t .
of.
Council Rock is
i basalt d i k e i n
Thoname
:'LS
derive6
o f t h i s landmark as a meeting place between
from t h e u s e
In$ians and settlers in the
1800's.
Physiographically, 'the Council
on t h e east.?rn edgeof
Rock d i s t r i c t is l o c a t e d
a lo;? north-'south divide which connects
t h eG a l l i n a sM o u n t a i n sw i t ht h eS a n
Widceo Mountains.Present
topography i s s t r o n g l y c o n t r o l l e d b y b l o c k f a u l t i n g t y p i c a l '
ofthoBasinand
Range province.Grabensoccuring
east and t o the west
Gulch trough,
of t h e G a l l i n a s u p l i f t a r e t h e
to the
bIullLgan
a l s o k n o m as t h e I k g d a l e n a P l a i n . , a n d t h e
2 i s a pan.oramic
1Jorth L&Q B a s i n ,r e s p e c t i v e l y .F i g u r e
view oE thestudyareaandthesu.rroundinglandforms.Relief
i s l e s s t h a n 900 f e e t i n t h e d i s t r i c t
divide;
and along
homever, a prouinentlandmark
justsouth
of the study area, rises
most of t h e
k n o ; ~as
~ Tres Montosas
a b o u t '1300 f e e t above t h e
surroundingterrain.,Phyoiographychangesrapidlytowardthe
north because of the proximity
o f the Colorado Plateau
p r o v i n c e .A n o t h e rs i g n i f i c a n ' ct e c t o n i cf e a t u r eo fg r e a t
importance, the north-south trending Rio
Grande r i f t , is
3
. .
5
l o c a t e d a b o u t 35 m i l e s east of Council
Roc!:,
is
Ea.sjr a c c e s s t o t h e n o r t h h a l f o f t h e d i s t r i c t
a
i ilnproved fo,-es% s e r v i c e r o a d w h i c h t r e n d s
provided by
S t a t e Road 52 to Horth Lalre v i a t h e C o u x c i l
westerly fcon
RockRanch.
o f t h e t h e s i s area i s
The s o u t h e r np o r t i o n
best reached
from
. .
U.S, €Ii.ghmy GO b y r a n c h r o a d s
Nontosas Cattle Company.
of the
Ylood c u t t e r s trails are numerous
throughout the area and allow four-wheel-drive vekicular
t r a v e l t o u i t h i n a mile of any point
in the district.
-
Nethods o f I n v e s t i g a t i z
.
of 1971, s u r f a c eg e o l o g y
D u r i n gt h es u n w ra n df a l l
was mappedon
p o r t i o n s o'f t h e Tres lkfontosas a n d G a l l i n a s
. .
Peak 7.5 minute quadrangles
( 1 :24000) p u b l i s h e d by t.he
United. States GeologicalSurvey.
t h e Geolo@xal
5 :31,
'
'
Aerzal photographsfrom
Survey (VABJ s e r i e s , 3-20-63) a t a s c a l e o f
680 proved t o be v a l u a b l e i
n locating and outlininz
~ k r eo f t e n 1x31 d i s p l a y e d
o u t c r o p s .S t r u c t u r a ll i n e a m n i x
on t h e s e a e r i a l photographs,
mylar vas u s e d f o r p r e p a r a t i o n
k topographicbase
of %he f i n a l g e o l o g i c map,
One h u n d r e d a n d s e v e n t e e n t h i n s e c t i o n s
from roc!;
map on
were made
samples c o l l e c t e d i n t!le area f o r t h e purposes
of correla.ting
o r characterizfngrock
hydrothermalalteration,
ImitS,
studying
and d e t e r m i n i n g t h e p a r a g e n e s i s
of vein m i n e r a l s and p e t r o g e n e s i s o f i n t r u s i v e r o c k s .
Nodal analyses o f e i g h t e e n t . h i n s e c t i o n s o f s a m p l e s
from
t h e Tres i,iontosas s t o c k were p r e p r e d by Janes Bruning
u s i n g a Z e i s s z i c r o s c o p e equipp2:
.
.th a Swift automatic
6
poin-tcounter.
W.H.
Ten of thesesamples
were c o l l e c t e d by
Willdnson,fromthesouthborderofthestock.Another
.
Tres Montosas stock were i a t e r
eight thin sections of the
s t a i n e d t o a i d i n the i d e n t i f i c a t i o n o f potassium feldspar
by u s i n g t h e methodofChaycs(1952).
Fabricdescription,preliminarymineralidentification
were completed on a Z e i s s
andestimatesofmodalmineralogy
. .
binocularpetrographicmicroscope.Microscopicvisu.al
t o c l a s s i f y rocks
estimates of modal mineralogy were used
a c c o r d i n gt oT r a v i s
(195.5).
The cornposition o f p l a g i o c l a s e
was determinedusingtheFouque'method(Troger,1959,
and' by t h e R i t t m m n m n e
p. 101)
method (Enmons, 1911-3, p.. 115-133)
u t i l i z i n g a f i v e - a x i su n i v e r s a l
stage.
The u n i v e r s a l stage
was ale0 u s e d t o m e a s u r e o p t i c a x i a l . . q g l e s .
of a l k a l i f e l d s p a r ,
of clinopyroxenes
p l a g i o c l a s ea n dp y r o x e n e s .E x t i n c t i o na n g l e s
(ZAG) mere a l s o measured u t i l i z i n g t h e u n i v e r s a l s t a g e .
O p t i c a l d a t a wereused
andcomposition
t o determinethestructural
of f e l d s p a r s a n d t h e c o m p o s i t i o n
state
of pyroxenes.
are d e s c r i b e d i n t h e
The methods of these determinations
appendix e
P r e v i o u s Work
Previous geological investigations in the Council
area havebeen
o f a reconnaissancenature.
recorded investigation in the vicinity
Rock
The first
was made by Herrick
' ( 1 9 0 0 ) Eis p a r t of a r e c o n n a i s s a n c e s u r v e y i n p o r t i o n s o f
Socorroand'ValenciaCounties,
were t h a t t h e G a l l i n a s ,
.
His b r i e fo b s e r v a t i o n s
Datil. andBearNountains
composed o f t r a c h y - t e a n d r l ; y o l i t e . i n t r u a i v c c .
were
::'inchester
7
,
I
( 1920) o b s e r i w l v o l c a n i c
r o c k s overlyirlg formations of
Cretaceous age along the hlamosa
He named t h e s e v o l c a n i c u n i t s t h e
Creek Valley ( X i 0 S a l a d o ) .
ttDatil Formationtt .and
d e s c r i b e d a. t y p e - l o c a l i t y a t t h e n o r t h e n d
o f t h e Bear
Mountains.
Loughlin and
Koschmann (1942) began a d e t a i l e d
comprehensive geological investigation
of t h e Magdalena
raining d i s t r i c t i n 1945; t h e i r work was later p u b l i s h e d as
U.S.
GeologicalSurveyProfessionalPaper
200.
t h a t t h e i r banded r h y o l i t e u n i t ( e q u i v a l e n t .
They stated
t o t h e lower
A-L Peals. Formation o f t h i s m p o r t ) i s an e x t e n s i v e formation
and Tonking (1957) made
mt3t ofMagdalena.Givens(1957)
independent studies of the Tertiary volcanic rocks
5-11.
the
Dog S p r i n g s and P u e r t e c i t o 15-minutequ-adrangles,renpectively.
TGnking s u b d i v i d e d t h e Datil F o r m a t i o n i n t o t h r e emelnbbers
'
whichhe
named itSpearstt, 6113[ellsNesa" and "La Jara Pezktt
( f r o mo l d e s t
t o youngest),Givenssubdividedtha
He1l.g'
Nesa Nember i n t o s e v e n mappable s u b u n i t s i n t h e G a l l i n a s
Kountains,Equivalents
notobserved
of t h eL a
Jara Peak Mc;aber were
by Tonking, Givens o r t h e writer i n t h e
G a l l i n a s 1-Iountains.
,
I'lillard ( 1 9 5 9 ) c o n d i t i o n a l l y c o r r e l a t e d
t h e L a Jara Peds Member wi.th a post-Datil basalt sequence i n
the Datil Mountains.Later,
Weber (1963) excludedtheLa
Jara Peak Member from t h e Datil Formation.
t h e ne l e v a t e d
Weber ( I 971 )
t h e Datil Formation t o g r o u ps t a t u s .P a r k
(1971 ) has conducted a d e t a i l e d p e t r o l o g i c s t u d y
Anchor Canyon s t o c k l o c a t e d a t t h e n o r t h e n d
range.
of t h e
of t h e llagdalena
8
Brown (1972)andC,E.,Chapin
provided detailed descriptions
( o r a l ccmmun., 1972)
and a n a l y s e s o f measnred
( 1 972) subdivided the Hells
Brown
Mesa Formations,
sectionsintheSpearsandHells
Mesa Formation i n t o a loyrer.
t u f f ofGoat
c r y s t a l - r i c h member, i n f o r m a l l y t e r m e d t h e
S p r i n g s , and a n u p p e r c r y s t a l - p o o r
'
member, t e r m e d t h e t u f ?
of Bear Springs.DealandRhodes(1974,
i np r e s s )s u b s e q u e n t l y
renamed t h e t u f f of Eear S p r i n g s as t h e A-L PeakFormation
on A-L Peak i n t h e n o r t h e r n
f o r a 2000-foot section exposed
S a n I4ateo Mountains.
Deal and Rhodes(1974,
boundariesof
i n p r e s s ) havedelineatedthe
a cauldron(caldera)inthe
Mount Vlithington
a r e a o f t h e San Mateo Mountains.They
cauldron i s t h e s o u r c e
b e l i e v e t'hat t h i s
f o r b o t h A-L Peak Formationand
thhr'sequence of overiying tuffs which they
P o t a t o Canyon Rhyolite.
,
E l s t o na n do t h e r s
a
named t h e
(1968, 1970, 1973)
a r e now i n t h e p r o c e s s o f s t u d y i n g e r u p t i v e p a t t e r n s a n d
their tectonic relationships in the Datib-Mogollon vol.cznic
province.Simon(1973)recentlycompletedaninvestigation
of t h e s t r a t i g r a p h y , s t r u c t u r e a n d m i n e r a l i z a t i o n o f t h z
S i l v e r Hill a r e a .
\YoHe Wilkinson ( i np r e p a r a t i o n )h a s
c o m p l e t e d d e t a i l e d mapping o f t h e Tres Montosas a r e a c o n t i g u o u s
t o , and s o u t ho f ,t h eC o u n c i l
Rock d i s t r i c t .
( i n p r e p a r a t i o n ) has mapped t h e c e n t r a l N a g d a l e n a
suggests the presence
of another cauldron in the
Canyon a r e a , which may a l s o . havebeen
Krewedl
D.A,
Range and
Sarnni.11
a vent area for the
A-L Penk Formation.
Numerous radiometric(mostly
K-Ar)
datesonthe
Datil-
.
9
Mogollon volca.nic units and intru.sions
b ys e v e r a li u t h o r s .
havebeenprovided
Weber and Bassett'(1963) o b t a i n e da g e s
.
f o r t h e Nitt and Anchor Canyon s t o c k s l o c a t i d i n t h e K e l l y
mining district
and f o r t h e basal 'welded ash-flom of t h e
Hells Mesa Member a t Tonking'stypesection,
o t h e r s ( 1 963) dated
Burkeand
a l a t i t e boulder from t h e b a s e o f t h e
S p e a r s Member and two welded t u f f s of t h e Hells Mesa Member.
Kottlowski, V!ebex. and Willard (1969)
.
l i s t e d Lt9 r a d i o m e t r i c
i n the
d a t e s of C r e t a c e o u s a n d T e r t i a r y i g n e o u s . r o c k s
New Mexico r e g i o n a n d s u m m a r i z e d t h e i r r e l a t i o n s h i p s t o
mineralizationepisodes,FiveprevS.ouslyunpubllshed
age
d e t e r m i n a t i o n s were l i s t e d by Weber (1971) f o r T e r t i a r y
i g n e o u s r o c k s f r o mc e n t r a l
New Mexico,Recently,Chapin
a date f o r t h e La Jam Peak Formation and
(7971) published
e v a l u a t e d its s i g n i f i c a n c e i n regard t o r a l n e r a l e x p l o r a t i o n .
E.1,
Smith and o t h e r s (197kr i n press) havedatedthe
A-L Peak and P o t a t o
Mountainsuslng
Canyon Formations i n t h e San Mateo
t h c f i s s i o n t r a c k method,
(1 973) have sumarized
I3ogollon-Datilprovince,
a l l I<-Ar dates a v a i l i i h l e f o r
the
The r o c k s d a t e d are i n t e r p r e t e d
t o r e p r e s e n t 3 s e p a r a t ev o l c a n i cc y c l e s .
u n i t o f ArroyoMontosa
E l s t o n a n do t h n r s
A new date on t h e
(SZnIon, 1973)provides
a narrow
b r a c k e t i n g f o r t h e onset o f b a s i n and. r m g e f a u l t i n g i n t h e
Magdalena area.
Acknomledeements
-
..
I g r a t e f u l l y acfr.nowledge t h e h e l p o f
who a s s i e t e d me d u r i n gt h ec o u r s e
the many i n d i v i d u a l s
of t h i s p r o j e c t ,
W.E,
Arnold
and N.J. Jaworslri provided eqaiproent' and
p r e p a r a t i o n of t h eg e o l o g i c
K+Ce Condieand
nap.
i d e a s used i n t h e
Budding, R,H.
A.J,
Weber,
W0H. I ' l i l k i n s o n f u r n i s h e d t h e a u t h o r . w i t h
helpful ide'as and information concerning the study during
,~
ply wife Louise was a great
i n f o r m a lo f f i c ed i s c u s s i o n s .
h e l pi nt h ep r e p a r a t i o no ft h em a n u s c r i p t .T h a n k s
go t o Pete Evans, a r a n c h e r i n t h e
permitted access through his
of Santa
also
thesis area, who g r a c i o u s l y
private l a n d s a n d V0Ee Walsh,
Fe, New Mexico, vrho p r o v i d e d i n f o r m a t i o n on t h e
magnetite nlineralization
a t t h e B i g John mine.
To Charles E. Chapin, my t h e s i s a d v i s o r ,
gratitude for
I owe s p e c i a l
the o r i g i n a l s u g g e s t i o n o f t h e s t u d y area and
f o r h i s . v z l u a b l e ad.vice and keen observations
a n d o f f i c e .F i n a n c i a la s s i s t a n c e
in both field
f o r f i e l d work was p r o v i d e d
...
through the New bkxico S t a t e Bureau ' o f Iti.nes and Miner&
Resources.
STRATIGRAPHY AND PEllROLOGY
Prevokanic
Pravolcanic sedimentary
t o t h en o r t h
rocks c r o p o u t a b o u t
and I mile t o t h es o u t ho f
To t h e n o r t h ,
1% m i l e s
t h e t h e s i s area.
. .
(9957) i n d i c a t e s t h a t t h e
mappingbyTonlring
age r e s t s conformably
base of the Spears Formation of Oligocene
on t h e BacaFormationofEoceneage.Fluvia.1sandstones,
mudstones and limestone-cobble conglomerates of 'the Baca
Forma'tion mere o b s e r v e d j u s t n o r t h o f
the s t u d y area.at t h e
Deep \!'ell Canyon by the writer and C.E.
headof
one mile s o u t h e a s t o f t h e T r e s M o n t o s a s s t o c k ,
Chapin.
About
Wilkinson
W.H.
( i n p r e p a r a t i o n ) has mapped a paleo-topographic 'high of
t!le
Ab0 Formation of Permian age surrounded by outcrops of the
Fornation.
'Spears
. ....
Similar n o r t h t o s o u t h s t r a t i g r a p h i c r e l a t i o n s h i p s
f r o m conforrnity t o unconformity have been observed
(1957) and Brovm (197%) i n t h e BearNountxinsand
by Tonking
i n the
n o r t h e r nH a g d a l e n aH o u n t a i n s .T h e s er e l a t i o n s h i p ss u g g e s t
that both the Council
Rock and the Bear Plountains
l o c a t e d on t h e n o r t h f l a n k o f
ageof
are
a pre-Oligocene u p l i f t .
The
t h e u p l i f t is Lararni.de ( l a t e C r e t a c e o u s t o m i d d l e
Eocene).
D.O.
Snyder (197'1 1, conducted a p a l e o c u r r e n t
d i r e c t i o n a n a l y s i s on e x p o s u r e s o f t h e B a c a F o r n a t i o n i n
Socorro and Catron Counties and concluded that
d e p o s i t e di na ne a s t - t r e n d i n gb a s i n .
a
In a d d i t i o n ,S n y d e r
suggestst.hatthemajorsourceofdetritius
areas t o t h e s o u t h a n d w e s t o f
i t was
t h e basin,
was f r o m p o s i t i v e
12
I n t h e l a t e Eocene, j u s t p r i o r t o the onsetofOligocene
New Mexico was eroded t o a s u r f a c e
volcanism, west-central
of moderately lorn r e l i e f ( C h a i i n ,
1973)
1971b;
.
Epis and Chapin,
Hence, t h e base of t h eO l i g o c e n ev o l c a n i c
'pile
v a r i e s from a c o n f o r m a b i e r e l a t i o n s h i p w i t h t h e E o c e n e ,
BacaFormation
in the east-trending
Laram2de.basi.n t o an
angular unconformity which p l a c e s t h e base of t h e Spears
Formation i n c o n t a c t w i t h p r o g r e s s i v e l y o l d e r r o c k s s o u t h v a r d
The t r a n s i t i o n from a conformable t o
towardTresMontosas.
u n c o n f o r n a b l e r e l a t i o n s h i p i s not exposed within t h e study
a r e a and cannot be a c c u r a t e l y de-termirvxl from available dataa,
Tert5.arx VolcaniL go?%%
Bodsockexposures
a s h - f l o wt u f f s
in the thesis area are chiefly
o f T e r t i a r ya g e .I n t e r b e d d e d
volcaniclastic sedimentary rocks
wevelded
lava flowsand
are minor i n o u t c r o p a r e a ,
Oligocene volcanic rocks and volcaniclastic rocks
d i v i d e d i n t o three formations vhich
are
a r e fxom o l d e s t t o
youngest, the Spears, I.Iel.18 Xesa.and A-L Peak FormatLons.
A maximum total thLclmess f o r these t h r e e f o r m a t i o n s i n
t h e C o u n c i l Rock d i s t r i c t i s e s t i c l a t e d t o be 4425 f e e t
(Fige
3 ) . About 400 f e e to fO l i g o c e n eb a s a l t i c - a n d e s i t e
f l o w s c a p t h e A-L Peak Formation i n t h e study area and a r e
t e n t a t i v e l y c o r r e l a t e d with t h e a n d e s i t e . of Landavaso
Reservoir
(Simon, 1973, Po 3 4 ) .
B r o m (4972) h a sp r e s e n t e d
d p t a i l e d d e s c r i p t i o n s and moclal a n a l y s e s of samples from
measuredsections
f o r most of' t h e s e saxe r o c k units i n t h e
southern Bear Kountains.
DESCRIPTION
purplish-gray,aphanitic,platy,flow-folded,
b a s a l t i c - a n d e s i t e f l o w at ba.se t o redd:Lsho r p h y r i t i c ,b l o c k y ,
flo\:.s o f a n d e s i t e
aIt?
O"*m
t o p a.ndesite o f bandavaso .Reservoir)
P
1-ight-gray t o w h i t e , s i m p l e c o o l i n g u n i t
densely t o moderatelywelded,moderately
c r y s t a l - r i c h , r h y o l i t e ash-f.:.xJ t u f f
of
.
reddish-brovm and purpl.ish-gray, compound
c o o l i n g unit of densely vrelded, c r y s t a l - g o o r
and flaggy .(so~netines pumice-rich) , 1 i t V - c p o o r ,r h y o l i t ea s h - f l o wt u f f
'
basalticd a r k gray, p o r p h y r i t i c ,( o l i v i n e )
a n d e s i t e flows a t base . t o reddish-brom,
p o r p h y r i t i c s' (pyroxene 1. p a r t i a l l y amygdaloidal.
basaltic-andesite flows at top
"I.-/
~
l i g h t - g r a y and massive t o f l a s g y arid purplishbrown,fluidal-banded,contorted,
compound
coolingunitofcrystal-poor,densely
\welded,
r h y o l i t e ash-flow t u f f s
\
p i n kt op u r p l i s h - g r a y ,m u l t i p l a - f l o v ,
simple
c o o l i n g v m i t ofdenselywelded,massive,
b l o c k y ,c l i f f - f o r . m i n g ,c r y s t a l - r i c h ,
qua:rtzs ' i c h , q u m t z l a t i t e t o rhyoliteash-flowtuffs
purplish-toreddish-gray,(greenish-gray
where
p r o p y l l t i x e d ) corapound c o o l i n gu n i t of p o r l y
t od e n s e l yw e l d e d ,c r u d e l yb e d d e d ,c r y s t a l r i c h ( l i t h i c - r i c h at b a s e ) , l a t i t e ash-flosr 'iu-ff
dark p u r p l e , a n d e s i t i c
mudflow b r e c c i a - s a-nd
conglomerates with minor reddish-brom,
a p h a n i t i ca n dp o f p h y r i t i c ,t r a c h y t i ca n d e s i t e
floitls(uppercontact
is g r a d a t i o n a l )
l i g h tp i n kt og r a y i s h - w h - i t e ,
cornpomnd c q o l i u g
u n i t ofpoorly t o noderatelyVelded,massive,
c r y s t a l - p o o r ,s c r a e t i w sl i t h i c - r i c h ,l a t i t e
(?)
a s h - f l o w t u f f s with nedium-gray, coarsely
p o r p h y r i t i c ,a m y g d a l o i d a la n d e s i t ef l o w s
at
base ( t u f f ofNippleKountain)
purpleandreddish-brown(greenish-gray
t;;Lere
p r o p y l i t i z e d )l a t i t i ca n dm d e s i t i cc o n g l o m e r a t e
and s a n d s t o n e s
?
Regional Unconformity
*
800 f o o t i n t e r v a l n o t
6 h om
1
I
I
,5P0'
S cf(aeilenet )
I
22Y0
14
Spears Fornation
named t h e t h i c k basal s e c t i o n o f l a $ i t i c
Tonking(1957)
q d ' l e s s volurninous
and a n d e s i t i c e p i c l a s t i c v o l c a n i c r o c k s
Member of t h e
interbedded ash-flow tuffs
and l a v a s t h e S p e a r s
Datil Formation,Rocks'of
t h i s t y p e mark .thebeginningof
volcanism i n many Oligocene volcanic provinces of the
Southern Rocky Mountainregion
Burkeand
others(1963)reported
(Lipman and others, 1970).
a dateof
for a latite tuff breccia collected
the Spears in the Joyita
of Council Rock.ReceutXy
37.1 m.y, ,(K-.Ar)
from t h e u p p e r p a r t
Hills a b o u t f o r t y miles n o r t h e a s t
Weber (1971 ) r a i s e d t h e
to formational status,
BrOVM
since subdivided the Spears Formatio.n,.into
member and an upper
Datil
i n .turn has
Formation t o group sta.tus and Chapin (1971a)
raised t h eS p e a r s
of
(1972) has
a lower e p i c l a s t i c
member o f v o l c a n i c r o c k s
and minor
interbedd.ed volcaniclastic sedimentary rocks.
Exposures of the upper
member o f t h e S p e a r s F o r m a t i o n
.indicate that i t is a continuous unit throughout the length
The lovrer member, i s o n l y p a r t i a l l y
o ft h es t u d ya r e a .
exposed i n t h e a r e a b u t
d r i l l hole data i n d i c a t e t h a t i t i s
a thick, continuous unit in the subsurface
Rock d i s t r i c t .
of' t h e C o u n c i l
The m a x i m u m total t h i c k n e s s 'o ft h eS p e a r s
Formation i n t h e d i s t r i c t
Lower Member.
_
I
_
o u t c r o p o f the lower
is e s t i m a t e d a t 2200 f e e t .
W i t h i nt h es t u d y
area a s i n g l e s m a l l
member,,of the Spears Formation
l o c a t e d j u s t s o u t h e a s t of theTres14ontosasstock,
is
It
c o n s i s t s o f d a r kp u r p l e ,a n d e s i t i c ,p e b b l ec o n g l o m e r a t e s
a n d ' a few t h i n i n t e r b e d s
o f purplemedium-grainedsar:iatones.
An e s t i m a t e o f t h e t h i c k n e s s a n d l i t h o l o g y o f t h e
lower member of .the SpearsFormation
the Council
i nt h es u b s u r f a c eo f
Rock d i s t r i c t c a n be rnade from s e v e r a l s o u r c e s
areas,
17hich d e s c r i b e t h i s u n i t i n n e a r b y
A t H e l l s %sa,
Tonking (1957, p. 5 6 ) measuredanddescriibed
section comprising the
.
, ,
1025 f e e t o f
lower member andwhich
consists of
-
quartz latite tuffs, breccias
andagglomeratesinterbedded
withvolcanicconglomerates,sandstonesandnumerousth.in
beds o f s i l t s t o c e andclaystone.
Brown (1972, p. 10)
d e s c r i b e s marked f a c i e s c h a n g e s i n t h e S p e a r s F o r m a t i o n
a l o n g t h e E e a r Nountains and northern Nagdalena Mouatains.
ILis o b s e r v a t i o n s may besumnsrizedby
1.
V o l c a n i c l a s t i cs e d i m e n t a r yr o c k so f
llleraber g e n e r a l l y c o a r s e n
2.
ts?o g e n e r a l i t i e s :
the lower
toward t h e s o u t h ,
Lava f l o w andash-flowtuffs"'oftheupper
generallythickentowardthesouth,
member
.
Approximately 1200 f e e t of t h e lower Spears Forination
was p e n e t r a t e d i n t h e
p r o s p e c t sI n
Banner d r i l l hole at ,theSix;ty Copper
this d r i l l h o l e , t h e lower Spews c o n s i s t s
t o finer
predoainantlyofcoarseconglomeratosgrad-ing
conglomerates.lomer i nt h es e c t i o n .I n t e r b e d d e ds a n d s t o n e s
and s i l t s t o n e s a r e more abundant near t h e b a s e o f t h e s e c t i o n .
A few minor l a t i t e flox:is were also observed i n the. sequence.
C r o s s s e c t i o n sp r e s e n t e d
byGivens
(1957, p l a t e I ) s u g g e s t
t h i c k n e s s o f about /.tOOOfeet f o r t h e t o t a l S p e a r s F o r m a t i o n
..
a t t h en o r t h e r nb a s e
i
of G a l l i n a sP I o u n t a i n s .T h i st h i c k n e s s
i s probably exaggerated
by u n r e c o g n i z e d t r a n s v e r s e f a u l t s
r e p e a t i n gt h es e c t i o n ,
A thiclrness o f 1200 f e e t f o r t h e
a
16
lowr member o f the Spears Formation,
as indi.cated frorn
t h e Banner d r i l l hole i s probably the
most r e a s o n a b l e
e s t i m a t e f o r thestudyarea.
Upper Ikrnber.Volcanicandinterbeddedepiclastic-
volcanicrocksoftheupperportion'oftheSpearsare
found i n t h r e e s e p a r a t e a r e a s a l o n g t h e e a s t b o u n d a r y
t h e s i sa r e a .
of the
The w r i t e r h a s s u b d i v i d e dt h e s er o c k si n t o
t h r e e map u n i t s ,I nt h eB e a r
Mountxiins, t h e r e s i s t a n t
or
o v e r l y i n g Bells X e s a F o r m a t i o n h a s c r e a t e d c l i f f s
escarpments i7hich .have shed a mantle o f t a l u s o v e r t h e
upper Spears making a rflappable s u b d i v i s i o n o f t h e u p p e r
member impossible.Gentleslopes
formation of major
Rock a r e a h a v e n o t a l l o w d . t h e
c o n e s ,a n dt h e s eu n i t sa r e
cormon i nt h eC o u n c i l
ell exposed.
talus
I na s c e n d i n g
..
as f o l l o w s :
stratigraphicposition,theunitsaredesignated
( 1 ) t u f f of NippleMountain
S p e a r se p i c l a s t i c
(Brovx, 1972, p. Ilk)
(2)upper'
r o c k s , and ( 3 ) c r y s t a l - r i c h l a t i t e t u f f s
of t h e u.pper Spears,
Tuffof
Itipple Mountain.
The t u f f ofRippleMountain
as d e s c r i b e d by Brovm (1972, p. 14) i s h e r e r e d e f i n e d t o
i n c l u d et h e" t u r k e yt r a c k "a n d e s i t e
p. 13) com~nonly found a t t h e b a s e
flows (Brovn, 1972,
of t h e t u f f
o f Nipple
Mountainash-flowsequence,Reasonsforthischangeare
the occurrence of andesite lavas which are interbedded in
theashflow
t u f f s that forn the
the comon stratigraphic
" t u r k e yt r a c k f r " lavas
the Hagdalena, Bear
mainbodyof
t h e u n i t and
and s p a t i a l a s s o c i a k i o n
ir/j.131
t h e s et u f f s .R e g i o n a l
of the
mapping n
i
and G a l l i n a s bioumtains h a s shown t h e .
..
17
to be a n e x c e l l e n t m a r k e r h o r i z o n
t u f f of Nipple Xountain
between%heupperandlovler
members o f t h e
Spears
Formation.
Outcrops of the tuff of Nippie I4ountain cover approximately
three-quartersof
a square mile i n t h e f o l l o w i n g t h r e e
s e p a r a t el o c a l i t i e s :e a s to ft h e" r e sM o n t o s a ss t o c k ,s o u t h
of Council Rock Arroyo and north of Gallinas Springs
Outcrops of the ash-flow portion.weather. to
platyandsandy
'.
Canyon.
a buff-colored
soil and t h e " t u r k e y t r n c k f l a n d e s i t e s f o r m .
a dark g r a ys o i lw i t hd - i s t i n c t i v ep o r p h y r i t i c
u n i t characteristically forms hogbacks
clasts.
Tho
where s t e e p l y i n c l i n e d .
The t u f f of Ripple Mountain crops out discontinously along
t h e east side of t h e G a l l i n a s u p l i f t over a d.istance o f
approximately 12 miles.
the Kelly mining
E a s t o f GraniteKountainund
in
d i s t r i c t ; , t h e t u f f o f Nipple H o m t a i n fills
. ..
channelscutinthelower
member o f t h e S p e a r s .
r e l a t i o n s h i p may b e r e s p o n s i b l e f o r
A similar
the d i s c o n t i n o u s n a t u r e
o f t h i s unit a l o n g t h e G a l l i n a s u p l i f t . N t e r n a t i v e l y t h i s
d i s c o n t i n u i t y may be s t r u c t u r a l l y d e r i v e d ,
The i f t u r k e y t r a c k " a n d e s i t e r a n g e s
from a thickness of
zero f e e t a t Gallinas S p r i n g s Canyon t o a p p r o x i m a t e l y 100
f e e t southofHontosaArroyo.
Montosasstock,
I n the v i c i n i t y o ft h e
Tres
i t containsamygdulesoutlinedbypistachio-
green e p i d o t e a n d f i l l e d w i t h e u h e d r a l b r o m a n d r a d i t e garnet.
Metamorphism of t h i s r o c k u n i t
will be d i s c u s s e d
later i n a section concerning the Tres Hontosas stock.
E a s t o ft h eT r e s
andesite varies
Montosas s t o c k , t h e " t u r k e y t r a c k "
from a blue-gray color, mottled with white
plagioclasephenocrysts,to
a denseblackrockwithfresh,
18
as r e f l e c t i o n s a l o n g c l e a v a g e
clear plagioclase visible only
planes.
t o the c o n t a c t
The l a t t e rv a r i a t i o no c c u r s . a d . j a c e n t
One o t h e r small t r i a n g u l a r - s h a p e d- o u t c r o p
with the s t o c k ,
mile
i s found about 'one half
of the 'Iturkey track" andesite
west-northwestoftheCouncil
Here again, i t
Rock Ranch.
i s a blue-graycolor,buttheamygdulesconsist
o f white
c a l c i t eg r a d i n go u t w a r dt op i n kh e m a t i t i cc a l c i t e ,
Sn t h i n
s e c t i o n , t h i s r o c k c o n t a i n s from 15 t o . 2 0 p e r c e n t p h e n o c r y s t s
o fp l a g i o c l a s e ,
Ang3 (Fouqe'method,one
g r a i n ) ,a v e r a g i n g
approximately 4 rn i n l e n g t h i n a groundmass of c a l c i c
andesine (Michel-Levymethod,
20 me.asurements).Both
phenocrysts and groundmass p l a g i o c l a s e are o f t e n p a r t i a l l y
altered to low-birefringent clay minerals.
flows were observed
Cooling breaks occupied by andesite
.
n e a r t h e middle o'f t h e t u f f
ofNippleMountain
in t h e B a n n e r
i n a hole
d r i l l h o l e a t t h e "Sixty Copper prospect" and
on t h e n o r t h e a s t s i d e o f t h e T r e s M o n t o s a s s t o c k
Bear Creek Wining Company.
d r i l l e d by
Thesebreaks,together
with
v a r i a t i o n s i n abundance of . l i t h i c fragments and pulnice
i n d i c a t e that the ash-flow sequence of tuff of
Nipple
Mountain i s a multiple-flow compound c o o l i n g u n i t .
flow portion of the tuff
o.f Nipple Kountain
The ash-
is estimated to
have' a t h i c k n e s s of 400 f e e t .
P u r p l e a n d brokn a n d e s i t i c l i t h i c fragments c o n t a i n i n g .
w h i t ea r g i L l i z e dp l a g i o c l a s ep h e n o c r y s t s
a t t h et o pa n db a s e
are f a i r l y abundant
o f t h e t u f f s (Fig. 4).
a d e v i t r i f i e d matrix cf poorly welded
0.2 t o 0.3 mm i n l e n g t h , w i t h
In t h i ns e c t i o n ,
glass s h a r d s a v e r a g i n g
a few unbrokenoval-shaped
3
figure 4.
Outcrop of t h e t u f f ofKip2leEiountah.Note
t h e p r e s e n c e of t y p i c a l Spears lit!xicfragments
of p u r p l e2 o r p h y r i t i ca n d e s i t e .
The f13.$aT
f r a c t u r e s n d l i g h t gray c o l o r s?e c o m o n .to this
u n i t i n tLeCouncil Rock a r e a .
20
85 p e r c e n t o f t h e r o c k s .I n
glassbubbles,comprises'about
fills the
most s e c t i o n s , v e r y f i n e - g r a i n e d c h e r t y q u a r t z
and a h i g h - b i r e f r i n g e n t c l a y m i n e r a l ,
remaining pore space
p r o b a b i g i l l i - t e 7 commonly fills c a v i t i e s i n u n c o l l a p s e d .
pumice.
T h i s c h e r t y ceiaent i s p r o b a b l yt h ec o n t r o l l i n g
of
factor in the resistant, massive vieathering character
this unit.Disseminated
opaque d u s tw i t h i nt h ea x i o l i t i c -
d e v i t r i f i e d g l a s s , s h a r d s makes t h e n r e a d i l y d i s c e r n i b l e f r o m
t h ec l e a rs i l i c e o u s
t o havebeen
cement.Thisash-flowsequenceappears
a p r e f e r e n t i a l channelway for hydrothermal
o f its h i g h
solutions and(or)meteoric maters, because
initial permeability.
C r y s t a l s n o r m a l l y make up l e s s t h a n two t o t h r e e p e r c e n t
o f t h e t u f f o f Nipple Mount:l.in,
Small
m i c r o p e r t h i cs a n i d i n e
.. .;.
.
crystals containing exsolved strings
and patches
polysynthetically tvinned albite(?) appear to
be approximately
e q u a l i n volume t o th3.t o f a l t e r e d p l a g i o c l a s e ,
of a singleplagioclasephenocryst
I n many t h i n s e c t i o n s ,
was
.An,
J7
of
The composition
..
(Pouque'methoci)
however, b o t h f e l d s p a r s
are completely
a l t e r e d t o a gray opaque c l a y .
Thinsections
from dense gray blocky-jointed outcrops
of the tuff of Nipple Mountain
where i t ' a b u t s t h e Tres
Montosas stock are
a felsic,allotriomorphic-
composedof
g r a n u l a r ,v e r y - f i n eg r a i n e d
(0.05 t o 0.05 m
a
i n diameter)
matrix with a few small g r a i n s o f m a g n e t i t e w i t h h a l o e s
b i o t i t e and a fer1 c o m p l e t e l y a r g i l l i z e d p h e n o c r y s t s o f
feldspar.
The v i t r o - c l a s t i c n a t u r e
completely erased
o f t h i s r o c k hasbeen
by h o r n f e l s i c r e c r y s t a l l i z a t i o n ,
of
'
21
rocks
Upper S p e a r s E p i c l a s t i c Rocks.Outcropsofthese
consist primarily
of crudely bedded well-indurated
mud-flo:rr
b r e c c i a s w i t h no r e c o g n i z a b l e i n t e r n a l s t r a t i f i c a t i o n
i m b r i c a t i o n of l i i t h i c fragments.
:A few thinreddish-brovm
flows a r e
a p h a n i t i c ,a n ds o m e t i m e sp o r p h y r i t i c ,a n d e s i t e
a t Montosa and Council
interbeded with the laharic breccias
,RockArroyos,
A t G a l l i n a s S p r i n g s ,Canyon, a n d e s i t e
flOTiS
a r e n e a r l y e q u a l t o them
cap the epiclastic breccias and
i n volume.
or
Lit1xi.c fragments i n t h e mud-flow breccias are
vrell-rounded t o sub-angular and vary from pebbles
t o boulders
of a n d e s i t e axd l a t i t e (Fig, 5).
Thesem~1dfloi~/sand andesite flovis crop out .ra,ther poorly
,
compared t o t h e t u f f of Nipple Nountain, o r t h e d e n s e l y
I.relded c r y s t a l - r i c h l a t i t e t u f f s i m m e d i a t e l y a b o v e
..
...
them,
a dark reddish-brown t o
The e p i c l a s t i c r o c k s norma.llyform
p u r p l e s o i l containing abundant well-rounded, c'obbles
pebbles of l a t i t e s and a n d e s i t e s ,
Theonly
exception t o this
w e a t h e r i n g h a b i t i s found i n the baked periphery
I4ontosas stock where rounded clasts
soil,In
this area, S p e a r sr o c k s
the mafic border facies of
of lithic fragments
and
o f the Tres
do n o t w e a t h e r l n t o
may be d i s t i n g u i s h e d from
t h e s t o c k onlywhere vague o u t l i n e s
c2.n be'observed.
Although the upper Spears
epiclastic rocks could vary
significantly in thickness by nature of their origin,
appsars that the relatively
thicknessof
i n a relatively
vihich v a r i e s from 130 t o 200 f e e t .
369 € e e t
$01'
it
smooth erosion surface developed
on t h e t u f f of Nipple Mountain resulted
uniformthic!mess
the
this unit in the
A
Bear Creek d r i l l
n
Figure 5,
3
Outcropof massive mudflom b r e c c i a s o f t h e
upper member o f the Spears Formation i n
G a l l i n a sS n r i n g s Canyon. A greenish-black
p r o p y l i t i z e d n a f i c d i k e , a b o u tf i v ef e e t
thick,cutstheS2earsepiclasticrocks
on
t h e l e f t s i d e o ft h ep r i n t .
hole may be exaggerated by a lLigigh-angle r e v e r s e f a u l t
( C r o s ss e c t i o n
Pl, 1).
%"I,
The u p p e rc o n t a c to ft h e s e
r o c k s w i t h the o v e r l y i n g c r y s t a l - r i c h l i t h i c - r i c h l a t i t e ,
ash f1w.w o f t e n c o n s i s t s o f
markedby
o f white f e l d s p a rp h e n o c r y s t si n
a ni n c r e a s i n ga b u n d a n c e
.
a gradational change
t h e matrix of a l i t h i c - r i c h f l o x of mud and ash.
in
Upper Specam C r y s t a l - r i c h L a t i t e T u f f s .V a r i a t i o n s
composition, grain s i z e a n d d e g r e e o f v e l d i n g i n d i c a t e t h a t
a u u l t i p l e - f l o w compound c o o l i n g
t h e s ea s h - f l o wt u f f sa r e
'
unit.
Lateral t h i c k n e s s variati0n.s i n this uppermostportion
o f t h e S p a r s are normallyminore
occurs i n t h e v i c i n i t y
One n o t a b l ee x c e p t i o n
ofGallinasSprixgs
Canyon where
aporoxinately half of the section appears to
a p p a r e n t l y due t o non-+position of the
ash flows i n t h e v i c i n i t y o f
Estinatesofthethickness
be missing,
lower l i t h - i c - r i c h
a pa1e.o-topographichigh.
o f t h e s e t u f f s range from a
ninimum o f 100 f e e t t o a maximum of 300 f e e t ,
'
Outcrops of t h e basal poorly vrelded l i t h i c - r i c h a s h
I
flow o f t h i s u n i t at Council Rock a r r o y o are cormonly pale
greenish-graydue
t o pervasive propylitic alteration.
E'lsevrhere, f r e s h handspecimens
gray colors spckled
vri.th abundantchalky-rrhi'tephenocrysts
o f s a n i d i n ea n dp l a g i o c l a s e
'
Quartz i s
are c h a r a c t e r i z e d by p u r p l i s b -
i
n approximately equal p r o p o r t i o n s .
g e n e r a l l ya b s e n te x c e p ti nt h et r a n s i t i o n
t o the
ash f l o u t u f f s o f t h e o v e r l y i n g I b l l s lhsa li'ormation.
e
Copper-colored, b i o t i t e i s common i n d e u t e r i c a l l y a l t e r e d
.
zones near the top
of the crystal-rich
latite tuffs.
.
24
Crystalsgenerallycomprise
11.0
t o 50 percent of these roclrs.
Bells Mesa
The u p p e r c o n t a c t o f t h e s e t u f f s w i t h t h e
Formation is q u i t e s u b t l o .
A tMnhematite-stained'conglomerate
(Brown, 1972, p. 17) s e p a r a t e s th.e S p e a r s from the Reells
Hesa i n the Bear Nountains; unfortunately, the conglomerate
i s m i s s i n g i n the Council Rock d i s t r i c t ,C o n s e q u e n t l y ,
forriiationboundary
i s p l a c e d a t t h o first appearance of
abundant quartz phenocrysts,
wMch approximately concides
t o welding
with a consistent morphological change related
characteristics.
ashflowsof
the'
The moderatelytodenselywelded
latite
the Spears o f t e n h a v e a c r u d e l y bedded appearance,
weather t o roundedoutcrops,andform
an e q u i g r a n u l a r s o i l .
On theotherhand,outcropsofthedenselywelded
IIells
Mesa t u f f s have a massive bloclry-jointed- ..appearance and form
large a n g u l a r b l o c k s
of talus.
Outcrops differ greatly in their appearance in the
hornfels zone around the Tres Montosas stock from their
e q u i v a l e n t so u t s i d et h i sz o n e .L i t h i c - p o o rc r y s t a l - r i c h
l a t i t e t u f f s of the upper
contact with
part of t h e S p e a r s are found i n
the east b o r d e r o f t h e s t o c k n e a r P i n e
Viell,
I n t h i s area h o r n f e l s e d c r y s t a l - r i c h l a t i t e t u f f s a r e d a r k gray massive blocky- jointed rocks which strongly resemble
monzoniteborderfaciesrocksfoundalongthewestand
southboundaryof
t h e stoclr.
hornfelsed tuffs have
0.f
these
a riaxy-gray matrix c o n t a i n i n g f i n e -
grained clear plagioclase,
a b u n d a n tc l o t s
Hand specimensof
white p o t a s h feldspar and
of m e t a l l i cb l a c km a g n e t i t e .
andesiticlithicfragmentsoutlined
The o b s e r v a t i o n
by p o t a s h ( ? ) f e l d s p a r
25
r e a c t i o n rim i n some outcrops mas t h e o n l y . f i e l d i n d i c a t i o n s
t o the Spears Formation rather
that these outcrops belonged
t h i n section, a h o r n f e l s e dl a t i t e
t h a nt ot h es t o c k .I n
t u f f sample consisted
of about 20 percent brolien phenocrysts
o f p o t a s h f e l d s p a r and n e a r l y eqv.al amounts of plagioclase
f e l d s p a r (An- Fouque' method,averageofthreemeasurements)
.
32'
i n a matrix o f e q u i g r a n u l a r , v e r y f i n e - g r a i n e d p o t a s h f e l d s p a r .
a n d clinopyroxene.
a seriatetextureare
Broken c r y s t a l s a n d
its pyroclastic origin.
primary textures rernaining to reveal
latite t u f f s
P e t r o g r a p h i c a l l y ,s a m p l e so fc r y s t a l - r i c h
from t h e v i c i n i t y o f Council Rock arroyo vary widely
t h e i rd e g r e e
in
of a l t e r a t i o n , which i s g e n e r a l l yi n v e r s e l y
p r o p o r t i o n a l t o thedegree
.
a t t h eb a s e
o f welding,Tuffs
are poorly melded and.. a. r g i l l i c a l t e r a t i o n of
both plagioclase and sanidine to
a g r a y opaque c l a y i s n e a r l y
of the section
,
c o m p l e t e .T u f f sh i g h e ri nt h es e c t i o nu s u a l l ye x h i b i t
varyingdegreesofpropyliticreplacement
c a l c i t e andreplacementofbiotite
colored biotites
of s a n i d i n e by
by c h l o r i t e .
Copper-
near t h e t o p o f t h e s e q u e n c e a p p a r e n t l y
formed by o x i d a t i o n p r o c e s s e s r e l a t e d
t o welding,
Conpositional variations in these tuffs appear
minor.Plagioclaseranges'
t o be
i n composition from AnZ8 t o An33
(Fouque' method) f o r six p h e n o c r y s t s i n 3 d i f f e r e n t t h i n
s e c t i o n s .S a n i d i n e
i s s l i g h t l y m o r ea b u n d a n tt h a np l a g i o c l a s e ,
and together these crystals
make up ab0v.t 40 i e r ' c e n t of t h e
..
r o c k .F e l d s p a rp h e n o c r y s t sv a r yi nl e n g t h
3 m.
'
Mafic minerals normally
fromab0u.t
1 to .
make up less t h a n 1 0 p e r c e n t
of t h e r o c k and, i n o r d e r of decreasing abundance, include
26
r c a g n e t i t e ,b i o t i t ea n d
of densely
'minorclinopyroxene.
The groundmass
velded t u f f s i n t h i s s e r i e s c o n s i s t s o f r e d d i s h -
brovrn, p a r t i a l l y t o c o m p l e . t e l y d e v i t r i f i e d
homogenous glass.
Quartzphenocrystsarenotentirelyabsentfromtheupper
t u f f s n e a r their gradational contact with
t h e Hells Nesa,
b u t t h e y . a r e much smaller i n s i z e and s p a r s e i n abundance.
Hells Mesa Formation
By agreement among g e o l o g i s t s p r e s e n t l y w o r k i n g i n
northeast Datil-Wogollon volcanic
f i e l d , t h e Hells Mesa
Formation has been restricted to'the
m e a s u r e ds e c t i o n
Nesa ?<ember o ft h e
the
basal u n i t of Tonking's
(1957, p. 5 6 ) ofwhathe
Datil Formation.
c a l l e d t h e Hells
In i t s new r e s t r i c t e d
s e n s e , the Hells Nesa i s a quartz-rich densely-welded
m u l t i p l e - f l o n simple' c o o l i n g u n i t o f q u a r t z l a t i t e t o r h y o l i t e
ash-flowtuffs,Olderterminologyapplied
o f t h i s formation a r e :
r h y o l i t ep o r p h y r y
and Koschmann, 1911.2,'p. 3 3 ) , a n d t u f f
1972, p. 19).
t oe q u i v a l e n t s
sill (Loughlin
o f Goat S p r i n g (Brown,
base of this
B i o t i t e from o u t c r o p sn e a rt h e
f o r x a t i o n a t H e l l s Iksa has been d a t e d by t h e IC-Ar method
as
30.6 2 2.8 m,y,('Veber
32.1 may, and 32.14 m.y.
a n dB a s s e t t ,
1963).
(Burke a n do t h e r s ,
r i c h t u f f s i n theGallinasXountainsand
correlated with the Hells
o t h e r d a t e d u n i t s ,G e o l o g i c
Dates of
1963) f o r c r y s t a l
t h e J o y i t a Hills
Mesa are i n b e t t e r agreement with
mapping i n t h eG a l l i n a s ,B e a r ,
Magdalena,SanXateoandLeiitarHountains,hasshowthe
I I e l l s Mesa Forlaation t o be a mappable u n i t o f r e g i o n a l
importance.
The Hells &sa F o r n a t i o n - t y p i c a l l y f o r m s s t e e p c l i f f s
i n t h eb l o c k - f a u l t e da n dt i l t e d
Bear 1-fountains, The Hells
Mesa a l s o f o r m m o s t of t h e s t e e p s l o p e s o f
Tres Montosas,
where 5-t. i s -e s t i m a t e d t o be about GOO f e e t t h i c k .
Hosrever,
t a l u s masks uuch of t h e l o w e r s l o p e s o f Tres Montosas malring
A t Galliias S p r i n g s a rflaximum
this e s t i m a t et e n a t i v e .
thiclrnessof
1200 f e e t was o r i g ; i n a l l y e s t i m a t e d
Hells Mesa,
F u r t h e ro b s e r v a t i o ni n d i c a t e d
for the
that at l e a s t
two t r a n s v e r s e f a u l t s may r e p e a t p o r t i o n s o f t h e s e c t i o n . .
A
a minimum t h i c h e s s h e r e o f
second calculation indicated
800 f e e t which i s considered by the w r i t e r t o be a r e a s o n a b l e
estimateof
i t s t r u et h i c k n e s s
study area, the
at t h i s l o c a l i t y ,
Hells Mesa Formation crops out
In t h e
as low rounded
hills w i t h a few s t e e p s l o p e s formed only where cut
by
..,.
major d r a i n a g e s
T h i s ' unusual geonorphic expression
D'
for
t h e Hells I4esa may be r e l a t e d t o s t r u c t u r a l c o m p l e x i t i e s
or
p o s s i b l y t o abrupt changes i n t h i c k n e s s f r o m Vest t o
East intheCcuncil
Bock a r e a ,
The l a t t e r conclus-ion i s
p r e f e r r e d by t h e m i t e r s i n c e a maimunl' t h i c k n e s s o f 503
. f e e t is indicated for the
justeast
Hells I k s a i n a v e r t i c a l s e c t i o n
o f theTresLiontosasstocli,
T h i s t o p i c will be
discussed in greater detai2. in the structural section
of
this r e p o r t .
The Bells Kesa Formation crops out
i n a semi-continuous
manner over apgroximately .six square miles i n t h e a r e a of
..
investigation.
The bestexposures
canyons of the Council
OCCUP
i ns t e e p - w a l l e d
Rock and Ga1.lina.s S p r i n g s d r a i n a g e s
Irlhere the massive homoseneous r e s i s t a n t n a t u r e
o ft h e
28
forma-tion i s b e s tr e v e a l e d ,
Pumice o f t e nw e a t h e r s
d i f f e r e n t i a l l y from some outcrops leaving bekind '!crescentlike!! cavities, convex-upmard at the t o p . a n d f l a t on t h e
bottom,
However, pumice i s o f t e na b s e n t
weldedoutcropsandattitudes
from t h e s ed e n s e l y
of c o m p a c t i o n f o l i a t i o n
are
oftend%fficulttodetermine.
B r o a d ,g e n t l ys l o p i n gs u r f a c e sd e v e l o p e d
on this u n i t
i n the Council Rock area are g e n e r a l l y mantled b y . l a r g e
s i x i n c h e s t o one f o o t i n
angular blocks averaging from
l e n g t h .I n t e r s t i c e sb e t w e e nb l o c k s
are commonly f i l l e d by
buff c o l o r e d s a n d ,
.
Freshhandspecimensofthe
Bells Nesa t u f f s e x h i b i t
v a r y i n g h u e s o f medium t o 1iS'ht-purplish-gray.plish~grayl i t h o i d a l
matrix comprising about 50 t o 60 p e r c e n t o f t h e
are normallyshades
therock,\'/eatheredsurfaces
o r gray.
P l a g i o c l a s eo f t e nh a s
s a n i d i n e i s nearlyalwaysmilky
feldsparcrystals
3 mm t o
0.4
KT;
shovr
o f brown
a greenish-gray cast and
of
amd white.Fragments
a continuum o f s i z e s fromabout
snaller g r a i n s t e n d t o
as c l o t s i n t h a matrix.
volume o f
becrowded
together
crystals,
Clearsub-equantquartz
as much as 4. oln i n diameter, commonly compriseabout
t o 10 p e r c e n t of t h er o c k .O u t c r o p sn e a rt h e
formation, however,. contain
top of t h e
as much as I5 percent quartzs.
Black t o copper-colored ' b i o t i t e is t h e o n l y o t h e r
recognized phenocryst
commonly
.)
The upper contact zone
o f t h e Hells Hesa Formation is
often bleached by hydrothermal solutions
groundwaters,
5
The c o n t a c to ft h e
o r p o s k i b l y by
Hells Ibfesa Formation s i t 1 1
.
.
t h e o v e r l y i n g A-L Peak Fornation i s not exposed
i&
along
inthestudyarea.Usuallythecontact
~ ~ u v i a t edrainage
d
channels.
anyvrhere
small
A f r i a b l eb e n t o n i t el a y e r
about t y o f e e t t h i c k i s exposed i n a r o a d c u t at t h i s
c o n t a c t on Gray IJLill, f o u r m i l e s t o t h e s o u t h
area (I'lilkins&,
purple lithic fragments
g e n e r a l l ya b s e n t .
of Hells Mesa
i np r e p a r a t i o n ) ,O u t c r o p s
axe uniformly.anddensely
melded,
of t h e t h e s i s
Pumice a n da l j h a n i t i c
'
'
,
become a b u n d a n t l o c a l l y b u t a r e
o f t h eH e l l s
Some o u t c r o p s, n e a rt h eb a s e
Mesa c o n t a i n 10 p e r c e n t o r more l i t h i c fragments,These
observations indicate that the Hells
&sa Formation i s a
multiple-flow simple c o o l i n g u n i t ,
T h i n s e c t i o n s shovr t h a t r o c k s i n t h e
€reIells Mesa Formation
a l l have a p o r p h y r i t i ca n ds e r i a t et e x t u r e ,E x p l o s i v e
p u l v e r i z a t i o n and (or) i n t r a - f l o w a < & c i o n r e s u l t e d i n
conminution of t h ep h e n o c r y s t s .B o t hs a n i d i n ea n dp l a g i o c l a s e
e x h i b i t rninor degrees of d e u t e r i c a l t e r a t i o n
is g e n e r a l l y ' t h e least a l t e r e d of
clay minerals; sanidine
i s o f t e nm i c r o p e r t h i t i c ,c o n t a i n i n g
t h ep a i r .S a n i d i n e
p a t c h ye x s o l u t i o n s
t o low-birefringent
of albite-.tvLinnedplagioclase
Some
s a n i d i n e s have undergone patchy replacement by calcite
which may be r e p l a c i n g e x s a l v e d p l a g i o c l a s e ; h o w e v e r ,
intermediate stages of plagioclase replacement confirming
this were notobserved.In
most t h i n s e c t i o n s ,s a n i d i n e
i s a b o u t t v i c e as ,abundant as p l a g i o c l a s e and normally
comprises over 20 p e r c e n t o'f t h e t o t a l roc!< volume,Subhedral
quartz crystals often
have'inwardly mushroomingembagments
f i l l e d v i t h brown g l a s s .
A few p e r c e n t o f b i o t i t e . i s common
b i o t i t e may v a r y from a
i n r o c k s of t h e Iklls ;.lesa.The
a type strongly
pale green slightly pleochroic variety to
o f clinopyroxene
p l e o c h r o i c i n reds andbrolms.Traces
and hornblende
may be a c c i d e n t a l x e n o c r y s t s i n c o r p o r a t e d
from v e n t . r o c k s o r from t h e u n d e r l y i n g e r o s i o n s u r f a c e .
Densely welded gla'ss
i n t h e groun&%ass.o f t h e R e l l s
Mesa i s reddish-brown t o l i g h t brovm i n c o l o r .
is normallydevitrified;and
small s p h e r u l i t e s of r a d i a l l y
d i s p o s e d c r i s t o b a l i t e and p o t a s h f e l d s p a r
ma.trix, commonly o u t l i n ef u s e dg l a s s
shards,
arestronglymappedaroundphenocrysts
one type of
are cornmona
a r e abundant i n t h e
Dust-like inclusions of iron oxide which
compaction.Only
The g l a s s
Glass s h a r d s
due t o d i f f e r e n t i a l .
l i t h i c fragment is cormon
. t o pebble t;ized,
i n t h e H e l l s Hesa Formation. These granule
. ..
reddish-brovnlithicfrasmentsareporphyriticaphanitic
a n d e s i t e s composed of a r g i l l i z e d f e l d s p a r and d i s i n t e g r a t e d '
m a f i cm i n e r a l si n
a brormgroundmass,
The l i t h i c f r a g m e n t s
a.re v e r y lil.:eLg derived from the Spears Formation.
F i e l d and l a b r a t o r y o b s e r v a t i o n s s u g g e s t
vertical coapositional-variation in the
a consistent,
I l e l l s Mesa Formation,
Data p r e s e n t e d by Brown ( 1 972, p. 26-28, Table 1 , f i g u r e s
7 and 8) i n d i c a t e an upward overall i n c r e a s e i n t h e q u a r - t d
f e l d s p a r ratio.
Chemicalanalyses
o r a l c ormun. ) of two samples: 1.1-24-23 and X-24-33
measured section
(1972,
by DealandRhodes
o f Brown (1972, p. 2 7 ) i n d i c a t e a n
from t h e
upwerd
d e c r e a s e ' i n CaO f r o m 0,537; t o 0,277 and,unexpectedly,
s l i g h td e c r e a s ei n
a
Na20 i n t h e sane d i r e c t i . o n .T h ew r i t e r
has measured the anokthite content of
20 p l a g i o c l a s e c r y s t a l s
31
~i.thi11
tvro t h i n s e c t i o n s fron t h e
(Rittmgnnzonemethod)
Hells Hesa Fornlation i n t h e s t u d y
areao Sample 1-16 was
c o l l e c t e d a p p r o x i m t e l y 30 f e e t above t h e basal c o n t a c t o f
.
t h e Heils Ibiesa,
.
The r a n g e o f a n o r t h i t e c o n t e n t - f o r t e n
this sample i s An
p l a g i o c l a s ec r y s t a l si n
2-5 t o An32 w i t h
Anz8., I n Sample1-17,
an averagevalueof
approximately 40 f e e t below t h e t o p o f
collected
the FIells Mesa, . t e n
f r o m An22 t o
plagioclase crystals ranged in coinposition
I n addition,theaverage
An25 .and averaged
An
angleofsanidine
(measured. on t h e u n i v e r s a l s t a g e ] d e c r e a s e d
23
upviards from a 2Vx=460 (averageof
o f '5)
I ng e n e r a l
w i t hd e c r m s i n g
t h e 2V o f
1017
i?a content(Troger,
4) t o
a 2Vx=41"
(average
s a n i d i n e becomes s m a l l e r
19.56, p.
961, Howver,
o f A1 c a t i o n s is
anotherimportant
..
thedegreeofordering
2-V of s a n i d i n e .
factornllichcofitrolsthe
These observations
suggest a t r e n d o p p o s i t e o f
t y p i c a l l y c i t e d i n the. l i t e r a t u r e ( L i p a a n
Ratte'andSteven,
axial
that
and others, 1966;
1964; Smith and Bailey, 1966)
Llpman
(ope c i t . p. 18) s t a t e s t h a t t h e t y p i c a l c o m p o s i t i o n a l t r e n d
in thick ash-flow sheets
is from a c r y s t a l - p o o r r h y o l i t e
at
the base t o a more c r y s t a l - r i c h q u a r t z l a t i t e at the top.
The Hells Xesa a p p e m s t o .have a $rendfrom
latite t o rhyolite.
Brown (1972, p. 7 4 )h a sp o s t u l a t e dt h a t
fissures initially tapped
level, whichallowed
a zoned magma chamber at a
more mafic magma t o b e e r u p t e d
followed by i n c r e a s i n g l ys i 1 i c e o u . s
Would requ.ire more
latite to quartz
SiliCCOUS
of the v e n t i n g f i s s u r e s
magma,
1017
first
T h i sp r o c e s s
magma t o move down t o t h e Level
as t h e magua chamber was d r a i n e d ,
The r e s u l t
\70Uld
be compositionally zoned ash-flow sheet
whose zonation i s t h e r e v e r s e
of t h a t commonly observed,
A-L PeakFormation
.
.
Tke A-L Ped< Formation i s t h e f o r m a l name proposed
by Deal and Rhodes (1974) f o r a 2 0 0 0 - f o o t - t ~ ~ c k s e c t i o n
of ho!nogenous, densely-welded
, crystal-poor , rhyolite ash-
f l o w t u f f s exposed on t h e n o r t h e a s t
f l a n k of A-L Peak i n
Ma.teo Mountains,Deal.andRhodes(1974)
thenorthernSan
o f a ma.jor resu&pnt
have ' a l s o d e l i n e a t e d t h e b o u n d a r i e s
cauldron (20-2.5 m i l e s i n d i a m e t e r ) c e n t e r e d a b o u t
Mt,
\ t i t h i n g t o n , vr'nich i s l o c a t e d about 20 miles south of Council
They b e l i e v e t h e Mt, I'iithingtoncauldron- i s t h e
Rock.
s o u r c e area f o r t h e A-L Peak Formation and
a younger sequence
newly named t h e P o t a t o Canyon Rhyolite.
.. ..
Stratigraphic and petrologic equivalents
Peak Formation have been previously described
named i n e a x l i e r i n w s t i g a t i o n s .
o f t h e A-L
and i n f o r m a l l y
The f o r m a t i o nc o r r e l a t e s
r;rith t h e "banded r h y o l i t e I i o f Loughlin ancl Koschmann (1942),
middle 65 feet
of thetypesection
o f Tonkingrs 1Tell.s Hesa
(Brown,
Member (Todiing, 19.57), a n d t h e t u f f o f B e a r S p r i n g s
1972, p. 311, E.I.
Smith, and others(1974)havedated
A-L Pea.!< R h y o l i t e a t 31 -82 1.7 m,y.
the
using the fission track
method.
Outsidethe
Itt. Witkington cauldron,' the
Rhyolite grades laterally into
A-L Peak
a compositesheet
(R,L.
Smith,
1960, p. 158) i n which r h y o l i t e c r y s t a l - p o o r a s h - f l o w t u f f s
are i n t e r b e d d e d w i t h t h i n c r y s t a l - r i c h , q u a r t z 1 . a t i t e a s h -
33
fl0M
flovrs (Chapinand
t u f f s and b a s a l t i c - a n d e s i t e l a v a
r e a s o n i t i s r e f e r r e d t o here
F O ~
this
others,,inpreparation),
as t h e A-L Peak Formation rather
as rimed by Deal andRhodes,
than A-L Peak R h y o l i t e
The A-L Peak Formationhas
mappable u n i t s i n t h e C o u n c i l
been subdivided into four
Rock a r e a . T h r e e a s h - f l o w c o o l i n g u n i t s a r e r e c o g n i z e d ,
.
of whichthelowerandmiddlecoolingunitarequitosimi1a.r
p e t r o l o g i c a l l y bu$ a r e s e p a r a t e d by i n t e r v e n i n g b a s a l t i c andesite flows.
i s a s i m p l ec o o l i n g
The u p p e rc o o k i n gu n i t
u n i t of m o d e r a t e l y c r y s t a l - r i c h r h y o l i t e .
Lower Cooling
I__
m.
The loivest subdi.vision of the
o a t overaboutthree
A-L PeakFornationcrops
square nliles o ft h et h e s i s
area.o
It is exposedsemicontinously
a l o n g a .north-tre.ndingzone,aboutone
&royoHontosa
t o G a l l i n a s Canyon.
a b s e n ti na na r e an e a rC o u n c i l
. ..
mile wide, Prom
It is, hovever,noticeably
Rock,
Coincidentwith
its
i s a zone of c o n c e n t r a t e d f z u l t i n g
northerlyoutcroptrend
a n d h y d r o t h e r n a la l t e r a t i o n ,
A-L Pedr crops out
agd one h a l f
For t h i s reason.,thelovrer
as low rounded hills w h i c h w e a t h e r r e a d i l y
t o a s o i l of reddish-brosm and v r h i t e p l a t y
fragments of small
size.
A second area o f vide' exposure of these lower
t u f f s 'is found along
an e a s k r e s t t r e n d i n
c o r n e r of t h et h e s i sa r e a ,
A-L Pea!!
the n o r t h w e s t c
A t this l o c a l i t y , t h e i r c h a r a c t e r
is r e m a r k a b l y s i n i l a r t o t h a t d e s c r i b e d
by Brown (1972, p. 31 1
in t h e Bear I.Iountains.
The lower cooling
u n i t of t h e A-L Peak R h y o l i t e i s a
multiple-flow compound-cool-ing u n i t cornprisedof
two
.
34
r e c o g n i z a b l e members i n t h e s t u d y a r e a .
The basal nenber
c o n s i s t s of approximately 350 f e e t of l i g h t - p u r p l i s h - g r a y ,
6 )e
n a s s i v e ,c r y s t a l - p o o rr h y o l i t e( F i g -
Its massive
nature appears t o be r e l a t e d t o a s c a r c i t y o f ' pumice and a
moderate t o low degree.of welding,
VIelded t o t h e lover member i s the purplish-brown
member (Brown,
densely
velded,
fluidal-banded,
contorted
1972, p. 38)* Its t h i c k n e s s is e s t i m a t e d a t 250 f e e t .I n
t h eu p p e r
t h i r d o f thecontorted
warped t o form f l o w f o l d s .
f o l i a t i o nh a sb e e n
foldsoftenoccur
member, the compaction
The flow-
as f l a t - b o t t o m e d s y n c l i n e s a n d t i g h t
o:t about two f e e t a n d
V-shaped a n t i c l i n e s vrith amplitudes
n a v e l e n g t h s of aSout 10 f e e t (Figc 7)* Axial p l a n e s o f t h e s e
s t r i k e from
asymmetricfoldsdiptothenorthAdrangein
.. ...
IT 85' Ll' t o S 83'
L'j
(8 measurements).
r e g i o n a l d i p i s removedby
However, when t h e
rotation to the horizontal the
axial plzmes o f t h e s e f o l d s a
1 dip t o the south at m g l e s
a v e r a g i n g about 80 degrees.Schmincke
and Swvanson (1967,
similar s t r u c t u r e s i n w e l d e d a s h -
p. 656)havedescribed
f l o v t u f f s o f t h e Gran CanariaIsland.
thenoregent,le
sourcewad.
They observed that
limb of t h e f o l d s c o n s i s t e n t l y d i p p e d
Zrovrn's d a t a . (1 972 , p.
observations in regard
78) a a d t h e m i t e r s
t o the orientation
of t h e s e s t r u c t u r e s
are consistent with the proposed source
of t h e A-L Peak
Formation i n t h e SnnI4ateoHountains.
The o r i e n t a t i o n a n d
repeated occurrence
of t h e s e w i d e s p r e a d s t r u c t u r e s a r g u e s
f o r a pri:naryoriginoBecausethe
i t s e l f i s deformed,development
compaction f o l i a t i o n
o f t h e f l o w folds must
..
Figure 6.
Outcrop o f t h e basal gray-massive a m b e r of t h e
A-L Peak Formationnear Gallinas S s r i n g s . Columar
j o i n t i n g is r e a d i l y a p p a r e n t and a p p r o x i x a t e l y
a t r i g h t angles t o t h e cordpaction f o l i a t i o n .
36
~
f l o w had' l a r g e l y compacted and began
have occurred after the
t o weld,but
mhile t h e i n a t e r i a l
rianner (Loriel1andChapin,
Y
sSt i l l moving i n a lCminar .
~
1972)
Elongated pumice also f o r m a p r i m a r y l i n e a t i o n i n t h e
',
l o v e r part o ft h ec o n t o r t e d
Trends from f o u r o u t c r o p s
member.
v a r i e d from N 8 " W t o N 3" E, approx5mately a t r i g h t angles. . .
t o t h e axial p l a n e s of nearbyflom
probably formed by differential
t h e same t i m e t h e
folds,
Thepumice
lineation
at
movement w i t h i n t h e t u f f
flow f o l d s forned,
Megascopically, colors range
from l i g h t g r a y a n d
purplish gay in moderately velded portions
of the sequence
t o 'dark chocolate b r o m where densely~velded,.Phenocrysts
average about 1 .O 1:m i n l e n g t h andcomprisegenerally
than the average of approximately
. ....
10 percent phenocrysts
phase,followed
.
(Brown, 1972, p. 35)
observed i n t h e s o u t h e r n B e a r M o u n t a i n s
C h a l k yn h i t es a n i d i n e
.
Thris i s n o t a b l y less
o f therock.
t h a na b o u tf i v ep e r c e n t
less
( 0 0 5 - 2 8 0 I ~ Q ) i s thedominantrdneral.
by smoky q u a r t z (less than 1 .O mm) and t r a c e s
many s p e c i n e m s u b h e d r a l
of p l a g i o c l a s e a n d . b i o t i t e , I n
sanidine crystals appear
t o havebeen
"honey-combrl l i k ec r y s t a l s .
l e a c h e d t o form c e l l u l a r
c h a r a c t e r i s t i c a l l y s c a r c e in thegraymassioe
Petrographically,there
However,
member,
t o i 5 p e r c e n t pumiceand
the contorted unit my contain up
about 3 percentaphanitic,
are
Pumice and l i t h i c f r a p e n t s
.
b r o m a n d e s i t e l i t h i c fragments,
is l i t t l e d i f f e r e n c e i n
mineralogy of t h e s e ash f l o w .
t!re
E u t a x i t i cs t r u c t u r e i s
~
e v i d e n t from s u b - p a r a l l e l e l o n g a t e
glass s h a r d s a n d f r o 2
f l a t t e n e d pu:nice vhic!? ,is o f t e n d i f f e r e n t i a l l y
compacted
..
a o u n d c. r y s t a l s
Of
top
and l i t h i c fragments.
Pumice,
near
the
u n i t hasbeencompletelyreplaced
t h ec o n t o r t e d
c r y s t a l s of p o t a s h f e l d s p a r
anhedralquartz
37
by bladed
.
.
e o u n d t h e r i m vri.th equigranular,
crystals (0.05 t o 0.3 nun) f i l i i n g t h e c e n t e r .
The grou.ndmass o f t h e s e ' t u f f s i s c o n s i s t e n t l y d e v i t r i f i e d
as a x i o l i t e s
to very fine-$rained quartz and alkali feldspar
Tmo t ot h r e ep e r c e n to ft h eg r o u n d m a s s .
anduxorientedmasseso
in the massive
member i s h a i r l i k e h e m a t i t e c r y s t a l s ( l e s s
t h a n .0.1 mm long;).
A much g r e a t e r c o n c e n t r a t i o n o f v e r y
f i n ed i s s e m i n a t e dh e m a t i t e
(15 p e r c e n t ) i n t h e m a t r i x
contorted u n i t i s apparently the major reason
coior.
The g r e a t e r' d e g r e e
of the
f o r its d a r k e r
o f welding i n t h e c o n t o r t e d
member i s a l s o probably important
i
n this color change.
S a n i d i n e i s t h e most abundant phenocryst throughout the
..
l o w e rc o s l i n gu n i t ,
..
It i s e s t h t a t e d . t o be a b o u t 4 t o 5 t i m e s
as abundant as quartz and range
from 4 , t o 7 percent by volume,
Nearly all s a n i d i n e appears t o have been p e r t h i t i c , b u t
a l t e r a t i o n o f theexsolvedpotassicphasetosericite
and
i l l i t e ( ? ) masks the t r u er e l a t i o n s h i p ,S e c o n d a r yp o t a s h
f e l d s p a r prod?Jced during devitrification and
phase processes has
clay.
i n some i n s t a n c e s also b e e n a l t e r e d t o
f l u i d s , o r ground
T h i s f a c ts u g g e s t st h a th y d r o t h e r m a l
w a t e r s ,a r et h e
f r o m 'vapor
cause of a r g i l l i z a t i o n ,
c l a y s from t h e ' s a n i d i n e c r y s t a l s
Renloval o f t h e s e
i s t h ep r o b 2 b l eo r i g i n
for
t h el l h o n e y - c o n b ' tt e x t u r ed e s c r i b e de a r l i e r .
Quartzcrystals
are u s u a l l y a n h e d r a l a n d
more abundant i n the gray massive
member.
are s l i g h t l y
member t h a n i n t h e c o n t o r t e d
Eovrever, q u a r t z novel- exceeds one p e r c e n t by volume
'
38
was n o to b s e r v e d . i na n ys p e c i n e n s
i ne i t h e r .P l a g i o c l a s e
f r o m t h e 1o;Ter c o o l i n g u n i t ,
Interbedded
I n tlost o f t h e . s t u d y&ea
AndesiAeA.
thin'
f l o w s of de&. r e d d i s h - b r o w n a p h a n i t i c b a s a l t i c a n d e s i t e
separate t h e lovier a
d middle cooling units of the
A-L Peak
o f t u f f a c e o u ss a n d s t o n ea n dg r a n u l e s t o n e
Formation,Thinbeds
o c c u r l o c a l l y where t h e s e lavas t h i n o v e r a s t r u c t u r a l l y
.
'
The thiclrness i s h i g h l y v a r i a b l e
c o n t r o l l e dt o p o g r a p h i cU g h ,
r a n g i n g from 0 t o 200 f e e t .
t o a dark-brown clayey soil
Outcrops generally weather
a t the base
found along gentle slopes
hol?ovs b e t w e no u t c r o p so f
o f r i d g e s or as small
welded t u f f ,
The, b..e s te x p o s u r e s
are found i n small t r i b u t a r y d r a i n a g e s a l o n g t h e s o u t h s i d e
o f G a l l l n a s Canyon (Fige 8 ) ,
massivealthough
The a n d e s i t e s are g e n e r a l l y
..
.
sone a r e - a u t o b r e c c i a t e d .V e s i c u l a r
h o r i z o n s a r e r a r e l ye x p o s e d ,
b u t where f o u n d t h e v e s i c l e s
a r e n e a r l y a l l f i l l e d y.6th c h e r t y q u a r t z and may contain
some bl.ue--green c e l a d o n i t e
Iiand specimens are u s u a l l y a p h a a i t i c - p o r p l l y r i t i c
a l t h o u g h , a t first g l a n c e , t h e f i n e g r a i n s i z e . ( a p p r o x i m a t e l y
0.5 rm) anddarkcolorof
t o be overlool;ede
most
phenocrysts may causethem
Flows n e a rt h eb a s e
of t h e s e r i e s t e n d t o
be darker a r d more abundant i n ferromagnesian minerals.
. f i n e - g r a l n e dr e d d i s h - b r o w ni d d i n g s i t e( a f t e ro l i v i n e )
and
b l a c k pyroxene may comprise as much a.s 25 p e r c e n t by volume
of t h e s el a v a s .P h e n o c r y s t so fp l a g i o c l a s e
becane .abundant near the
(0.5-2.0
mn l o n g )
to12 o f this l a v a f l o w sequence.
Determination of e x t i n c t i o n a n g l e s ' f o r p l a g i o c l a s e
ll-0
phenocrysts and groundmass m i c r o l i t e s s u g g e s t e d t h a t t h e s e
lavas a r e b a s a l t i c a n d e s i t e s ,
15 p e r c e n t o l i v i n e
of t h i s u n i t c o n t a i n e d
a u g i t e <2Vx= 53" 3
4O
phenocrysts (0.5-1.5
A samoletaken
, ZAG= 44" ?:
mra').
and 10 p e r c e n t
20) as f i n e g r a i n e d .
*Olivineoccurs
euhedral hexagonal forms rimed
f r o m t h eb a s e
as subliedraland
by r e d d i s h - b r o w n i d d i n g s i t e ,
is
A n g i t o r i t es o m e t i m e so c c u r sa l o n gf r a c t u r e s .A u g i t e
p r e s e n t as f r e s h , . n e u t r a l - g r a y c r y s t a l s ( i n t h i n s e c t i o n )
that , a r e s m a l l e r t h a n t h e o l i v i n e p h e n o c r y s t s a n d
corhrnonly
l a b r a d o r i t e m i c r o l i t e s (An
glomeroporphyritic.Calcic
64'
average o f 5 d e t e r m i n a t i o n s by Rittmannzonemethod)
are
t h e dominant phase i n t h e groundmass and compriseabout
p e r c e n t o f t h er o c k s
volume.
augiteandmagnetite
form the remaining groundmass.
40
Approximatelyequalvolumesof
.. ...
Less mafic lavas, which f o r m t h e major volurne o f t h e
sequence,haveplagioclase
phenocrystsand
matrix,
as t h e dominantphase
P l a g i o c l a s ec r y s t a l s
zoned; one phenocrystrangedfrom
An
are normally
51 t o An,+G (Fouque'
m e t h o d ) .I ng e n e r a lp l a g i o c l a s ep h e n o c r y s t s
labradorite vhile
in both
are s o d i c
groundmassplagioclasemicrolitesare
calcicandesine.Augitephenocrysts
a n d t r a c e so fo l i v i n e
a r e a l s o p r e s e n t malring as much as 15 p e r c e n t o f t h e r o c k s
volume.
Small outcrops o f t u f f a c e o u s s a n d s t o n e a n d g r a n u l e s t o n e
occuradjacentto
a north-trendingfaultwhichtruncatesthe
basalticandesitesabout
Springs.
one-hal:f m i l e s o u t h
of Gallinas
Clasts i nt h eg r a n u l e s t o n ea r ep r e d o m i n a n t l y
d e r i v e d f r o m the gray massive
member of t h e lower c o o l i - . ;
47
c l a s t s derived f r o m t h ec o n t o r t e d
unit.Denselywelded
member a r e also p r e s e n t i n m a l l amounts ( l e s s t h a n 3
percent) e
Fragments are sub-angular t o sub-rounded 'and v a r y
from t l p e et of i v em i l l i m e t e r si nl e n g t h ,
"honey-combedft s a n i d i n e c r y s t a l s
Sub-rounded
and r n i c r o c r y s t a l l i n e c h e r t y
A tuffaceous
cementconprisetheremabderoftherock,
f i n e - t o medium-grainedsandstonewhichoverliesthe
'
.
g r a n u l e s t o n e i s c h a l k y - w h i t ea n dt h i n l yl a m i n a t e d ;I n
decreasingorderofabundancesand-sizedtuffclasts,.angular
are cenented by c h e r t y
qutirtz grains and perthitic sanidine
q u a r t z t o form t h i s rock.
'
Thinningofthebasaltic-andesitesaccompanied
by t h e
occurrenceoftheselocallyderivedsedimentsapproaching
the north-trending fault suggests the
minimuma.ge
of the
fault
. ..
t o be la.te .Oligocene (slightly younger than the
Peal< Formation).Thisconclusion
lower A-L
is confirmed by t h e
occurrence o f a welded f a u l t c o n t a c t b e t m e n t h e g r a y m a s s i v e
member a d t h e c o n t o r t e d nember about two hundred yards
dong the strike
of the fault
sandstoneoutcrop
t o t h e n o r t h of t h e t u f f a c e o u s
(Figo17, this r e p o r t ) ,.
f a u l t c o n t a c t imppies t h a t t h e f a u l t
The term%eldedfl
formedwhile
thecontorted
u n i t vas s t i l l h o t enough :to r e w e l d i t s e l f t o t h e f o v i e r
emplaced i n c o n t a c t u i t h i t across t h e f a u l t ,
member a f t e r b e i n g
Middle Cool5.ng
u,
The m i d d l ec o o l i n gu n i to ft h e
A-L Peds Formation i s a uniform multiple-flow sequence of
c r y s t a l - p o o rw l d e dr h y o l i t ea s h - f l o wt u f f s ,
Its e s t i m a t e d
maximun t h i c k e s s i s f i v e h u n d r e d f e e t . , E x p o s u r e s a r e
m o s t l yr e s t r i c t e d
t o a four-square-milearea
of r o l l i n g
.
.
quadrant of tilestudy
hills i n the nor';hmest
of d i s t i n c t z o n e s
.
42
areao The l a c k
a l o w d i p t o these t u f f s
o f welding and
are major c o n t r o l s i n its topograpllicexpression.
The lower
a topographic break, ivhich may be
contact normally follows
a t t r i b u t e d t o the r e l a t i v e l y s o f t n a t u r e o f t h e u n d e r l y i n g
andesites o r p o s s i b l y an unwelded basal zone i n t h e ash-flows,
The c h a r a c t e r i s t i c o c c u r r e n c e
z o n ew i t h i n
of a . s p h e r u l i t i c densely melded .
ten f e e t of t h e base
t h i n unmeldedzone,
i f presente
would n e c e s s i t a t e a very
A similar s p h e r u l i t e zone
i n t h e Bear Mountains ( B r o ~ m , 1972, p. 4 2 ) .
has been noted
t h e Iniddle c o o l i n g u n i t from densely
Hand specimensof
melded zones are u s u a l l g l i g h t p u r p l i s h - g r a y
and weather t o a l i g h t brovri,
associated ~
A few reddish-bro;:mzones
are
t moderately
h
to poor*ly welded p o r t i o n s o f the
..
of subhcdral., cha1.l.i~white s a n i d i n e
sequencePhenocrysts
(0.5-3.0
on . f r e s h s u r f a c e s
n m ) appear t o be s l i g h t l y more abundant than i n the
Lorver c o o l i n gu n i t .
The s a n i d i n e - q u a r t zr a t i op r o g r e s s i v e l y
inckeases upmrds in the unit
as a l s o . does t h e t o t & ' c r y s t a l
five
c o n t e n t ,E s t i m a t e so fp h e n o c r y s tc o n t e n tr a n g ef r o m
percent crystals at thebase
top.Pumice
t o abouteightpercent
i s mostabundantnear'the
middle of the section
where i t i s e s t i m a t e d t o bB f i v e t o ' t e n p e r c e n t
L i t h i cf r a g m e n t s
by volume.
are c o n s i s t e n t l y
2re rare andwherefoundthey
small (less t h a n 2.0 mm).
at the
They c o n s i s t o f a p h a n i t i c a n d e s i t e
and make up less t h a n one percent of the rock.
In thin section the
to densely welded
groundmass consists o f moderately
glass. shal>ds.and pumice.
A l l vitric
material has c r y s t a l . l i z e d t o f i n e g r a i n e d q u a r t z
and alkali
43
f e l d s p a r (0,1-0,5
Ghosts of g l a s ss h a r d s
mu)
under hi& magnificatlon only because
d u s t ( l e s st h a n
of v e r y f i n e h e m a t i t e
-005 m) which o n t l i n e s them.
u s u a l l yh a s p r o d u c e d
are v i s i b l e
Devitrification
a r,andom o r i e n t a t i o n o f t h eq u a r t z o -
. .
feldspathic in-tergrowth, although axiolites and spherulites
were also observed,
F r e s h . s a n i d i n ep h e n o c r y s t s
section.
.
were r & e l yo b s e r v e di nt l s i n
H o s t are c h a l k y w h i t e a n d a l t e r e d t o
a. lovr-birefringent
c l a y which is probably a p r o d u c t o f d e u t e r i c a l t e r a t i o n .
This
conclusion i s s u b s k n t i a t o d by t h e p e r s i s t e n t , w i d e s p r e a d
occurrenceofthealteration,
'
S a n i d i n e is t h e most abundantphenocryst,comprising
much as 5 p e r c e n t byvolume.Neasurements
f o r six s m i d i n e p h e n o c r y s t s , i n
as
o f axial a n g l e s
a s i n g l e s e c t i o n from t h e
f r o m 2Vx= 26' 2" t o 30" ?: 2".
m i d d l eo ft h eu n i t sv a r i e d
Smallalbi.te-twinnedplagiocl.asecrystalsareoftenenveloped
v z i t h i nl a r g ee u h e d r a ls a n i d i n e s .
The plagrioclase i s anoma1ousl;r
f r e s h compared t o t h e s a x i d i n e , e v e n
c r y s t a l s , E x t i n c t i o n.-
when p r e s e n t as Lnclividual.
angles and 2V measurements f o r t h e
i t is hightemperaturealbite
plagipclase ind.icate that
to
sodicoligoclase.Anorthitevaluesforthreemeasureruents
ranged ' f r o 3 s e v e n t o
eleveli percent.
The 2V of one c r y s t a l
vas measured as 2Vx= 48'
2" t o e l i m i n a t ea m b i g u i t i e si n v o l v e d
f o r e x t i n c t i o n a n g l e s ( X f A (010)) of l e s s t h a n 14" on t h e R i t t n a n n
zonecurves,
that determined
The a n o r t h i t ec o n t e n td e t e r m i n e d
by e x t i n c t i o n a n g l e d i f f e r e d
a n o r t h i t e .P l a g i o c l . a s e
by 2 V and
b~ one p e r c e n t
v m p r e s e n t i n a l l of s e v e nt h i n
s e c t i o n s from t h e nlAdd.le
c o o l i n gu n i t ;
however, e s t i m a t e s d i d
44
n o te x c e e d
one percentby
volume.
Tracesofreddish-brown
b i o t i t e were n o t e d i n a few sections,Subhedralandangular
brokenquartzcrystalscomprise
r o c k s from t h em i d d l c u n i t .Q u a r t z
' t o threepercentof
fromone
becomes s l i g h t l y more
abundanttowardthetopofthemiddlecoolingunit,
-
CoolTns Unit,
.
The youngestofthe
A-L Peak ash-
t o densely
flow t u f f s i s a s i m p l e c o o l i n g u n i t o f p o o r l y
ash-flow t u f f .
vrelded, m o d e r a t e l yc r y s t a l - r i c hr h y o l i t e
It
is exposed a t two small outcrops i n the northwest quadrant
The larger a n db e t t e re x p o s u r eo c c u r s
of the t h e s i s a r e a .
as a narrow n o r t h - t r e n d i n g s t r i p a b o u t
one mile i n l e n g t h
in,thevicinityofSouth
Welle A v a r i a b l e e s t i m a t e d t h i ' c k n e s s
t o 125 feet i s attributed.toerosionoftheupper
of 75
poorlymeldedzone
flow.
prior t o extrusion of the overlying lava
A t South Well, .where a l l b u t bhe p o o r l y vrelded basal
zone i s e x p o s e d , t h e u p p e r t u f f c r o p s o u t
roundedbloclry
w e a t h e r st o
as a s e r i e s o f
ledges from whichcompacted
form p l a t e l i k e vugs.
purrice e a s i l y
The s t r a t i g r a p h i c p o s i t i o n
and p e t r o l o g y o f t h i s u n i t s u g g e s t s t h a t
i t nay b e e q u i v a l e n t
t o the lower P o t a t o ' Canyon R h y o l i t e (Dea.1 and Rhodes,1974).
Hand s p e c i ~ n e n sof t h e u p p e r c o o l i n g u n i t
t o lightpinkishgray
are l i g h t gray
on a fresh s u r f a c e .W e a t h e r e ds u r f a c e s
a r e commonly l i g h t t a n o r may be sta.ined
by i r o n o x i d e l e a c h e d
dark r e d d i s h b r o w
from t h eo v e r l y i n ga n d e s i t e s .
The t u f f
c o n t a i n s 1 0 t o 20 p e r c e n t p h e n o c r y s t s w i t h q u a r t z s l i g h t l y
more abundantthansanidinewhich
Lithicfragments
i s sometimeschatoyant.
( 0 - 3 t o 1 .O cm) ofaphaniticreddish-brown
andesite and light-pink
.
welded t u f f a r e common throughout
..
c .
45
the unit and
may comprise 3 o e r c e n t o f t h e r o c k s
Pumice is mostabundant
vo1u:ne.
i n the upper half of the unit
where
i t r a n g e s from about 2 t o 3 cm i n l e n g t h .a n d. makes Up 10 t o
15 p e r c e n t o f t h e r o c k s
volume.
P e t r o g r a p h i ca n a l y s i so fs a n i d i n ea n dp l a g i o c l a s e
u t i l i z i n g a u n i v e r s a l stage s u g g e s t s a s l i g h t l y more mafic
c o m p o s i t i o nf o rt h eu p p e rc o o l i n gu n i ti nc o m p a r i s o nt ot h e
m i d d l ec o o l i n gu n f t .S a n i d i n e
c r y s t a l s )a n d
2Vx= 36"' ( f i v e
has anaverage
ranged from 32" t o 39".
o f a densely welded sample
t o . b e 11 p e r c e n tq u a r t z ,
7 p e r c e n ts a n i d i n e ,
plagioclase, a traceofbiotite,
zonemethod).
was estima,ted
1 percent
3 p e r c e n tl i t h i cf r a g m e n t s
and 10 p e r c e n t pumice i n a f i n e l y d e v i t r i f i e d groundmass.
C r p t a l s range i n s i z e from 0.2 n m fd.4.0 mm with an average
ofabout
1.0 mm,
On t h e a v e r a g e , s a n i d i n e
is o n l ys l i g h t l y
l a r g e r than q u a r t z but b o t h are s i g n i f i c a n t l y l a r g e r t h a n
p l a g i o c l a s ew
, h i c ha v e r a g e s
0.4 mm i nl e n g t h .
,
Andzsite of Landavaso Reservoir
in
The youngest of theOligocenevolcanicrocksexposed
t h e s t u d y a r e a . i s a s e r i e s of t h i n p u r p l i s h - g r a y
brovrn b a s a l t i c - a n d e s i t e f l o w
t o reddish-
(map designation: T a ) *
These
l a v a f l o w are e q u i v a l e n t i n s t r a t i g r a p h i c p o s i t i o n a n d o f
similar p e t r o l o g y t o t h e a n d e s i t e o f L a n d a v a s o R e s e r v o i r
mapped bySimon
(1973) a l o n g t h e e a s t s i d e o f t h e M u l l i g a n
Gulchgraben.Reconnissancetraversestothe
t h e s i s a r e a , i n thevicinityofLion
_.
The averagecomposition
of p l a g i o c l a s e i s Anl2 ( t h r e e c r y s t a l s , R i t t m a n n
The modalmineralogy
. .
west o ft h e
Houn'cain, i n d i c a t e . t h a t
..
46
these basaltic andesites
are o v e r l a i n by a t h i c k s e q u e n c e o f
"moonstonet1 ( c h a t o y a n ts a n i d i n e )m o d e r a t e l yc r y s t a l - r i c h
r h y o l i t ea s h - f l o wt u f f s .
wvelded
'
These tu:ffs are most l i k e l y e q u i v a l e n t ,
i n p a t , t o ' t h e P o t a t o Canyon Formation which also overlies the
Reservoir .at LandavasoReservoir.
a n d e s i t eo fL a d a v a s o
P o t a t o Canyon B'nyolite has been dated by
using the fission track
The
(1974.) ,
Smith a n d o t h e r s
method, a t 30s3 2 1.6.
C
I
~
;
~
.
~
~
Exposures of , t h e a n d e s i t e o f L a n d m a s o R e s e r v o i r c o n s i s t
of
area of approsimately
platy. t o massive lavas which cover an
one s q u a r e mile i n t h e n o r t h w s t q u a d r a n t
ofthethesis
areao
The estimated maximum t f i c l m e s s of t h e s e f l o w s i s 400 f e e t -
The l a r g e s t continous exposure i s centered around a topographi.-,
high, hill It7914" which i s i n t e r p r e t e d a s t h e s o u r c e v e n t
these l a v a s . T h i s v e n t
for
area i s here rimed t h e Gallinas S p r i n g s
. ...
i n t r u s i v ec e n t e r .N e a r l yv e r t i c a lf l o wf o l i a t i o no b s e r v e d
this h i l l t o p and a concentricarrangementof
on
f l o a f o l d axes
about t h e h i g h are t h e basis o $ . t h i s i n t e r p r e t a t i o n (fig" 9).
tks u n i t c h a r a c t e r i s t i c a l l y w e a t h e r
Tile'basal. 1ava.sof
l a r g e p l a t e - l i k e fragrflentsfrom
s i x i n c h e s t o one f o o t i n
l e n g t ha n da v e r a g i n ga b o u to n e - h a l fi n c hi nt h i c l m e s s .
platy fracture
The
of t h e s e r o c k s is a t t r i b u t e d t o t h e a c c u m d a t i o n
of vola.tiles along closely spaced
duringflovage.Lzvas
from the v e n t as
vfell
half of the andesite
shear p l a n e s d.eveloped
become m a s s i v ew i t hi n c r e a s i n gd i s t a n c e
as upward i n t h e s e c t i o n .
The upper
o f Lanclavaso R e s e r v o i r t e n d s t o
be
r e d d i s h broun i n c o l o r and blocky i n o u t c r o p c h m a c t e r .
Handspeciaens of the platy
andcontain
to
lavas are phenocryst poor
less than t h r e e p e r c e n t
o f pinkLsh-urhite
.
47
Figure 9.
Exposure of a l a r g e p l u n g i n g s y n c l i n a l flovr-fold
i n the basal p l a t y b a s a l t i c - a n d e s i t e florrs o f t h e
a n d e s i t e of LandavasoReservoir.Forscale,the
dog s t a n d s about one f o o t i n height.Notethe
abundance of l a r g e p l a t y f r a g n e n t s w h i c h a r e
t y p i c a l 0 2 t h e basal f l o m . The outcrop i s l o c a t e d
a p p r o x i n a t e l y t;vo miles southwest of G a l l i n a s
Springs.
48
plagioclase laths ranging
from 2 , O t o
11.0
ma i n l e n g t h .
A fern greenish-blackstubbypyroxenephenocrysts(average
l e n g t h a p p r o x i m a t e l y 1,O mm) may also b e . p r e s e n t i n specimens
of t h e p l a t y
flows.
Incomparlson,the
d i s t j n c t l yp o r p h y r i t i ca n d
51.Qcky flows are
may c o n t a i n from 10 t o 20 p e r c e n t
'
p l a g i o c l a s e l a t h s ( as much as 7.0 mm l o n g ) a n d a few
p e r c e n t ofpyroxene.Drusyquartzandblue-greenceladonite
.
may occur as a c a v i t y f i l l i n g i n v e s i c u l a r z o n e s .
-Petrographic analysis of the platy lavas
t o b eb a s a l t i ca n d e s i t e s .
The l a r g e rp h e n o c r y s t s
a r e 'commonly zoned i n a normaland
*
showed' them
t o An
The
o s c i l l a t i o r y manner.
composition of one s t r o n g l y zoned phenocryst -ranged
An70 i n t h e c o r e
of p l a g i o c l a s e
from
46 i n its' n a r r o w ,s h a r p l yd e f i n e d
rim.
M i c r o l i t e s o f p l a g i o c l a s e i n t h e same' r o c k v a r i e d - i n c o m p o s i t i o n
from An
45
t o An,,
>+
and averaged
by Rittmann Zone method).
An
51 ( f i v ed' e t e r m i n a t i o n s
this s m p l e
The groundmassof
f r o m t h e p l a t y f l o w s c o n s i s t s o f approximately 70 p e r c e n t
t r a c h y t i cp l a $ i o c l a s e ,
15 p e r c e n t i n t e r s e r t a l m a g n e t i t e c u b e s
and 1.5 percentstubbyneutral-brownclinopyroxene.This
sane
clinopyroxene a l s o occurs as glomeroporphyritic phenocrysts
and was determined t o be d i o p s i d i c a u g i t e
(2V
Z
= 55' 2 2',
znc= 39- ) "
Siztilax a n a l y s e s o f t h e b l o c k y l a v a s i n d i c a t e d t h a t t h e y
axe p r e d o m i n a n t l ya n d e s i t e s .E v ep l a g i o c l a s ep h e n o c r y s t si n
a sample of t h eb l o c k y
averaged An
l a v a s ranged from An
45 (Rittmann zone nethod).
42
t o Anb8 and
A z o n e dp l a g i o c l a s e
in t h i s sane s m p l e vas determined t o have a core of An
and a rim of An,
26
52
(Fouque' method).ClinopJroxenephenocrysts
49
ZAC=
43" )
.
49 a t -. 2 " ,
Plows were determined t o be aug;ite (2V,=
intheblocky
These r e l a t i o n s h i p s s u g g e s t , t h a t t h e b l o c k y
of t h e b a s a l t i p - a n d e s i t e
andesites are differentiates
magma.
varies ,from dark-brovm
The groundmass of' t h e b l o c k y a n d e s i t e s
glass to subtrachytic and felty plagioclase microlites vrith
minor nasnetite and clinopyroxene.
T e
r m Intrusive
m!~
MaficDikes
Two d i s t i n c t g r o u p s o f m a f i c d i k e s a r e r e c o g n i z e d
in the
s t u d y a r e a on t h e basis o f p e t r o l o g ya n dr e l a t i v ea g e ,D i k e s
Bear N o u n t a i n s a r e
of'similar occurrence and petrology'in the
.
d e s c r i b e d by Brown (1972, p. 55) andLoughlinand
( o r a n d e s i t e ) and l m p r o p h y r e
(1942, p. 4 3 ) *B a s a l t i ca n d e s i t e
on t h e g e o l o g i c map
dikes comprise the older group desihated
as
Tmd.
of o l i v i n e basalt dikes
The youngergroupconsists
flows capping
which cut pediment gravels and are the source of
t h e s eg r a v e l s ,
Koscfmann
i n a later
T h i s group *vi11bediscussed
basalt o f Council l?oclr,
sectionundertheheading,
Five individual mafic dikes belonging to the older 'group
were mapped.
of
Three o f t h e s e a r e a n d e s i t e s
q u i t e similar compositionand.tex-tures
interbedded within the
t o l a v a flows
A-L Peak Formation and
of LandavasoReservoir,There
t o the andesite
i s c o m p e l l i n ge v i d e n c ef o r
. l a t eO l i g o c e n ea g ef o rt h el o n g e s t
cropping out about
o r b a s a l t i ca n d e s i t e s
of t h e s ed i k e s ,T h i sd i k e , .
13 miles southwest
o f Gallinas S p r i n g s ,
i s remarkably similar i n t e x t u r e and composition
a n d e s i t e s from t h e a n d e s i t e
a
t o blocky
o f LandavasoReservoir.
It i s
.
50
nile from t h e p r o p o s e d v o l c a n i c v e n t ,
less thanone-half
5 percent)
Horeover it i s s i g n i f i c a n t l yv e s i c u l a r( a b o u t
level of e r o s i o n i s v e r y n e a r
suggesting that the present
t o t h a t of thegroundsurface
when t h e d i k e
d e s c r i b e d as l m p r o p h y r e s i n t h a t t h e y a r e
m i n e r a l (Figo 5, p. 22 of t h i s r e p o r t ) .
both lamprophyres are about two-thirds
movement a l o n g t h e
dark a p h a n i t i c
o f a ferromagnesian
rockscontainingsparsephenocrysts
on t h e n o r t h s i d e
was emplacedi
area a r e b e s t
The two o t h e r m a f i c d i k e s o b s e r v e d i n t h e
G a l l i n a sS p r i n g s
'.
The o u t c r o p s o f
,
o f a m i l e e a s t ' of
o f t h ea r r o y o ,R e c u r r i n g
f a u l t zone i n t o which they were injected
has i n t e n s e l y f r a c t u r e d t h e d i k e s a n d p e r m i t t e d s t r o n g
p r o p y l i t i c a l t e r a t i o n by h y d r o t h e r m a l s o l u t i o n s ,
The l a r q r o p h y r e s a r e g r o u p e d v r i t h t h e a n d e s i t e d i k e s
on t h e basis of similar composition.
lamprophyres consists of
p a r t i a l l yr e p l a c e d
'The groundmass of t h e
70 p e r c e n t s u b - t r a c h y t i c p l a g i o c l a s e ,
by c a l c i t e a1d quartz.Theremainder
is
comprised o f a n i n t e r s t i t i a l g r a n u l a r f e r r o m a , g a s i a n m i n e r a l ,
probablypyroxene,
which i s completely replaced
A semi-fresh plagioclase microlite
by c h l o r i t e ,
o f the lamprophyres was
determined t o have a composition o f An 42 (Bittmmnzonemethod).
are s o d i c l a b r a d o r i t e
Intheandesites,plagioclasephenocrysts
and groundmass m i c r o l i t e s are c a l c i ca n d e s i n e .
a n d e s i t e s and lamprophyres appear
Hence the
t o have similar compositions,
The a n d e s i t e s and b a s a l t i c a n d e s i t e s are d i s t i n c t l y
p o r p h y r i t i c and d i S f e r from t h e l a m p r o p h y r e s m a i n l y i n t e x t u r e .
The modal mineralogy of a t y p i c a l a n d e s i t e i s : 24 p e r c e n t
p l a g i o c l a s e (An5,-59),
3 percentiddingsiteafterolivine,
51
-
and 3 p e r c e n t a u g i t e (2V,= 5.
3
, + 2 ' , ZAG = Ll.1" 1. (all as
0
phenocrysts) in an intergranular
groumdmass o f p l a g i o c l a s e
(-hLk
clinopyroxene
6),
andminorolivine.
Latitelntrusives
The major c o n c e n t r a t i o n of l a t i t ei n t r u s i v e si n .t h e
study area occurs within
one mile .of t h e p e r i m e t e r o f t h e
Brovnl (1972, p. 6 4 ) ' h a sf o u n d
TresMontosasstock.
relationship of latite
dikes a s s o c i a t e d w i t h t h e
(monzonite)stockintheBearXountains.
of l a t i t e d i k e s i n
1
a similar
La Jencia
The d i s t r i b u t i o n
radial and concentric faults about the
in
Tres Montosas stock along with subtle similarities
p e t r o l o g i cc h a r a c t e rt ot h em o n z o n i t ef a c i e so ft h es t o c k
s u g g e s t a g e n e t i ct i e .
The d i k e s range from 1 f o o t t o 20 f e e t
i n t h i c k n e s s .and 50 t o 15.00 f e e t i n i e n g t h o f outcrop.
A n oval-shapedplug
.feet forms
o f l a t i t e porphyry 1200 by 1400
a topographic high 3/11 of a mile east o f Pine
Ilrel.lL. The o u t c r o p is' bloclry,andmassive
i n appearance.
Talus derived from tkis r . e s i s t a u t r o c k u n i t s u r r o u n d s
of the outcrop and obscures
Freshhandspecimens
rauch
most o f t h e i n t r u s i v e c o n t a c t .
o f l a t i t e are g e n e r a l l y l i g h t
brol*mish-gray t o l i g h t - b u f f i n c o l o r . G r e e n i s h - g r a y v a r i e t i e s
exhibit features
of p r o p y l i t i c a l t e r a t i o n i n t h i n s e c t i o n .
-
The l a t i t e d i k e s a r o u n d t h e T r e s
similar i n t e x t u r e andcomposition
Montosas s t o c k are q u i t e
t o a sample from t h e plug.
Phenocrystsafnormallyzonedplagioclaseformabout
p e r c e n t o f t h e l a t i t e s andoccur
e q u a n tc r y s t a l s
25
as s m a l l l a t h s and l a r g e r
from 1.0 t o 7.0 m m i n l e n g t h .
The most
52
a coreof
s t r o n g l y zonedphenocrystobservedhad
An
33 and
An,7 (Fouque' method). t I?any o ft h e
rim m a t e r i a lo f
p l a g i o c h s e c r y s - t a l s are mantled bynarrow
cloudy alkali f e l d s p a r .T h i sf e a t u r e
rims of untvdnned
i s a l s oq u i t ec o m o n
i n samples from t h em o n z o n i t ep o r p h y r yf a c i e so ft h e
.,
Montosasstock.Brovn
Tres
t od a r k - r e d d i s h - b r o mp l e o c h r o i c
b i o t i t e i s the o n l y o t h e r
cornrnon phenocryst and
may comprise
about 3 t o 7 p e r c e n t by volume o f the l a t i t i c r o c k s .
.
.
l a t i t e dikesare'connfionly
g r o u n d m a s sf e l d s p a r so ft h e s e
' a l t e r e dt ol o w - b i r e f r i n g e n tc l a ym i n e r a l s ,
The r e p e a t e d
t h i s feature i n what appear t o be u n a l t e r e d
occurrenceof
l a t i t e s s u g g e s t s some type of
process.
The
I n thesample
a selectivereaction(aiteration?)
from t h e l a t i t e plug,about
95 p e r c e n t
of t h e groundmass is stubby carlsbad tsrins o f alkali f e l d s p a r ,
p r o b a b l yo r t h o c l a s e ,
(0-02 t o
0.3 m i i o n g ) w i t h the remainder
a p a t i t e were.observedin.
c o n s i s t i n go fm a g n e t i t e ,T r a c e so f
.
s e v e r a ll a t i t e
.
samples,
Other l a t i t e d i k e s i n t h e , n o r t h v e s . t
a*-ea c r o p o u t w i t h i n
c e n t e r .I n
I + mile o f t h e G a l l i n a s S p r i n g s i n t r u s i v e
handspecimen,theyappearquite
latite dikes near
a p p a r e n t h a t h e i rc o m p o s i t i o n
fine-grained
make u pa b o u t
similar t o
t h e Tres Montosas stock except t h a t some
have a pinkishgroundmass,,
.
past of t h e ma-p
I n t h i n s e c t i o n homever, i t i s
i s markedly d i f f e r e n t .
(0,05 t o 0,2 mm) q u a r t za n h e d r a
Very-
i n t h e groundmass
10 p e r c e n t o f t h e r o c k . P l a g i o c l a s e m i c r o l i t e s
a r e also p r e s e n t - i n t h e groundmass (-15 percent),These
samples are more c o r r e c t l y d e s c r i b e d
as q u a r t z l a t i t e s .
compositional difference, along with
t h e .g e o g r a p h i c l o c a t i o n
.This
.
53
that t h e y are related. t o t h e
o ft h e s ed i k e s ,s u g g e s t s
Gallinas Springs intrusive center rather than to the Tres
Nontosas stock.
Phenocrystsofcalcictosodicoligoclase
and b i o t i t e
and i n a p p r o x i n a t e l y t h e
a r e coimon t o t h e q u a r t z L a t i t e s
Because of t h e , v e r y
same p r o p o r t i o n s as i n t h e l a t i t e s ,
small size ofthequartzcrystals,thisimportantdifference
i n composition was not reco,%ized
the latites and quartz latites
t h eg e o l o g i c
i nt h ef i e l d .T h e r e f o r e
are n o t d i f f e r e n t i a t e d
on
map.
R h y o l i t eI n t r u s i v e s
of q u a r t z l a t i t e t o
Felsicquartz-porphyrydikes
rhyolitecomposition
i n t r u s i v e rocks,
are proba.bly the youngest of the Oligocene
.. _..
These s i l i c i c i n t r u s i v e s are anomalous i n
t h a t t h e i r two major outcro, trends,
N
80" W and N 70" E,
are c l e a r l y di.scorda.ntwiththe
major s t r u c t u r a l f a b r i c i n
the s t u d y area.
i n width from 30 t o 200 f e e t
Outcropsrange
a n d i n l e n g t h from 400 t o lf.000 f e e t , H o s t a r e s e n i c o n t i n o u s
andlilzear
innature,howver,southwst
of G a l l i n a s S p r i n g s
a change t o an en echelon pattern occurs.
The r e l a t i v e a g e
bracketed.
o f these f e l s i c d i k e s i s f a i r l y well
They c u t t h e a n d e s i t e
southwest o f G a l l i n a s S p r i n g s a n d
. q u a r t zv e i n sn e a rC o u n c i l
Rock.
of LandavasoReservoir
are c u t a n d a l t e r e d
by
They a r e a l s o a p p a r e n t l y
t r u n c a t e d by major basin and range f a u l t s at Council Rock.
A s s t a t e d above t h e c o m p o s i t i o n o f t h e s e d i k e s v a r i e s
from q u a r t z l a t i t e t o r h y o l i t e .
t r e n d (11 80" Y!)
\
The l o n g e s ts u b - l i n e a r
extends about 3 m i l e s from t h e n o r t h v e s t
54
f o r k o f Council Rock Arroyoalmost
map a r e a .
t o t h e v;est edge o f t h e .
Field. observationsandpetrographicexamination
s u g g e s t s a progressive compositional change
t o r h y o l i t e movingfrom
from q u g r t z l a t i t e
this trend.
east to west along
are noted:
From east t o westthefol.losnngchanges
quartzphenocrystsprogressivelyincreasein
6 percent to I 3 percent;the
a.bundance from
matrix changes, from white t o
glassy black; t h e p l a g i o c l a s e / a l k a l i f e l d s p a r r a t i o d e c r e a s e s
from s l i g h t l y g r e a t e r t h a n
1,O
t o a b a u t 0.25;
plagioclase
from An
a t t h ee a s te n d ,t o
Anz7 n e a r
37
t h e m-iddle t o Anl3 a t the west end; and
a decrease i n
c o m p o s i t i o nv a r i e s
s a l i i d i n e 2vx- 42" t o 30" s u g g e s t s a n i n c r e a s e i n p o t a s s i u m
mest.
c o n t e n tt o w a r dt h e
S i n c et h ef e l s i c
dikes c u t
it
progressively higher stratigraphic units toward the west
is difficult to determine
lateral
i f thesechangesrepresent
c o m p o s i t i o n a l v a r i a t i o n s i n a magma chamber at d e p t h o r
verticalvariationsin
e v i d e n c ef a v o r st h e
a l a t e r a l l yu n i f o r m
latter situation,
Additional
dike.
The o p t i c a lp r o p e r t i e s
ofplagioclaseinsample
1-64 from t h e east e n d i n d i c a t e a
low-temperature, plutonic
I
X'A010= 20.5')
s t r u c t u r a l .state
(zv,= 90'
T h i s d a t as u g g e s t s
z0 ,
the
T h i s i s a ne n i g m a t i cs i t u a t i o ns i n c e
same specimen a l s o c o n t a i n s s a n i d i n e p h e n o c r y s t s
t oo r t h o c l a s e .
2
as opposed
a c o o l i n gr a t ei n t e r m e d i a t e
t o t h a t of t h ep l u t o n i ca n dv o l c a n i ce n v i r o n m e n t s .
Sample
1-67 from t h e w e s t e n d o f t h e d i k e t r e n d
-
is a b l a c k r h y o l i t i c
vitrophyre.Thissamplecontainshigh-t.ernperaturesodic
o l i g o c l a s e (2Vx= 52- 2 2'
, XrAO1O=
character of this vitrophyre
8" ).
The h i g h l y a l k a l i n e
i s revealed by
a t r a c e of
II
55
0
spodumene phenocrysts (2Vz= 511
.
- 3' , ZAC=
4
27" 1.
T h u st h e
changes i n composition described above are probably
to vertical variations in the transition
related
from t h e h y p a b y s s a l
t o the volcanicenvironment.
A small t r i a n g u l a r f e l s i c p l u g , a b o u t
s i d e , whichcropsout
300 f e e t , o n a
1% miles southj,iest of Gallinas Springs,
consists of multiple intrusion of
a coarsely porphyritic,
medium-gray d a c i t e , o r q u a r t z l a t i t e , and a l i g h t - g r a y
rhyolite.
An e x t e n s i o n o f t h i s i n t r u s i v e t o t h e s o u t h
the only rhyolite
Loughlinand
dike i n t h e area w i t h a n o r t h e r l y t r e n d .
ICoschmann (194.2, p. 43) and Brovm (1972,
p.' 6 5 ) d e s c r i b e f e l s i c d i k e s o f
relationships.
similar composition and
granite and monzoniteof
r e s p e c t i v e l y ,C h a p i n
cut t h e
the Anchor Canyon and N i t t s t o c k s ,
and o t h e r s( i np r e p a r a t i o n )h a v e
the extent of these white rhyolite
c o r r e l a t e s v e r y me11 w2th t h e l a t e r a l e x t e n t
dikes
of the ?4asdalena
A similar c o r r e l a t i o n o f f e l s i c d i k e s
compositepluton,
with a gravity-delineated granitic pluton has
i n t h e L i t t l e B e l t MountainsofNontana
7970, p. B&).
age
The geologic map o f LoughliuandKosckqann
(1942, P1, 2 ) i n d i c a t e s t h a t w h i t e r h y o l .i.t e d i k e s
recognizedthat
is
been observed
(Vi.tk;ind and o t h e r s ,
Thus i t s e e m v a l i d t o u s e t h e p r e s e n c e o f
f e l s i c d i k e sxarms t o d e l i n e a t e unexposedplutons..
The
g e o l o g i c map (Pl. 1 , t h i s r e p o r t ) s h o w t h e d i s t r i b u t i o n
o ft h er h y o l i t ed i k e s .T h e i rc l o s ea s s o c i a t i o n
with
h y d r o t h e r m a la l t e r a t i o n ,q u a r t zv e i n s ,o t h e ri n t r u s i v e s
and a v o l c a n i c v e n t
a l l support .thepresenceofunexposed
stocks below the Council
Roc!:
area a n d b e l o w t h e i n t r u s i v e
56
centerabout
one mile soathvfest o f G a l l i n a s S p r i n g s .
Tres Montosas Stock
In the southirlest corner
of t h e s t u d y 'area a r o u g h l y
circula pluton of interaedia.te to silicic ilitrusive rocks
c r o p s out discontinously from under pediment
blow sand,
gravel and
Approximately 90 p e r c e n t o f t h e s t o c k is veneere d
by s u r f i c i a l d e p o s i t s ;
however, t h e d i s t r i b u t i o n o f o u t c r o p s
along with topographic
and s t r u c t u r a l e x p r e s s i o n of t h e
i t i s a continuous cupola-like body beneath
stocli suggest that
t h es u r f a c e ,B e c a u s e
t o the
o f t h ei n t r u s i v e ' sp r o x i m i t y
peaks of Tres Xontosas, one mile t o t h e southvmst, i t i s
Tres
as t h e Tres Montosasstock.The
herereferredto
Montosas stock i s the only exposed pluton
..
Gallinas Mountains.
,
knovrn i n t h e
.
The Tres Montosas stock i s q u i t e similar i n composition,
l e v e l ofemplacenent
and c r o s s - c u t t i n g , r e l a t i o n s h i p s with
o f Nagdalena.
c o u n t r y roc!rs t o s t o c k s d e s c r i b e d i n t h e v i c i n i t y
B e s t 'I m o m of t h e s e are themonzonitic
t o graniticstocks
o f t h e Kelly d i s t r i c t d e s c r i b e d by Loughlin and
(1942, p. 36-l@j.
The Nitt andAnchor
K e l l y d i s t r i c t have been dated
m.y.
and 28-3
1953, p.
220).
-+
1 .it m.y.,
.
Ifoschmann
Canyon s t o c k s of t h e
by IC-Ar methods a t 28.0 2 1.4
r e s p e c t i v e l y (Weber and Eassett,
These ages are i n good.agreement with t h o s e
o f t h e A-L Pede R h y o l i t e (31.8 m.y.,
which i s t h e y o u n g e s t r o c k u n i t
NagdalGna p l u t o n , a n d t h e
Smith and others, 1974)
Imosm t o be cut b y t h e
.LaJara Peak Andesite (23.8 m.y.,
Chapin, 1971-a) vhich i s youngerthanthepluton.
a s s o c i a t i o n , a l a t e OlLgocene a.ge
V I ~ So r i g i n a l l y
By
inferred
57
by t h e writer f o r t h e Tres Hontosas stock.
was
A sample of metasomatic hedenbergite syenite
c o l l e c t e d by t h e m i t e r from the margin of the
s t o c k a t t h e Big Jolmmine
Tres .I.Iontosas
f o r K-Ar d a t i n g .
The sample
'
was s e l e c t e d on the basis of i t ' s fresh appearance (though
closely .associated with apparent hydrothermal magnetitev e i nm i n e r a l i z a t i o n
a t themine).
A whole r o c k IC-Ar a n a l y s i s ,
-
a d a t e o f 33.9 n.y.
by Geochron L a b o r a t o r i e s , I n c . y i e l d e d
which i s o l d e r t h a n t h e A-L Peak Formation which the stock
intrudes.
T h i s a p p a r e n t l yh i g h
age may be explained by t h e
followingobservation,Petrographicexaminationofthe
as much as 10 percent pyroxene
datedsamplereveals
According t o Damon (1968, p.
1Lt)
pyroxenesfrom
environments commonly y i e l d h i g h a g e s
intrusive
due t o e x c e s s a r g o n .
t h a t may be dram from t h e date is
A positive conclusion
that the stock
by v o l m e .
is definitely Oligocene in age.
Most, o u t c r o p s o f t h s T r e s Montosas stock are comprised
by i t s m a f i c b o r d e r f a c i e s w h i c h d i s c o n t i n u o u s l y o u t l i n e s t h e
s t o c k by a s e r i e s o f
low arcuateridges.Borderexposures
range f r o m mass-ive,bloclry
c l i f f s t o lovi h i l l s mantled by
d a r k bro;.rn s o i l w i t ha b u n d a n tm a f i c ,a p h a n i t i cf l o a t .
central core rocks crop
beneathpediuentgravels
The
o u t as small ledges p r o j e c t i n g f r o m
and as very l o w flat lrills d o t t e d
with a few rounded boulders.
Most o f t h e c o n t x t b e t w e e n t h e s t o c k
i s approximately locatbd because
o f colluviumand
An e x c e p t i o n is found along the mest margin
monzoniteporphyry
snd c o u n t r y r o c k
soil covert'.
vhere q u a r t z
is c l e a r l y v i s i b l e i n c o n t a c t
with
.
58
.
c r y s t a l - r i c hE I e l l s
Nesa t u f f s .
B1thouC;’n t h e c o n t a c t is an
i r r e g u l a r s u r f a c e , i t i s e s t i n a t e d t o d i p a t about 30 degrees
t ot h ev e s t
at t h i sl o c a t i o n .T h i ss u g g e s t st h a tt h ea p e x
..
o f t h e r o o f of t h e s t o c k w.s approximately 1500 f e e t above
t h ep r e s e n te r o s i o nl e v e l .
However, t h e d i p o ft h ec o n t a c t
this conclusion
hasnotbeenobservedelsewheremaking
tenative.
Marked doming andfau-ltingof
theperiphery
v a l l rocksaround
of thestock,alongwiththe
of latite dikes,
vr?despread d i s t r i b u t i o n -
suggest t h a t t h e T r e s M o n t o s a s s t o c k
Vras
forcefully intruded t o a high level in the volcanic pile.
Probably less t h a n 2000 f e e t of Oligocenevolcanic
strata
separatedthepresentleveloferosionandthegroundsurface
i t s e e mr e a s o n a b l e
a t thetimeofemplacement.Therefore,
t o assumetha.t
some type of v o l c a n i c e r u p t i o n o c c u r r e d i n
stock,
associationwithintruisionofthe
F a c i e s .T h r e ep r i n c i p a lf a c i e sa r ep r e s e n ti nt h e
Montosasstoc!~:andesite
Tres
t o g r a n o d i o r i t e ,p y r o x e n el a t i t ea n d
\
monzonite t oq u a r t zm o n z o n i t e , ,a n dg r a n i t e .G r a n i t eo c c u r s
i n thecoreofthepluton
w i t h intermediate composition rocks
o c c u r r i n g a t t h e b o r d e r .F e r r o - p y r o x e n es y e n i t e sa l s oo c c u r
l o c a l l y i n border facies rocks
as a product o f c o n t a c t
m e t a s o m a t i s m ,S p a t i d .r e l a t i o n s h i p s
o f outcropssuggest
t h a t most o f t h e s y e n i t e s v e r e o r i g i n a l . l y g r a n o d i o r i t e s .
The i n f e r r e d d i s t r i b u t i o n o f
andsamplelocationsdiscussed
by Figure 10.
the t h r e e p r i n c i p a l f a c i s s
later i n t h e t e x t are s h o m
The sin!ple r e l a t i o n s h i p s betTween f a c i e s as
p o r t r a y e d on t h i s !nap a r e h i g h l y s i m p l i f i e d b e c a u s e o f
poor exposures.
.
. .
.
59
.
GO
the o l d e s t i n t h e
Andesiteandgranodioriteare
i n t r u s i v es e q u e n c e .
Andesites form a c h i l l e d s h e a t h a b o u t
of t h e s t o c k a t t h e northwest,southvrest
theoutersurface
andsouthmargins.
Xear t h e Big Johnmine,andesiteporphyry
is apparently gradational into fine-
t o mediurn-grained
are b l a c k t o dark-
granodiorite.Andesitehandspecimens
g r a yd e n s ep o r p h y r i t i cr o c k sw i t hs m a l l ' p h e n o c r y s t so fc l e a r
.
plagioclase and greenish-black pyroxene.
Under t h e m i c r o s c o p e t h e a n d e s i t e p o r p h y r y
is s e e n t o
consistofphenocrystsofsodiclabradoritetocalcicandesine,
endiopside (Fig. 3
trixhytic to felty
appendix), and s p a r s e m a g n e t i t e , i n
groundmassof
a
calcicandesinewithminor
amountsofma.gnetiteandpyroxene.Plagioclasephenocrysts
areeuhedral,
. ..
one c a s e ,p o s s e s s
compositionof
-.
An43 8
in
e x h i b i t o s c i l l a t o r ya n dn o r m a lz o n i n ga n d ,
a narrowreverse-zoned
I 3 plagioclasephenocrysts
Modal a n a l y s e so f
samples for the
rim.
.The a v e r a g e
i n sample TX-12 i s
T?.1-12 a n do t h e rr e p r e s e n t a t i v e
Tres Montosas stock are l i s t e d i n Table 1 .
Porphyriticgranodiorite
handspecimensrangefrom
b l a c k t o gray-brwm with Fhenocrysts of lath-shaped
plagioclase,
2.0 m t o 1 .O cm i n l e n g t h and medium grained pyroxene.
fine-grainedgroundaass
c o n p r i s e sa b o u t
o f a l k a l i f e l d s p a ra n dq u a r t z ,
20 t o 30 p e r c e n to ft h er o c k .T h i ns e c t i o n
examination of samplenumber
*
1-88 s h o w i t t o c o n s i s t o f 65
p e r c e n tc a l c i ca n d e s i n e( h i g h - t e m p e r a t u r e )a n d
d i o p s i d i ca u g i t e
;A
BE
phenocrysts,
of a granophyricintergrovth
i n a r a t i o of about 4 t o 1.
15 p e r c e n t
Groundmass m i n e r a l sc o n s i s t
of alkali feldspar and quartz
GI
35019
qmr'cz
..
..
62
Granophyricgroundmass
t h ep r i n c i p a lf a c i e si n
i s a f e a t u r e c,ommon-to all.
Barker (1970, p. 3342)
the stock.
i n e p i z o n a l i n t r u s i o n s and
ind-icates granophyres only occur
extrusiveigneousrocksandsuggeststhat
rapid c r y s t a l l i z a t i o n
feature
is a c r i t i c a l f a c t o r i n t h e i r f o r m a t i o n , ' A n o t h e r
common t o r o c k s o f t h e b o r d e r f a c i e s
is the mantling of'
high-temperature (volcanic) plagioclase
feldspar.
..
'
by rims o f a l k a l i
A l k a l i f e l d s p a r rims o c c u r r i n g i n sample number
1-13 have a 2Vf=47" and g r o u n d m a s s f e l d s p a r h a s
a 2VX=53O.
The same t h i n s e c t i o n vas s t a i n e d f o r p o t a s s i u m f e l d s p a r
(Chayes,1952).
T h i s e l i m i n a t e dt h ep o s s i b i l i t y
.
'that t h e r Z m s
!Che measured 2 V ' s are
c o u l d be untxinnedhigholigoclase,
compatible with a n o r t h o c l a s ea n do r t h o c l a s e .
cloudedappearanceofthe
.
The white
. r i m material f a v o r s t h e l a t t e r
. ,,.
but
t h i s i s n o td i a g n o s t i c .P h o t o m i c r o g r a p h so ft h i ns e c t i o n s
1-88 and 1-13 which i l l u s t r a t e t e x t u r e s a n d . o t h e r f e a t u r e s
d e s c r i b e da b o v ea r e
shown i n Figure 1 1
Latites and monzonites with
a wide v a r i a t i o n i n composition
and t e x t u r e s are i n t i m a t e l y a s s o c i a t s d w i t h
g r a n o d i o r i t e so ft h e
border f a c i e s .
intrudes granodiorite near the
Even though l a t i t e
B i g John mine, c o n t a c t s
between a.ndesite and pyroxene monzonite
.
.
-
gradational.
are probably mostly
A n o t a b l ee x c e p t i o no c c u r sa l o n g
margin of t h e s t o c k where quartz monzonite
with pyroxene syenite suggesting
related to
2
the andesites and
the west
is in sharp contact
t h e quartzmonzonite
is
later intrwive event.
Viewed i n t h i n s e c t i o n , t h e
gro,undmass t e x t u r e s of
l a t i t e s vary froln s e r i a - t e , a l l o t r i o m o r p h i c g r a n u l a r i n
11A.
11B.
I
Figure 1 1 .
f
.o
r?lm
I
A.
Granodiorite p o r p h y r y ( 1 -88) w i t h hightenperature p l a s i o c l a s e p h e n o c r y s t s . Hote
t h e g r a n o p h y r i cq u a r t za n d
z l k a l i feldspar
i n t h e groundmass; C r o s s e d nicols,
B.
G r a n o d i o r i t e p o r p h y r y (1-13) w i t h rim o f a l k a l i
f e l d s p a r mantling a h i g h - a n d e s i n e p h e n o c r y s t .
S e c t i o ns t a i n e d for K - f e l d s g a r .
X i n e r a l phases:
A, a n d e s i n e ; P , pyroxene; E , hornblende; Q,
q u a r t z ; Or, alkali f e l d s p a rC
. rossed
nicola.
specimenstakenneartheoutercontacttohgpidiomorphic
g r a n u l a r i n r o c l - sc l o s e rt ot h ec o r eo ft h ep l u t o n .
allotriom_op!iic granular textures
The
may b e r e l a t e d t o p r o t o c l a s i s ,
o r contzmination of t h e melt by wall roclrs.Althoughno
x e n o l i t h s o f vall rock were observed there seems
c'orrelation of poorly welded
t o be a
l a t i t e ash-flom t u f f s or"
t h e Spears Formation as wall r o c k s and. t h e m i c r o g r a n u l a r
textures.
Some phenocrysts ( ? ) ofoligoclasehavesharp
narrow rims oflzornallyzonedandesinetooligoclasesuggesting
t h e c o r e s a.re a c t u a l l y x e n o c r y s t s ,
Two d i s t i n c t t y p e s
of monzonites are recognized:a.ugite
X t could. n o t be d e f i n i t e l y
monzonitesandquartzmonzonites,
a normal
established whether the quartz monzonites represent
differentiationproductofthepyroxenemonzonites
from a d i f f e r e n t i a t i n g
whether they represent separate intrusions
magma body at depth.
.
or
A s i n d i c a t e de a r l i e r ,q u a r t zm o n z o n i t e .
does have a s h a r pc o n t a c t
notsufficientlydefinitive
with p y r o x e n e s y e n i t e , b u t t h i s
is'
t o r e s o l v et h ea b o v eq u e s t i o n .
It i n clear f r o n s e v e r a l modal a n a l y s e s t h a t a n z n t i p a t h e t i c
r e l a t i o n s h i p e x i s t s be'cveenpyroxeneandquartz,thussuggesting
a co.mon p a r e n t .
Hodesofthreepyroxenemonzonites
.
from t h e s o u t h an.d
following limits:
east b o r d e rf a c i e sr a n g eb e t w e e nt h e
p l a g i o c l a s e , 20-29 p e r c e n t ;a l k a l i - f e l d s p a r ,
pyroxene, 22-31 p e r c e n t ;q u a r t z
(mostlymagnetite)
0.5-5
42-45 p e r c e n t ;
percent;opaque6
1-G p e r c e n t ; a n d b i o t i t e
0-0.1
P l a g i o c l a s e i s found a.s s u b h e d r a l l a t h s from 1-5
l e n g t h v i t h o s c i l l a t o r y 2nd normalzoning
common.
percent,
111111
in
The
.
65
compositional range o f p l a g i o c l a s e
wit11 a n average of about
Lt5
A l k a l i f e l d s p a r o c c u r s as
An
32
s t u b b yc a r i s b a dt w i n si nt h e
i s from An2,{, t o A n
groundmassandsometimes
a narrow rim around plagioclase phenocrysts'
as
.
o r as a micrographic
i n t e r g r o w t h w i t h q u a r t z .S u b h e d r a ln e u t r a l - g r a ya u g i t eg r a i n s
-
range i n s i z e from O,O5 nun t o 2 mm i n diarneter and average
about 0.3 m.
*.
B i o t i t e a n d apatite sometimes.occur i n trace
amounts.
The modal a n a l y s i s o f s a m p l e
1-80 shovm i n Table 1 appears
.
t o be r e p r e s e n t a t i v e o f quartz monzonites which occur
'
mainly
a l o n g t h e viest andsouthvrestmarginofthestock.Pyroxene
ranges from a b o u t 0 . 5 , t o
5 percentwiththepresence
absence o f u r a l i t i c h o r n b l e n d e ( f o r m e d
pyroxene) as t h ep r i m a r yc a u s e
or
a t theexpense
of t h i sv a r i a t i o n ,
of
The q u a r t z
monzonite is d i s t i n c t l y p o r p h y r i t i c - i n c o m p a r i s o n t o t h e
pyroxenemonzoaite.Plagioclasephenocrysts
. i nl e n g t hc o m p r i s ea b o u t
10 t o
as much as 2 c n
25 p e r c e n t 'oy volume; a n o r t h i t e
8 g r a i n sa v e r a g e d An.37* R i m o f a.lkali-
v a l u e sm e a s u r e df o r
feldspar a r o u n d p l a g i o c l a s e a r e - m o r e a b u n d a n t t h a n i n t h e
The 2Vx of onegroundmass
pyroxenemonzonites.
c r y s t a l of
a l k a l i - f e l d s p a r i n sample 1-80 vas measured t o be 80 degrees,
e l i m i n a t i n ga n o r t h o c l a s e .Q u a r t zo c c u r s
i n t e r s t i c e so ft h e
and sphene appear
as grains i n t h e
groundmassorthoclase.Traces
of r u t i l e .
as l a t e - s t a g e c r y s t a l s i n a s s o c i a t i o n
vrith
quartz.
A histograa illustrating the distribution
of p l a g i o c l a s e
compositions i n b o r d e r f a c i e s r o c k s ( e x c l u d i n z s y e n.i.t e s )
shown i n Figure 12.
is
The d i s t r i b u t i o n appears t o be bimodal
.
"I
..
pSRCZN?' AXORTIIITE
FSi,wre
12.
Frequencydiagram
of a n o r t h i t ec o n t e n t
f o r 50 plagLoclase phenocrysts i n 9 . .
samples of b o r d e r f a c i e s r o c l c s
of t h e
T r e s Hontosasstock.
The . d i s t r i b u t i o n ..
i s suggestive .of'.a bimodal population.
67
b u t nore measurements a r e needed t o b es u r e .
A bimodal
d i s t r i b u t i o n would suggest a m u l t i p l ei n t r u s i o no r i g i n .
might be expected, plagioclase
i n border rocks with
matrix have.high-temperatureoptics.
As
.an a p h a n i t i c
However, g r a n o d i o r i t e s
(1-88 and1-13)andmonzonites(1-80
a n d TM-27),
also contain
high-temperature plagioclase phenocrysts (1
0 checked). some o f
which are i n a s s o c i a t i o n w i t h o r t h o c l a s e ,
It i s assumed here
t h a t most border facies rocks
i n t h e Tres MontosasStock
t o theoccurrenceof
containhigh-plagXoclase.Reference
high-.temperatwe plagioclase
i n thechilledbordersof
epizonal stocks and batholiths has been
a u t h o r s( T u t t l ea n d
made b y s e v e r a l
Bowen, 19.58, p. 116; and Cater, 1964.,
P. 45).
The seeminglyenigmaticassociationdescribedaboveof
,
high-temperatureandesineandlorn-temperatureorth'oclase
be e x p l a i n e d b y l a t e - s t a g e d e u ' t e r i c a l t e r a t i o n
t oo r t h o c l a s e ,
The c a t a l y t i c e f f e c t
opposed t os a n i d i n e .E x p e r i m e n t a l
of plagioclase
ofmater
criticalfactorinallowingtheformation
may
i s p r o b a b l yt h e
o f o r t h o c l a s e as
173)
data (Donnay,'1960,p.
i n d i c a t e s tkiat t h e r a t e o f c o o l i n g a n d c a t a l y t i c e f f e c t s o f
water a r e t h e d o m i n a n t f a c t o r s c o n t r o l l i n g t h e t r a n s f o r m a t i o n
fromhigh-temperature
t o low-temperaturefeldspar.
tempe-ature of crystal growth does not appear to be
c r i t i c a lf a c t o r .C a t e r( 1 9 6 9 ,
similar l a t e s t a g e p o t a s s i c
The
a
p. 4 2 )h a sa l s oo b s e r v e d
metasomatism i n l a b r a d o r i t e
g r a n o d i o r i t e s a n d quartzmoilzonites
e p i z o n a lb a t h o l i t hi nt h eN o r t h e r n
o f t h e CloudyPass
CascadeMountai.ns.
He
c i t e s Kennedy ( 1 95.5) t o e x p l a i n the phenomenon by t h e "upward
.
.
.
.
m i g r a t i o n of a l k a l i - r i c h s o l u t i o n s . . . s h o u l d c o n c e n t r a t e
alkalis in the upper parts
68
,
of i n t r u k i o n s where they 'could
c r y s t a l l i z e . as o r t h o c l a s e a n d o t h e r a l l r a l i - c i c h
Insupport
o f t h i sc o n c l u s i o n ,t h eh e d e n b e r g i t e - o r t h o c l a s e
s y e n i t e( s a m p l e s
BJ'P,l-Lt
m e t a s o m a t i co r i g i n ,
and 1-79), which i s c l e a r l y of
also c o n t a i n s r e l i c t c o r e s o f h i g h -
temperatureandesine.
Eoone (1962.) hasobserved
similar
r e l a t i o n s h i p s i n a compositepluton
i n Maine.
However he
suggests that "potassic enrichen%"
may a l s o o c c u r b y v o l a t i l e
d i f f u s i o n i n t h e nagma i t s e l f p r i o r t o c o m p l e t e c r y s t a y l i z a t i o n .
form t h e
Leucocraticgraniteandgraniteporphyry
c e n t r a lc o r e
of the TresMontosasStock,Granitehasbeen
Tank (Wilkinson, i n
observed cutting monzonite near Evans
p r e p a r a t i o n ) .I na d d i t i o n ,f l o a to ff i n e - g r a i n e da p l i t i c
.. ....
g r a n i t e mas found on t h e p o o r e x p o s u r e o f l t i t i t e . .
east o f Pine
\yell Suggesting an i n t r u s i v e r e l a t i o n s h i p ' f o r g r a n i t e e x i s t s t h e r e
also.
The l a r g e s to u t c r o p
o f nediun- t oc o a r s e - g r a i n e d
g r a n i t eo c c u r s
j u s t s o u t h o f PineWell,Texturesvaryfrom
lssPidiomorpl&
t o a.llotriomorphic granular and
.
from e q u i g r a n u l a
t oc o a r s e l yp o r p h y r i t i c .C h a l k yp i n ko r t h o c l a s ep h e n o c r y s t s
as much as 3 crn i n l e n g t h c o m p r i s e a b o u t
p o r p h y r i t i cs p e c i m e n s ,
Of
A P i n e - g r a i n e dp o r p h y r i t i cg r a n i t e ,
found as abundant f l o a t n o r t h of PineWell
unusual in that the large phenocrysts
sanidineinstead
15 t o 25 p e r c e n t
of o r t h o c l a s e e
(sample I-lLt), is
of a l k a l i - f e l d s p a r a r e
Hineralsidentifiable
megascopically i n t h e g r a n i t e s a r e : p i n k o r t h o c l a s e ,
snoky
q u a r t z , some c h a l k y v k i t e p l a g i o c l a s e , a n d s p a r s e h e m a t i t e ,
liraoniteandhornblende.
Liost of t h el i l n o n i t e
i s pseudonorphic
69
after pyrite.
Nodalanalysis
r e p r e s e n t a t i v eo f
.
of sar:lpie 1-46 (Table 1 ) i s f a i r l y
mediun-grainedequigranularrock.
The p r o p o r t i o no fo r t h o c l a s e
in porphyritic granites increases
Over e q u i g r a n u l a r r o c k s b u t t h e
by as much as 10 perc,ent
sum o f o r t h o c l a s e
i n all s p e c i m n s r e m a i n s a p p r o x i m a t e l y c o n s t a n t
i t e x h i b i t s a v e r yf i n ea l b i t et w i n n i n g .
zone-2Vzmethod
may be overlooked.
An36
(2Vx=
the section
The combinedRittmann
indicates that plagioclase
is low-temperatureoligoclase
The compositionof
a t 95 percent,:
Even i ng r a i n s
o r i e n t e d nth the (010) n e a r l y p e r p e n d i c u l a r t o
t h ea l b i t et w i n n i n g
and q u a r t z
is difficult to recognize because
Inthinsection,plagioclase
'
It'is a
g r w i t e s of t h e' c e n t r a lc o r e .
85" 2
of the granites
2', X 1 h O 1 O = 17")
6 measuredgrainsrangedfrom
Anz8 to
..
Orthoclase i s c o n s i s t e n t l y c l o u d e d a n d o c c u r s
a n h e d r a lt os u b h e d r a lg r a i n s ,L a t es t a g eg r a n o p h y r i c
'
of o r t h o c l a s ea n de u h e d r a lq u a r t z
are common.
from 2V,=
a n g l e so f, o r t h o c l a s er a n g e d
as
masses
Optic axial
65" t o 75". The 2 V ' s
s m p l e 1-14 have a w i d e v a r i a t i o n ;
of sanidine phenocrysts in
2Vx= 22, 33 and 38 d e g r e e s s u g g e s t i n g a r m g e i n Or c o n t e n t
o f 45 t o 85 p e r c e n t (Fig. 2
appendix).
T h i s mide range i n
a p p a e n t c o s p o s i t i o n i s p r o b a b l y i n v a l i d and s u g g e s t s t h a t o t h e r
'
f a c t o r s are p l a y i n g a p a r ti nc o n t r o l l i n g
t h e s e a l k a l i feldspars,
t h e axial. a n g l e s of
V a r y i n gd e g r e e so ft r i c l i n i t y
unmixing are two s u c hf a c t o r s .
t h i s r o c k is t h e p e s e n c e o f
and as i r r e g u l a rm a n t l e sa b o u t
axd
Another i m p o r t a n ta s p e c t
of
orthcclase in the groundnass
t h e s a n i d i n e .T h i so b s e r v a t i o n
s h i m i n Figure 1 3 s u g g e s t s t h a t t h e c a t a l y t i c e f f e c t
of
.
5.0mm
Figure 13.
J
S a n i d i n e g r a n i t e porphyry (1-14) a f t h e central
c u r e in the D e s rilontssas s t o c k .O r t h s c l a s c
is
' r e p l a c i n g ' sanidilre as i r r e g u l a r rizls a r o u ~ dthe
l a r g e r phenocrysts. iTf3te t h e elonsate x e n o l i t h
o f q u a r t z i t e vmrped a r o u z d the lar,.;e s a n i d i n e
phenccryst.
S, sanidine; Cr, o r t h o c l a s e ; Q,
quartz. C r o s s e d nicDls.
71
water vapor
of
is of critical importance in the inversion
sanidine t o orthoclase.
Q u a r t zo c c u r si ns e v e r a l
forms i nt h eg r a n i t e s * 'S i n g l e
g r a i n s f r o n 0.05 t o 2.0 n m ~ a r e common I n a n h e d r a l forms.
.
Granophyricquartz
i s well developed i n t h e more p o r p h y r i t i c
v a r i e t i e s .Q u a r t z
m a y a l s o occur as q u a r t z i t e x e n o l i t h s
(fig* 13.).
TAiCh a r e c l e a r l y b e i n g r e s o r b e d b y t h e ' r r l e l t -
Rutile nicrolites, mostly associated with late stage
common t o a l l t h eg r a n i t es a n p l e s I. l m e n i t e
q u a r t , z a, r e
'
i n samples from t h e west c e n t e r o f t h e
and leucoxene are found
pluton.Traces
of b i o t i t e ,h o r n b l e n d ea n dl i m o n i t e\ ? e r e
a l s o noted.
Strongargillic
(?) a l t e r a t i o n
o f orthoclaseand
p l a g i o c l a s e i s widespread i n t h e g r a n i t i c r o c k s .
r e l a t i o n s h i p ofminoramountsof
n o tc l e a r .
'
The
p y r i t e t o this process i s
The i n t e n s i t y and d i s t r i b u t i o n of t h i s a l t e r a t i o n
s u g g e s t a d e u t e r i co r i g i n .
IIedenbergitesyenite
along the
at. theBig
John mine and
.
:
vest margin of t h e stoc!i d i s p l a y s e v e r a l l i n e s
o f evidence f o r ferro-potassicmetasomatism.?,Iagnetite
in
v e i n l e t s as much as 5 cc1 t h i c k and i n d i s s e m i n a t e d g r a i n s
comprises 5 t o 2 5 p e r c e n t o f ' r o c k s , b y volume on t h e B i g
'
John dump.
as a p e r i p h e r a l
Dark greenhedenbergiteoccurs
mineral around magnetite veins and randomly
rock as a n h e d r a lg r a i n sa v e r a g i n ga b o u t
thin section, the texture
. .
of t h e r o c k
in the host
0.3 mm a c r o s s . I n
i s a '!classic1!
a l l o t r i o m o p h i c ~ r m u l a r . A s i l l . u s t r a t e di nF i g u r e
am0~1ct.so f Ugh-tenpera.ture andesine are found
14 minor
-as c o r e s o f
Figure 14.
Allctriomorphic-granular hedenbEzgitc s y e n i t e
(RTM-4) f r o n t h e E i g J o h n Zine-Y'res i4ontosas
s t o c k . iiotethe l a r g e aqhedml o r t h o c l a s e
c r y s t a l at t h e cer,ter a h i c h has a c o r e o f hightemperature andesine. Or, o r t h o c l a s e ; A, a n d e s i n e ;
H, hedenbergite, S l a c k grains zre magnetite.
Crossed ; l i c o l s .
73
some o f t h e l a r g e r o r t h o c l a s ec r y s t a l s .I n
some c a s e s
carlsbad twin planes reuain comon to the plagioclase and
..
t h es e c o n d a r yo r t h o c l a s e .
r-aetasomatism i s e v i d e n t as 'a
Contact metamorphism and
wide a u r e o l e o f h o r n f e l s e d
wall r o c k s a b o u t t h e
Tres biontosas
stockV
. i t r o c l a s t i ct e x t u r e si na s h - f l o wu n i t sh a mb e e n ,
'
r e c r y s t a l l i z e d t o a very-fine-grainedgranoblastic
(granular)
mixture of alkali.feldspar and quartz recognizable in,liand
s p e c i n e n as a dull t o m x y g r a y a p p e a r i n g g r o u l d m a s s , U b i q u i t o u s
at theexpenseof
very-fine-grainedbiotiteformed
i s cornlaon t o wall r o c k samples taken from within
the
magnetite
500 f e e t o f
c o n t a .c t. .
S3-1 I )
An amygdaloidal t"tv.rlxey track" andesite, (sample
has r e c r y s t a l l i z e d t o a s p , o t t e d h o r n f e l s a n d
i s especially
.'
. ..
r e v e a l i n g as t o t h e c o n d i t i o n s
o f metamorphism.
Fine-grained
b i o t i t e vrhich permeates the groundmass of
this rock is
i n d i c a t i v e o f potassic metasomatism since
it i s u n l i k e l y
t h a t t h e r e vas s u f f i c i e n t p o t a s s i u m i n t h e o r i g i n a l r o c k
t o form t h i s much b i o t i t e .
centers are recognized
Tao t y p e s of c r y s t a l l i z a t i o n
bg d i f f e r e n t m i n e r a l
c o n c e n t r i cz o n e sa b o u tt h ec e n t e r s .
assemblages i n
From t h e c e n t e r outvrard
the asse~lbla~e:andradi';;e,
~ o l l a s t o n i t e ,f e r r o s a l i t e
.apendix),diopside,nagnetite,biotite
(r7ig. 3 B
i s inferredtohave
o r i g i n a t e d from r e c r y s t a l l i z a t i o n o f h e m a t i t i c c a l c i t e
a u y g d u l e sc o m o ni n
this u n i t a t o t h e r l o c a l i t i e s .
a s s e a b l a g ed i o p s i d e ,i d o c r a s e
and r u t i l e , o l i g o c l a s e ,
magnetite,biotitehasprobablycrystallized
ferromagnesian
phenocrysts.
The
from t i t a n i f e r o u s
Zone r e l a t i o n s h i p as n dt h e
.
'
.
.
d i f f e r e n ta s s e m b l a g e s
are sholvm i n l?+ure
15.
. 74
.
The m i n e r a l
assemblageslistedaboveareind.icativeof'upperhornblende-
1965, p. 59)
h o r n f e l st op y r o x e n e - h o r n f e l sf a . c i e s( W i n k l e r ,
e q u i v a l e n t t o a minimum temperature o f about 550" 2 20 "C
a t 250 bars ( 1 km depth).Anothersampleofthe
%n"cy
3500
track" andesite (SR-l2).mas collectkd approximately
browh
f e e t f r o mt h ee a s t e r nc o n t a c to ft h es t o c k .R e s i n o u s
are v i s i b l e i n h a n d s p e c i m e n
garnet and calcite
filling
as a
Seen i n t h i n s e c t i o n , e p i d o t e o c c u r s
r o u n dv e s i c l e s .
cpncentrationofbladedcrystalsaboutthe
amygdules,Pyroxenephenocrysts
a n dc h l o r i t e .T h i sa s s e m b l a g e
(?>
perimeter o f t h e
are r e p l a c e d by e p i d o t e
i s i n d i c a t i v e of t h e albite-
epidote hornfels facies which begins
a t 380" ?: I O "C a t 250
bars.
Petro,yenes&.
The c l o s ea s s o c i h t i o no fh i g h - t e m p e r a t u r e
and l o w t e m p e r a t u r ef e l d s p a r si nt h e
sane r o c k s ,a l o n g
with
now be
other petrograghic data and field relationships can
i n t e r p r e t e d i n . t e r m s of t h e magmatic h i s t o r y of t h e T r e s
P l o n t o s a ss t o c k .T h e qa r es e v e r a l
a s s o c i a t e d p r o c e s s e s by which an
f i n a l make up.
Processessuch
contaminationand
magmaticand
closely
i n t r u s i o n may a c h i e v e its
as: d i f f e r e n t i a t i o n ,p r o g r e s s i v e
( o r ) a s s i m i l a t i o n ,d e u t e r i ca l t e r a t i o n ,
metasomatism and s i n g l e i n t r u s i o n v e r s u s m u l t i p l e i n t r u s i o n
are evaluated here in
formationofthe
regard t o t h e i r i m p o r t a n c e i n t h e
Tres N o n t o s a ss t o c k .D i f f e r e n t i a t i o n
heresubdiviciedinto
two p r o c e s s e s : c r y s t a l s e t t l i n g ' a n d
is
(or)
zoning,anddiffusionofanalkali-richvaporphase.
The T r e s 1-iontosas s t o c k i s probably a rare s p e c i m e n , o f
'
'75
I
=&re
1.5.
5.0 mm
1
S p o t t e dh o r n f e l s (SR-12) a f t e ra x y g d a l o i d a l
"turkey trac!<" a n d e s i t e of t h e Spears gornation.
Two t y s e s o f c r y s t a l l i z a t i o n c e a t e r s a r e
recognized: ( A ) a f t e r c a l c i t e amygdules,and
( B ) a f t e r ferromagnesianphenocrysts.
5 % ~ lic;:?t
brovm t i n t o f much o f the grGundiJass i s c r e a t e d
by v e r y - f i n e - g r a i n e db i o t i t e .
iii.:ols i n c l i n e d
at 70 degrees.
.
a magmaticproduct
are found.
76
.
i n mhich t r a c e s of t h e o r i g i n a l r e a c t a n t s
The writer d o e s n o t p r e t e n d t h a t t h e i n t e r p r e t a t i o n
which follows i s completely documented n o r a p p l i c a b l e t o
magmaticenvironments.
Tres
However, I b e l i e v et h a tt h e
Nontosas stock i s a c r i t i c a l e x p o s u r e
a11
of a r a p i d l y c r y s t a l l i z i n g
p l u t o n i n which hypabyssal magmatic,processes are evident
becausethey
mere f l f r o z e n l l b e f o r e t h e p r o c e s s e s c o u l d
be
completed.
Tres Nontosas
A model of the magmatic history of the
stock shich summarizes conclusions reached
earlier i n t h i s
is as f o l l o w s :
section,andinothersectionsofthisreport,
1.
The p a r e n t magma, a l k a l i ( ? ) andesite,. (p, 82) was
i n t r u d e d t o a shallom l e v e l , p r o b a b l y l e s s t h a n
p i l e ( p a 56'
500 f e e t from t h e t o p o f t h e v o l c a n i c
..
2.
Volcanic'plagioclaseandgranophyre
>.
intheborder
f a c i e s (p. 62,67) i n d i c a t e c r y s t a l l i z a t i o n of these
rocks uust have been
3.
extremely r a p i d .
Event)-~ough c r y s t a l l i z a t i o n was r a p i d , components
o f a l k a l i f e l d s p a r nere a l s o b e i n g r a p i d l y i n t r o d u c e d
by a n aqueous
vapor
phase
(p.67
This process
began p r i o r t o c o m p l e t e c r y s t a l l i z a t i o n
as i n d i c a t e d
by rims o f o r t h o c l a s e m a n t l i n g p h e n o c r y s t s
a n d e s i n e i n monzonites(and
of high
latitedikes)sometimes
-in a s s o c i a t i o n with e u h e d r a l o r t h o c l a . s e m i c r o l i t e s
i n t h e groundslass (p.
65)*'
4. -Enrichment of t h e rnakma w i t h p o t a s h c o n t i n u e d p a s t
t h e time o fc o m p l e t ec r y s t a l l i z a t i o n .B o t h
roclrs(p.
73) and
bel-der
f a c i e sr o c k s( p .
wall
75) viere
77
metasonatized by
5.
a potash-rich aqueous phase.
(p. 59),cross-
C o n t a c t sw i t h i nb o r d e rf a c i e sr o c k s
c u t t i n gr e l a t i o n s h i p s( p .
62, 613), a n d t h e d i s t r i b u t i o n
of plagioclasecompositions(p.
mulki.ple i n t r u s i o n .
6 6 ) 3.11 s u g g e s t .
Hoviever, t h er a n g ei np l a g i o c l a s e
composition (p. 60, 65, 693 f o r -b ' o r d e rf a c i e sr o c k s
a l s os u g g e s t st h a tf r a k t i o n a lc r y s t a l l i z a t i o np l a y e d
a
less i m p o r t a n t role i n t h e developmentofthemajorrock
.
'
typesthandidaqueousdiffusion.
6.
Contmcinationofthe
melt occurred adjacent
less i n d u r a t e d wall r o c k s b u t t h e p r e s e n c e
andesine rims aroundxenocrysts
t o some
of
o f o l i g o c l a s e( p -
64)
i n d i c a t e s t h e r e was l i t t l e chance f o r a s s i m i l a t i o n ,
7.
C r y s t a l l i z a t i o n b e g a n a t t h e border and moved
towardsthecore.
The g r a n i t i cc o r ec r y s t a l l i z e d
last w i t h r e l a t i v e l y h i g h water p r e s s u r e s being t h e dominant
f a c t o r i n t h e i n v e r s i o n of h i g h - t e m p e r a t u r e f e l d s i a r s
t o l o w - t e m p e r a t w ep l a g i o c l a s ea n do r t h o c l a s e
That t h e s e m i n e r a l s c r y s t a l l i z e d i n i t i a l l y
.temperaturephases
(p. 6 9 )
a s kigh-
i s i n d i c a t e d by t h e presenceof
s a n i d i n e i n some g r a n i t e s (p. 69).
8.
The g r a n i t e core e i t h e r formed i n s i t u by t h e a c t i o n
of the aforementioned process
o r by l a t e r i n t r u s i o n ,E v i d e n c e
of a l k a l i e n r i c h m e n t
f o r fractional
c r y s t a l l i z a t i o n i s l a c k i n g .P l a g i o c l a s e
i s unzoned
and similar i n composition t o monzonites o f t h e
b o r d e rf a c i e sr o c k s
(p. 69).Thusformation
in
s i t u by t'aqueo-a2.kalitt enriclment appears
t o be,
'
?G
a l i k e l yp r o c e s s .F l o a t i n go fq u a r t zx e n o l i t h s
d e r i v e d from qua.rtz-rich
wall r o c k s a t depth may
be a. majorsourceofqv.artz
a h do t h e r s ,
in the, granite (Chapin
i n preparation),
The role of vrater I n t h e d i f f e r e n t i a t i o n o f magmas h a s
b e e nd - i s c u s s e db ys e v e r a li n v e s t i g a t o r s .
Kennedy ( 1 955)
presented a detailed theoretical discu.ssion of
andOrville
i t s importance
(1963) ad.ded some experimental data i n t h e s y s t e m :
alkali f e l d s p a r s , alkali c h l o r i d e s ,a n d
vrater.
Orville
(1963, p. 201 ) c o n c l u d e s t h a t tlalBali metasomatism will take
n a t u r a lc o n s e q u e n c e
as a
of an alkali-bearing vapor phase
place in the presence
of temperature, pressure andcompositional
g r a d i e n t si nt h eE a r t h ' sc r u s t " ,S e v e r a la u t h o r sh a v er e p o r t e d
evidence o f t h i s p r o c e s s i n t h e
1962; h d e r s o n , 1948)
pointthatthesource
magma i t s e l f .
Boolzs
f i e l d ( C a t e r , 1969; lioone,
.
Boom (1962,' p. 1474) s t r e s s e s t h e
o f potassium i n this p r o c e s s e s i s t h e
p a '14.52) a l s o p o i n t s o u t t h a t
one o f '
t h e m a j o r unkuolms is t h e r a t e of t h i s d i f f u s i o n p r o c e s s .
Q u a l i t a t i v e l y , t h e Tres 14.ontosa.s s t o c k i n d i c a t e s t h a t t h e
uprmrd d i f f u s i o n o f
an ltaqueo-alkali" phase tovrards the top
of a magma chamber i s r e l a t i v e l y much faster t h a n l i q u i d solid fractionation processes.
Post-OligoceneRocks
Tertiary-Quaternary Gravels
Two t y p e s of v o l c a n i c - r i c h f l u v i a l s e d i m e n t a r y r o c k s a n d
g r a v e l s are recognized i n t h e s t u d y a r e a a l t h o u g h u n d i f f e r e n t i a t e d
on t h eg e o l o g i c
map;
The o l d e rt y p e
i s found i n f a u l t c o n k c t
.79
.
\Lath Oligocene volcanic
roc!rs a t Council Rock a n d e a s t o f
t h e Tres Kontosas stoc!r,
I t c o n s i s t s ofmoderately--vrell-
cerpented,pink-to-tan,sandstones
4.nd conglomerates,
The
congloaerates contain abundant sub-rounded clasts recogniza.ble
as d e r i v e d f r o 3 t h e A-L Peak and Hells Hesa Formations.'
I.Iany clasts are hydrothermally a l t e r e d .
Andesiticrocksform
a very minor volune of the clasts,
Excellentoutcropsof
this s e r i e s are found i n t h e
southernmosttributaryoftheCouncil
i n t h e s e roc!& d i p s t o t h e v e s t
Rock Arroyo,Bedding
a t about 10 t o 15 degrees,
T h i s d i p Bust be r e l a t e d t o r o t a t i o n a l rnovernent of basin and
range f a u l t b l o c l c s s i n c e p e b b l e i m b r i c a t i o n s i n d i c a t e t h a t
t h e d i r e c t i o n o f sedimenttransport
vas t o t h e e a s t - s o u t h e a s t .
An angular unconformity between t h e s e r o c k s ,and youlger
.
psdirnentgravels
,
i s assumed,
in a
Outcrops of block-faulted pediment gravels occur
narrow graben trend.ing northeastv~ard from
stock,
t h e Tres Montosas
T h i s s t r u c t u r e i s one o ft h ef e wi n d i c a t i o n so f
n c r t h e a s t - s t r i k i n g basin and range f a u l t s I n the s t u d y area,
Outcrops o f pediment gravels
also occur. on the doun-'faulted
side 'of transverse faults about
Springs.
14 miles s o u t h o f G a l l i n a s
For t h i s r e a s o n a Ifioceneoryoungerage
is
i n f e r r e d f o r t h e s e f a u l t s ...
Large cobbles of magnetite
t h e east endof
are abundant i n g r a v e l s a t
the narsovgrabendescribed
above,
Thtlir
s o u r c e is i n f e r r e d t o be i n c o n t a c t m e t a s o m a t i c i r o n d e p o s i t s
of t h e Tres Xontosas stock such
as found a t , the Eig John mine,
A few bould.zrs o f g r a n i t i c r o c k s . a s s o c i a t e d w i t h t h e m a g n e t i t e
~.
80
I t i s asr;umed t h a t t h e s eg r a v e l s
h e l pc o n f i r mt h i ss o u r c e .
at t h e mouth of a s t e e p ,
represent alluvial fan deposits
structurally controlled northeast-flowing drainage.
lov: primary ( ? > 'dips o f a b o u t
The younger gravels have
.
5 degrees which are generally .to the east in agreement
are
s e d i m e n t a r yt r a n s p o r td i r e c t i o n s .' Y o u n g e rg r a v e l s
cobbles m a t h e r e d o u t
as f l o a t .
c l a s t s is generzlly the
.
t o outcrops o f
r e c o g n i z a b l e by t h e i r o n l a p p i n g r e l a t i o n s h i p
Oligocene u n i t s andbyabundant
vnth
r i n d s of white c a l i c h e on
The dominant l i t h o l o g y of
same as t h a t o f nearby outcrops
of
bedrock,.
TertSary-Quaternary .Ba.salt
O l i v i n e basalt occurs as d i k e s a n d flovrs c u t t i n g a n d
cappingpediraentgravels
inthethesis.area;
dike i n t h e area
is thenorthernextensionofthelongest
i7hich trends north-northwest bver
C,ouncil Rock
a l e n g t h of about 13 mi1e.s.
$he, basalt dikes have two major t r e n d s : n o r t h t o n o r t h -
northwest and n o r t h e a s t ,
northeast-trending
The g r e a t e r p o r t i o n
dike i n t h i s v i c i n i t y c r o p s o u t
east of t h e s t u d y a r e a .
of a b o u t 3000 f e e t .A n o t h e rs e r i e s
in en echelon pattern along
a n o r t h e r l y d i r e c t i o n i n an
of I r o n Mountain.
b eP l i o c e n e .S e v e r a lo u t c r o p s . o fn e a r l yh o r i z o n t a l
flows of
for a d i s t a n c e
of basalt d i k e so c c u r
The basalts have n o t b e e n d a t e d b u t ' a r e
0
just
It i s exposed f r o m i t s westend
( n o r t h e a s t o f Iron Kountain) northeastward
area southviest
o f theonly
assumedhere
to
basalt
similar composition and occurrence' have been mapped
.
81
1972) on t h eX a g d a l e n aP l a i n
by Chapin (oral conmun.,
s o u t h e a s t o f Tres Montosas.
Olivine basalt f l o w of this
basalt ofCouncil
type have been informally termed the
Rock by C,E.
Chapin f o r t h e i r on1.y
sourcearea.
from d a r k gra.y t o b l a c k i n c o l o r
F a d specimens range
and a r ed e n s e
~CIIOYM
t o vesicular,Finegrainsofreddish-bropm
i d d i n g s i t e , pseudomorphs a f t e r o l i v i n e , a r e t h e o n l y
phenocrysts readily visible
therock.Vesicularspecimens
calcite,
2 t o 3 percent of
andcomprise
may c o n t a i na n y g d a l o i d a l
Viev;ed u n d e rt h em i c r o s c o p e ,l a b r a d o r i t e
t h e major phenocrysticphase
i s e v i d e n t as
(-10 p e r c e n t ) .P l a g i o c l a s ep h e n o c r y s t s ,
2 t o 5 mm in l e n g t h , a r e n o r m a l l y z o n e d ;
one rceasured An6,
t o An
a t t h e rim. O l i v i n e - a l s oe x h i b i t s
k8
zoningwhichappearc
t o be o s c i l l a t o r y .O l i v i n ep h e n o c r y s t s ,
i nt h ec o r e
-
. ,.
from 0-3 t o 1.5 r n ~a c r o s s
mostly altered t o iddingsite, range
and appear t o be magnesium r i c h (Fo
90'
2V= 90"
2" )
Sparse
also occurs as phenocrysts,
neutral-gray clinopyroxene
O p t i c a l data (2V,=' 50" 2 2" , ZAC= 38" ) i n d i c a t e s t h i s i s
endiopside.
Groundmass t e x t u r e s a.re p r e d o n i n a n t l yt r a c h y t i c
t o i n t e r s e r t a l ~ A t h a n d e s i n e (An ) thedonlnant'phase.
46
G r a n u l a rm i n e r a l s ,i d d i n g s i t e ,c l i n o p y r o x e n ea n dm a g n e t i t e ,
c o n p r i s e about 25 p e r c e n t of t h e mat2:ix.
Kuno (1959, p.
45)
hasobserve'dthatstronglyzoned
o l i v i . n e ,t i t a n i f e r o u s - a u g i t e ,d i o p s i d i c - a u g i t e
Kg-
and t h e
absence of r e a c t i o n c o r o n a s
around o l i v i n e ars c h a r a c t e r i s t i c
of a l k a . l i - o l i v i n e basalts,
B a s a l t i c a n d e s i t e s and a n d e s i t e s
of theSpearsFormation,
A-L Peak F o r n a t i o n , a n d e s i t e o f
Lmdavaso Reservoir am? t h e T r e s , M o n t o s a s s t o c k
anr! t h e
'
8‘
basalt o f Council Rock a l l possess one o r more of t h e s e .
ferronagnesian minerals characteristic
magma.
of a n a l k a l i c parent
F e r r o m z g n e s i a nm i n e r a l sc h a r a c t e r i s t i co ft h o l e i i t i c
basalts (Kuno, 1959 p. 42) were notobserved‘inanyrocks
o f t h e s t u d y area.
writer t e n a t i v e l y ’ p r o p o s e s
T h e r e f o r et h e
a n a l k a l i c p a r e n t magna. for the “DatiYt v o l c m i c rocks.
(1 970, p. 2347) i n d i c a t e a n . a l k a l i c p a r e n t
Lipman a n d o t h e r s
nagma f o r v o l c a n i c r o c k s o f t h e S a n J u m I4ount.a.iins based on
o f a similar magma g e n e s i s
numerous c h e a i c a la n a l y s e s .P r o o f
for.t h e Datil v o l c a n i c f i e l d avraits s u f f i c i e n t c h e m i c a l
analyses,
Quaternary Deposits
Talus and Colluvium,
A few small d e p o s i t s of c o a r s e
a n g u l a r debris d e r i v e d from Inass wasting vtere mapped i n
t h e stud.y area,
Suchdepositsoccur
a t the base of c l i f f - -
f o r m i n gu n i t ss u c h
as t h e Hells &sa Formation, t u f f o f
NipplePiountainand
a l a t i t e porphJryplug.
.
The rari’cg of
t h e s e d e p o s i t s i s a t t r i b u t e d t o t h e lov r e l i e f I n t h e a r e a .
The two larger d - e p o s i t s o c c u r v h e r e r e s i s t a n t u n i t s , u n d e r l a i n
by weak u n i t s , are c u t b y major d r a i n a g e s , S o i l a n d v e g e t a t i o n
h e l p s t a b i l i z e t h e upper p o r t i o n s o f t h e s ed e p o s i t s .
PleistocenetoHolocene
Eolian
Sand.
A
age i s i n f e r r e d .
A ’ t h i nv e n e e r
most of the s o u t h v e s tq u a d r a n t
v~ii1d.s a n d t h e d i s t r i b u t i o n
o f vind-blovm sand b l a n k e t s
o f t h e t h e s i s area.
Prevailing
ofthesesandsindicatestheir
s o u r c e i s i n t h e d r y bed o f a P l e i s t o c e n e lake, now known as
t h e P l a i n s o f SanAugustin.
The e o l i a n s a n d s a r e c l e a r l y
d i s c e r n a b l e on a e r i a l p h o t o s as ILglzt gray a r e a s , d o t t e d by
Itrater
largetrees(Ponderosapines).Shallowground
available in these 'sands
t h i c ks t a n d s
is evidentlycapable
o f Ponderosa,
of s u p p o r t i n g
a.ge is
A P l e i s t o c e n et o . R e c e n t
i n f e r r e d f o r theeoliansands.
Alluvium.Recentstreamgravelsfillingmajordrainages
o f alluviun.Weathered,
a r eh e r eg r o u p e du n d e rt h et i t l e
dark-gray s o i l s o f p r o b a b l e P l e i s t o c e n e a g e , a l s o
valley fill, are
found. as
in
groupedwiththecoarsermaterialsfound
i s contemporaneous i n p a r t w i t h t h a t
streambeds,Theirage
of theeoliansand,
Petrogenesis
Oligocehe
Igneous
Rocks
a g e n e t i c . model w i t h
B r o m (1972, p. 74) has presented
regard t o the crystallization history and
f o r two major- a s h - f l o w s h e e t s i n t h e
mode o f . e r u p t i o n
Magdalena a r e a : H e l l s
..
Mesa F o r m a t i o n ( t u f f
oP Goat Spring) and
( t u f f of BearSprings).
A-L Peds Bornation
The s f r i t e s p r o p o s e s t h a t c o m p o s i t i o n a l
data for feldspars in volcanic rocks
of t h e s t u d y a r e a a n d
conclusions o f - a petrogenetic nature reached in earlier
s e c t i o n s of t h i s r e p o r t c a n
be u s e d t o mdxe a d d i t i o n s t o
B r o m s model
The use o f c o e x i s t i n g p l a g i o c l a s e and alkali f e l d s p a r
pairs in igneous rocks
temperature of
1962,1968).
as a geothermometer t o e s t i m a t e t h e
c r y s t = i L l i z a t i o n has beensuggestedby
Bar-ih (1951,
I n his 1968 p u b l i c a t i o n ,B a r t hp r o v i d e s
a graph
(p. 306, f i g e 1 ) by mhich t h e e q u i l i b r i u m c r y s t a l l i z a t i o n
temperature o f f e l d s p a r p a i r s m a y b e c a l c u l a t e d t o
2 50°C
on t h e basis o f cheiaicalanalyses
o f t h ef e l d s p a r s .T h i s
w r i t e rh a su s e dt h i sg r a p h
e x t r a p o l a t i o n .a l o n gw i t h
and
. 84
OPtical-ly determined compositons of feldspar pairs
Hells &sa and A-L Peak a s h - f l o w t u f f s
in the
t o i n d i c a t e a trend
of crystallization temperatures for these volcanic units.
Accurateestinates
of thesetemperaturesarenotpossible
from t h e a v a i l a b l e
d a t a because o f t h e s i g n i f i c a n t error
".
involved in estimating 'the composition
and p l a g i o c l a s e f r o m o p t i c a l d a t a .
A s d i s c u s s e di nt h e
e r r o r f o r opticallydetermined
appendix(p.125)theprobable
alkali feldspar compositions
O r , and ?:
o f alkali f e l d s p a r
i n v o l c a n i c r o c k i s 2 20 p e r c e n t
3 p e r c e n t An f o r p l a g i o c l a s e .
This
i n c r e a s e s the
p o s s i b l e e r r o r range f o r c r y s t a l l i z a t i o n t e m p e r a t u r e e s t i m a t e s
t o as much as 2 225°C.
CrystallLzation temperatures calculated under the
r e s t r i c t i o n s d e s c r i b e d a b o v e do kow?ver
..,. s u g g e s t a trend i n
agreement vith the cormonly accepted relationship, namely
t h a t t h e more s i l i c e o u s a magma t h s l o v e r i'c's c r y s t a l l i z a t i o a
temperature,Compositional
data a n dt h ec a l c u l a t e de q u i l i b r i u m
crystallization tempsratures
Nesaand
of f o u r sanp%cs fromthe. Hells
A-L Peak Forrnations are l i s t e d i n Table 2.
ofthelarge
Because
errors involved,thecalculatedcrystallization
temperatures should only
be i n t e r p r e t e d w i t h r e s p e c t t o
the
trend they indicate and not with respect to their absolute
values.
In the discussion
o f t h e Tres Plontosas stock i t vas
suggested that differentiation of alkali
g r a n i t e byupward
(?) a n d e s i t e i n t o
d i f f u s i o n of a potassium-rich aqueous vapor
phase i s a v e r yr a p i da n de f f i c i e n tp r o c e s s .R e c e n t l y
andFriedman
(1974) haveconcludedfromoxygen
Lipman
isotope data
0
L
C
.
'
0
0
0
0
3 %
c
k
0
LnK
k
0
o n
O
N
N(\I
"0 "0
n n
v"
coo
0
fi*
I b L n
kM
0
(u
:
P
I
c .
W
c
85
.
.
that compositionally zoned ash-flox sheets
i n Nevada r e p r e s e n t
a %ajar i n t e r a c t i o n of meteoric ground vrater a n d b a t h o l i t h sized.bodiesofsilicic
important observation,
ma.gna p r i o r t o esup.tion".
This
plus c o n c l u s i o n s r e a c h e d n
i earlier
s e c t i o n s of this report.and
the t r e n d . o fc r y s t a l l i z a t i o n
temperatures (Table 2) are nowcombPned
.
int'o t h e following
i n t h e 1~b.gdalena .
petrogeneticmodelforOligoceneigneousrocks
area:
1,
I n i t i a l magmatic a c t i v i t y began i n t h e mantle w i t h
o f alkali o l i v i n e
theformationoflargevolumes
basalt (?) o r a l l c a l i (?) a n d e s i t e (Kuno,
1959; A o k i
and O j i , 1966).
2.
D i a p i r i c r i s e o f t h i s body i n t o t h e lower c r u s t t h e n
followed, Some f r a c t i o n a t i o n o c c u r r e d b e f o r e
or
.. ...
duringtheascent
t o produce intermediate-cornposition
magmas which were enlplaced as b a t h o l i t h i c b o d i e s .
was the e x t r u s i o n
The s u r f a c e e x p r e s s i o n o f i n t r u s i o n
ofvoluminous
allcali ( ? ) a n d e s i t e s and d e r i v a t i v e
l a t i t e s t o form the Spears Formation.
3 . At some l a t e r time, t h e r i s i n g u p p e r s u r f a c e
of t h i s
b a t h o l i t h r e a c h e d a l e v e l ' o f abundant meteoric water
(LipDan, 1974) and. large volumesof
i n c o r p o r a t e dt h r o u g h o u tt h em e l t
water w r e
by d i f f u s i o n .
tkis o c c u r r e d , t h e u p p e r s u r f a c e p r o b a b l y
As
mushroomed
(Thorn, 1955) to. form a v i d e , s h a l l o w nlagma c'hamber
i7hich i n c r e a s e d t h e r a t e o f i n c o r p o r a t i o n o f
into the relatively dry
4-
vater
a l k a l i c magma,
A s water d i f f u s e d i n i o t h e
chamber, SiOz, a l k a l i e s ,
.
..
trace metals, and volatile eiements (initially part
of t h e n e l t ) were p r e f e r e n t i a l l y p a r t i o n e d i n t o
thevaporphase
5.
(Kennedy,1955)
I n .r e s p o n s e t o a n o r m a lp r e s s u r eg r a d i e n tt h ev a p o r
phase moved towards t h e t o p
of t h e magma chamber
(Kennedy, 1955; Boone, 1962; O r v i l l e , . 1963)
potassium and,
near the top
Thus
t o a lesser e x t e n t , . s i l i c a a r e c o n c e n t r a t e d
of t h e magma along with the vapor
t o for;a
vapor-rich a l k a l i r h y o l i t e magna,. The magma then
started to cool
at t h e t o p
o f t h e chamber i n response
t o thenormalgeother~nalgradientbutcrystallization
vias
Sl017
t o beg'ln b e c a u s e t h e i n c r e a s i n g w a t e r c o n t e n t
depressed the liquidus temperature
(Kennedy,1955)
e
Thus a marked t h e r m a l g r a d i e n t d e v e l o p e d v e r t i c a l l y
i n t h e magma as a r e s u l t o f - t h e c o m p o s i t i o n a l
6.
gradient.
C r y s t a l l i z a t i o nb e g a na l m o s ti m m e d i a t e l yn e a rt h e
base o f t h e c h a n b e r b e c a u s e t h e d i f f u s i o n o f v o l a c i l e s
vias resoiring K20 a n d S i 0 2 t h e r e b y r a i s i n g t h e l i q u i d u s
t e m p e r a t u r eC
, r y s t a l - l i q u i df r a c t i o n a t i o nt h e n
became a n i u p o r t a n t d i f f e r e n t i a t i o n p r o c e s s i n t h e
1OWOS
p o r t i o no ft h e
chamber,
However, s i n c e
crystallization did not begin until
t h e t o p of the
much l a t e r a t
cfiamber i t was n o t a s i g n i f i c a n t
..
processthere,
7.
A t thispoint,thelatterpart
ofBrown's
'p. 74, p a r t 2 , 3 and 4) c o n c e r n i n gt h e
model ( 1 972 y
mechanisms o f
eruption of crystal-rich quartz latite and crystal.p o o rr h y o i i t e
i s p e r t i n e n t :I n
summary Ehgy a r e :
58
a.
. .
Fissures t a p p e d t h e lower level. of t h e magiia chanber
r e s u l t i n g i n eruption of a r e l a t i v e l y h o t , ( c f .
Table 2 ) c r y s t a l - r i c h , q u a r t z - l a t i t e a s h - f l o w s h e e t
o f t h e magma
(Hells.MesaFornatim).Draining
c h m b e r , from t h i s l e v e l u p u a r d s , p r o d u c e d
a
t h a t i s "upside
c o n p o s i t i o n a l l y zonedash-flow
d o m " i n comparison t o most compositionally
z o n e da s h - f l o w
(Liljman and o t h e r s , 1966; Ratte'
a n d Steven, 1964).
b.
L a t e r ,a n o t h e r
set offissuresintersectedthe
upper p o r t i o n of t h e nagma chamber which peruritted
the eruption of
a relatively cool crystal-poor
(A-L Peak Formation)
rhyolite ash-flow sheet
C.
e
Frox t i n e t o time t h e first s e t o f f i s s u r e s mere
reopned to permit almost .simultaneous eruption
of crystal-rich
8.
and c r y s t a l - p o o r . a s h flow.
After e r u p t i o no ft h ea s h - f l o ws h e e t s ,a l k a l i - a n d e s i t e
magnas c o n t i n u e d t o aove a p v a r d s i n t o t h e v o l c a n i c
pileto
form h y p a b y s s a li n t r u s i o n s .
I n t h es t u d y
a r e a ,t h eT r e sM o n t o s a ss t o c k ,a n d e s i t i c ,l a t i t i c
an e a s t - t r e n d i n g r h y o l i t e d i k e
'
and
formed at. t h i s time.
Erosion of the r h y o l i t ed i k et o
different verticallevels
has r e v e a l e d c o 3 p o s i t i o n a l v a r i a t i o n s i n d i c a t i n g s t r o n s
c o ~ p o s i t i o n a l . g r a d i e n t s were reiewed i n t h e younger
s h a l l o v ri n t r u s i o n s .
It i s assumed t h a t e v e n t s s i u i l a r
t o t h o s e as s t a t e d i n s t e p s I t o
r e p e a t e dt op r o d u c e
6 o f t h i s model
t h i s relationship.
s t o c k was i n t r u d e d t o s u c h
mere
The T r e s PIontosas
a h i g h level t h a t e v i d e n c e o f
t h e p r o c e s s o f allraki.-enric'ment
was lefrozenlf i n t o t h e rocks.
STRUCTURE
Regional
'
..
Structu.re
A detailedanalysis
in the
s o u t h v e s t e r n New Hexico has not yet been reported
l i t e r a t u r e .E p i s o d e s
in
of r e g i o n a l s t r u c t u r a l p a t t e r n s
o f a t l e a s t 2, a n d . p r o b a b l y 3 , s t a g e s
o f middle and l a t e C e n o z o i c f a u l t i n ; a r e a p p a r e n t i n t h e
s t u d ya r e a .K a p p i n go u t s i d et h es t u d ya r e ai n d i c a t ' e s
additional
deformational periods during the Late Cretaceous-early Eocene
(Laramide)and
i nt h ee a r l yO l i g o c e n e .U n r a v e l i n gt h i s
conlplex
s t r u c t u r a l p a t t e r n i s made even more d i f f i c u l t by t h e p r e s e n c e
and a l a r g e p r o p o r t i o n o f
o f two r e g i o n a l u n c o n f o r m i t i e s
s u r f i c i a lc o v e r( C h a p i n ,
i n p r e p a r a t i o n ) .I n
and o t h e r s ,
. c h r o n o l o g i c a lo r d e r ,m a j o rt e c t o n i ce v e n t s
vrhichhave
taken
p l a c ei nt h er e g i o n( p o s t - C r e t a c e o u s )a r e :L a r a m i d ef o i d i n g
.
. ..
a n d u p l i f t ,e a r l yO l i g o c e n et r a n s v e r s e
O l i g o c e n el o n g i t u d i n a l
(NM'I) f a u l t i n g , b a s i n a n d r a n g e
t r a n s v e r s e (XE) f a u l t s , b a s i n
faults.
(WNW)f a u l t i n g , l a t e
(N)
and r a n g e l o n g i t u d i n a l
The last tmo o v e r l a p i n t i m e
and space.
According t o H ~ n l ; ' . ( 1 9 6 7p.
~ 31 1 ), t h e Magdalena r e g i o n
i s p a r t o f t h e PiexicanIiighlandssection
Range province.
haslongbeen
o f the Basin and
The s t r u c t u r a l framework o f this province
a t o p i c o f c o n t e n t i o n among g e o l o g i s t s .S t c v r a r t
( 1 971 ) s u m m a r i z e s r e c e n t l y a v a i l a b l e g e . o p h y s i c a 1 a n d g e o l o g i c a l
data.
He i n t . e r p r e t s t h e b a s i n a n dr a n g es t r u c t u r e
of h o r s t s and grabensformedover
substratucl.
Xackin(1960,
accepted origin related
as a s e r i e s
a plastically extending
p. 108) p o s t u l a t e sa n o t h e r
t o tilted fault blocks
conmonlg .
formed i n a
90
t e n s i o n a l environment.Bothoriginsappearcompatiblevith
thestructure
(-29 m,y.)
A l a t e Oligocene age
o f t h e Magdalenaregion,
b a s h and r a n g e f a u l t i n g i n
fortheonsetof
New
Mexico i s r e p o r t e d by C h r i s t i a n s e n a n d Lipman (1972) and
Chapin ( o r a l commun.,
.
1974).
,are
In t h e N a g d a l e n a r e g i o n , l a t e r , p e r i o d s o f f a u l t i n g
o v e r p r i n t e d on a L a r a m i d eu p l i f t ( s ? ) .
E r o s i o n i n late-Eocene
time beveled this u p l i f t .to a s u r f a c e o f . s u b d u e d r e l i e f
(Chapin, 1971 b, p. l9Lt)
The EacaFornati.on
i n an east-trending basin
was d e p o s i t e d
t o t h e n o r t h of t h e s t u & area as
a r e s u l t of this erosion(Snyder,1971).
The Council Rock
d i s t r i c t i s l o c a t e d on t h e t r a n s i t i o n b e t w e e n u . p l i f t
and ba.sin,
Locally,aneast-northeasttrendappearslike1.yforthis
p o r t i o n of t h e u p l i f t ;
meant t o implyan
hovrever, t h i s o b s e r v a t i o n is n o t
.
. ,.
east-west t r e n d f o r t h e u p l i f t .
interpretation that the region
.svide n o r t h - p l u n g i n g . u p l i f t
is
OII
The
t h e broad noseof
a
is equally justifiable,
Larami-de s t r u c t u r e s v i s i b l e t o t h e n o r t h on t h e Colorado
P l a t e a u are d e s c r i b e d ' as broadopen
f o l d s and ,thrustbelts
(Relley and Wood, 1946; Tonlring, 1957; KelleyandClinton,
1960).
Most e a r l y i n v e s t i g a . t o r sh a v ea t t r i b u t e dt h e s e
features to regional compression during
e a r l yT e r t i a r y
time.
1960, p. 118; Tho*,
force of
l a t e Cretaceous and
O t h e r s (Eardley, 1962, p. 402; Mackin,
1955, p. 369)suggest'chatthebuoyant
a r i s i n & body of magma i s also a r e a s o n a b l e mechanism
t o form broad u p l i f t s ,
The importanceofgxtsvitysliding
t h e flanks o f ' s t e e p u p l i f t s t o p r o d u c e t h r u s t f a u l t s
d e n o n s t r a t e d by P i e r c e ( 1 957).
on
is
.
.
.
. 91
A major est-northmest-trending f a u l t c r o s s e s t h e
Magdalena Nountains a t North Baldy and crosses the Gallinas
of Highway 60 (Chapin and others,
uplift in the vicinity
Its l o c a t i o n on t h e G a l l i n a s u p l i f t
inpreparation),'
i n f e r r e d l a r g e l y from aeromagnetic and su.bsurface
North o f t h e f a u l t ,
a paleo-togographichigh
\?ilkinson, o r a l commun.,
composedof
1 9 7 2 ) ,S o u t ho ft h ef a u l t ,
o r Paleozoic age
carbonaceous siltstones of Fiesozoic
i n t e r s e c t e d i n a d r i l l hole(BannerMining
1300 f e e t( C h a p i n ,
data.
o u t from beneath %he SpearsFormtition
the Abo Formation crops
-(YI,H.
is
o r a l commun.,
rests on pre-volcanic rocks
nere
Company) at about
1972).SincetheSpears
,at b o t h l o c a l i t i e s , an a b r u p t
lomer Spears Formation may be
change' i n t h i c k n e s s o f t h e
the t u f f o f Nipple.Mountainacros.s
inferred.Continuityin
..
the fault implies that
....
mostmovement'hadceased
by that tiroe,
The i m p o r t a n c e o f a n c i e n . t s t r u c t u r a l t r e n d s i n c r y s t a l l i n e
basement rocks should
n o t be underestimated
a n a l y s i s .R e a c t i v a t i o n
of zonesofvreakxess
(1970)
i n basement
i s a. comonly accepted concept.
r o c k s by younger tectonism
Elston
i n any s%ructural
hasobservedevidence
f o r f r e q u e n tr e a c t i v a t i o n
of Late Greataceous o r o l d e r f a u l t z o n e s w i t h
He n o t e s t h a t v o l c a n i c r o c k s
a north trend.
of t h e PiogollonMountains a r e
d i s t i n c t l y d i f f e r e n t from those of the western Black
and i n t e r p r e t s t h e c a u s e
Range
of this d i f f e r e n c e as a. north-trending
with
structural-topographic barrier approximately continous
thG SaEta Rita/Hanover axis (op:
(1973, p. 2 2 6 0 )d e s c r i b e st h e
the surface expression
c i t , , p. A-VI-2).
Elston
Mogollon volcanicprovince
of a najor mid-Tertiary batholith.
as
It seems reasonable t o t h e n r i t e r t h a t a d j u s t m e n t s i n t h e
thin crust between the top
of such a b a t h o l i t h and t h e
s u r f a c e n-ould be necessary
i n response t o clianges i n magna
of n e w l y e r u p t e d m a t e r i a l .
pressureandtheadditionalweight
P r e - e x i s t i r g basement f a u l t s are t h e most l i k e l y p l a c e s f o r
compensatingadjustments
t o occurr
contenporaneous with eruption
the Kagdalena area mill
evidence f o r f a u l t i n g
of major a s h - f l o w . s h e e t s i n
be discussed under the heading:
OligoceneTectonics,
'&m
Structure
I
The r e l a t i o n s l . i p o f s t r u c t u r a l t r e n d s i n t h e s t u d y area
t o m j o r s t r u . c t u r e s of ' t h e 14s.gdalena r e g i o n i s i l l u s t r a t e d
by f i g u r e 16,
thestudy
P.70
dominant f a u l t t r e n d s a r e r e c o g n i z e d i n
area: l o n g i t u d i n a l f a u l t s g e n e r a l l y t r e n d n o r t h
(north-northvest t o north-northeast) and
most t r a n s v e r s e
A minor west-
f a u l t st r e n dn o r t h e a s tt oe a s t - n o r t . h e a s t .
northsresttrend
i s a l s o apparent.Recurreat
movement of
t h e s e fazlt b l o c k s a t d i f f e r o n t t i m e s h a s p r o d u c e d
f a u l t p a t t e r n ,S t r o n gd e f o r m a t i o n
a complex
of wall r o c k sa r o u n dt h e
Tres N o n t o s a s s t o c k have g r e a t l y i n c r e a s e d t h e s t r u c t u r a l
complexity i n this area.
S e v e r a lg e o l o g i c
and t o p o g r a p h i c f e a t u r e s
o f t h eC o u n c i l
Rock d i s t r i c t , a n d t h e G a l l i n a s u p l i f t i n g e n e r a 1 , a p p e a r
anomalous i n .comparison t o o t h e r b l o c k u p l i f t s i n t h e H a g d a l e n a
region.
Erst and m o s t n o t a b l e of t h e s e f e a t u r e s is a n a r r o n
zoneoftfgrabenmardt'dippingvolcanic
strata which follows
t h e e a s t edge of the Gallinas .uplift
from Abbey S p r i n g s t o
.. . .
M g u r e 16.
Piajor s t r u c t u r a l f e a t u r e s i n t h e
Magdalena reg-ion
w i t h r e s p e c tt ot h e s i s
area. Exposed o r i n f e r r e d
s t o c k sw i t h i n thesis area: 1, Tres Wontosas;-2,
C o u n c i l Xoclr; j , GallinasSprings.
. 94
from 30 t o 80 degrees i n an
Cat 1-lountain.Dipsrange
most d i p s n e a r
e a s t e r l yd i r e c t i o nx % t h
GO degrees,
The
i s a pronounced s t r u c t u r a l asymmetry
second of t h e s e f e a t u r e s
t o the Gallinas horst vith the structurally highest portion
i m m e d i a t e l y a d j a c e n t t o t h e e a s t boundaryof
P1. I ) ,
s t r u c t u r e s e c t i o n D-D'
the uplift (see
A thirdfeature
doming of vrall rocks around the Tres Montosas
i s thhe stlaong
skock accompanied
by c o n c e n t r i c faults dormthrom toward the stock and numerous
radial faults.
A l s o , v e r t i c a l t o overturnedd.ipsoccur
locallyintheareaeast
o ft h e
Tres Montosas s t o c k ,F o u r t h ,
p r o g r e s s i v e l y o l d e r strata a r e e n c o u n t e r e d a l o n g
a regional southerly
south traverse in spite of
units,
a north to
dip t o the
over most o f t h e
PAd f i n a l l y ,t o p o g r a p h i cr e l i e f
u p l i f t i s q u i t e small i n comparison t o. i t s s t r u c t u r a l r e l i e f ,
T h e s e . f e a t u r e s s u g g e s t a complex d e f o r m a t i o n a l h i s t o r y
i
f o r t h ea r e ai n v o l v i n g
However
many periodsoftectonism.
the vrriter b e l i e v e s t h e y c a n
be i n t e r p r e t e d l o g i c a l l y as t h e
r e s u l t of t v o d i s t i n c t l y d i f f e r e n t t e c t o n i c e v e n t s .
m a y be summarily described
'
The o l d e r
as magmatic u p l i f t a n d i n t r u s i o n
.171Lich t o o k p l a c ed u r i n gt h em i d d l et ol a t eO l i g o c e n e .T h e ,
younger i s a t t r i b u t e d t o block faui:king of
thhe basin 'and range
s t y l e .D e t a i l e dd i s c u s s i o no ft h e s ee v e n t sf o l l o v r s s
OligoceneTectonhs
Faulti1x'.Deformation
inthe
formofdovm-to-the-Mest,
north-trending,high-anglefaultsbeganinmid-Oligocene
time.
Some o f this deformationoccurredprior
concurrent with, extrusion
t o , and ( o r )
of .the Hells Nesa and lower
.
95
A-L Peal: ash-flowsheets.Evidencefornearconternporaneity
w i t h e r u p t i o n o f the lower
A-L PeakFormation
i s found i n
of Gallinas
excellent exposure about one-half mile southwest
A s shorrn by Figure 9 7, t h e e x p o s u r e r e v e a l s
Springs.
v e r t i c a l Tault c o n t a c t b e t a e e n t h e
'
a
on
gray-.massive-member
t h e e a s t and the. chocolate-brom contorted
member dovrnfaulted
on thewest,
nearlycontemporaneous
The a g e o f t h e f a u l t
s&th t h a t o f t h e c o n t o r t e d
i s welded t o t h e g r a y
rnustbe
member s i n c e t h e c o n t o r t e d
member
massive-uember,acrossthefault.
This
same f a u l t formed a topographic barrier t o easterly flowing
A-L Peal: Formation.
basaltic-andesite lavas interbedded in the
Evidence f o r e a r l i e r f a u l t i n g i s found i n t h i c k n e s s v a r i a t i o n s
of 'the Hells
'MesaFormation,There
areonly
i n t h e t h e s i s a r e a ' where d e p o s i t i o n a l u p p e r
.. ....
of t h e H e l l s Mesa a r e e x p o s e d t o - p e r m i t
two L o c a l i t i e s
and lovrer c o n t a c h
an e s t i m a t e o f t h e
I n a v e r t i c a l 4 i p p i n go u t c r o pa b o u tt w o - t h i r d so f
thickness.
a n i l e n o r t h e a s t o f Pine \']ell t h e t h i c k a e s s is estimated a t
500 f e e t .
F r o m a south-dippingexposure
Gallinas Springs the
o f ' H e l l s Mesa a t
'tlxi.clmess was first e s t i m a t e d a t 1200
f e e t . . T h i s v a l u e seemed anomalously high,and up-to-the-south
t r a n s v e r s e f a u l t s were i n f e r r e d t o be p r e s e n t a l o n g t o p o g r a p h i c
breaks.
However, evenwith
possiblethichess
this assumptionthe
minimum
i s 800 f e e t . T h i s r a p i d t h i c k e n i n g
to
t h e n o r t h and west i s i n t e r p r e t e d h e r e t o be caused by downto-the-vestdisplacement
t o , o r d u r i n g ,e r u p t i o n
s e c t i o n s A-A',
C-C'
on t h e G a l l i n a s S p r i n g s
o f t h e Hells NesaFosmation.Structure
and D-D' i l l u s t r a t e t h i s i n c r e a s e i n
t h i c k n e s s fr.on e a s t t o west,
I
fault prior
It c a nn o t
be reasonablyassumed
.
96
,%Sure 17.
Loolungnorth a
d dwm on a v s r t i c a l f a u l t
contactbetween the ;?rag-nassive member (risht)
and t h e c o n t o r t e d ~::e;.!ber ( l e f t ) o f t h e A-L Peal!
Formation. The ciomf2ultac: cor.torted mezber
i s welded t o the g r a y - m s s i v e o e n b e r i n d i c a t i n g
t h e f a u l t formec! s i l o r x l y a f t e r , o r during
e x t r u s i c n of t h e c o n t o r t e d member.
97
:Look p l a c e
that the entire 300-foot decrease in thickness
a c r o s s a s i n g l e s t r u c t u r a l and topographichigh.
change i n t h i c l m e s s across t h e G a l l i n a s S p r i n g s
s h o m , i s p r o b a b l ya no v e rs i m p l i f i c a t i o n ,
f a u l t , as
. .
The Hells Mesa .
t o vreathering and i s an
Formation i s e x t r e m e l y r e s i s t a n t
excellent cliff former in the
The a b r u p t
Bear Mountains and Northern'
The Hells Hesa f o r m s ' h i g h peaks at T r e s
Gallinas Kountains.
Montosas arid G a l l i n a s Peak both t o the west of the Gallinas
S p r i n g sF a u l t ,
Hells Mesa forms low, r o l l i n g
To t h e e a s t , t h e
h i l l s i n thevicinity
of Council Rock.
An abrupt'thinning
f a u l t may c o n t r i b u t e t o t h i s
across the Gallinas Springs
change i n topographic expression,
An Oligocene age
b y , m a f i ct or h y o l i t i c
is also inferred f o r faults intruded
These d i k e s are r e l a t e d t o
@,ires.
be similar i n composition and
unexposed stocks inferred to
age t o theOligocene
are recognized:
Tres Montosasstack.
Two d i k et r e n d s
N 10°!V t o N 20" Vi9 and N 75"Vi t o S 7O"W.
All
but one of the north-trending dikes occur in down-to-the-vest
faults.
a l l n e a r l y v e r t i c a l and
Dips on t h e s e d i k e s a r e
o r r e v e r s e f a u l t s is a moot
whether they represent normal
question.Evidence
f o r I . a t e r movement a l o n gt h e s ef a u l t s
,was observed i n only one ca6e.Reconnaissancetraverses
thewest
of t h e s t u d y area i n d i c a t e t h a t h i g h - a n g l e
d o m t h r o m on t h e n e s t a r e
Gallinasuplift.
to
faults
a consistent pattern' across the
Sone o f t h e s e may be r e l a t e d . t o basin and
range faulting associated with the
Transverserhyolitedikes
stresses developed over the
N o r t h Lake graben.
are p r o b a b l y r e l a t e d t o . l o c a 1
back of a l a r g e T e l s i c i n t r u s i o n
98
. .
e x t e n d i n g a l o n g a similar t r e n d from Council Rock to one m i l e
southrrest of GallinasSprings.Up-to-the-southnortheasttrending transverse faultizlg of Oligocene
o ft h e
age a t t h e l a t i t u d e
also seemsnecessary,This
Tres Xontosasstock
conclusion i s based on t h e f a c t t h a t t h e
stock i n t r u d e s t h e
Spears Formation around the south margin but intrudes the
A-L Peals Formation on the northwest
e l e v a t i o n , ' Simon (1973,
the Hagdalena area
a t approximtitely the sam
p. 67) c i t e s s e v e r a l l o c a l i t i e s
in
where northeast-striking mid-Oligocene
tuff of
f a u l t s have c o n t r o l l e d ' t h e d i s t r i b u t i o n o f t h e
La J e n c i a Creels which i s i n t e r b e d d e d i n t h e A-L Peal< Formation.
B r o m (1972, p. 8%) r e p o r t s a 2.5-mile-long
of longitudinal norEal faults which
emplacement of t h e s t o c k s n e a r
Oligocenezone
was a m a j o r f a c t o r i n t h e
Magdalena..
The G a l l i n a s
S p r i n g s f a u l t vas tlm most i r n p 6 r t a n . t c o n t r o l l i n g f a c t o r i n
, t h e emplacement o f t h e T r e s PIontosas s t o c k and a t l e a s t t h r e e
-
o t h e ri n f e r r e ds t o c k sa l o n g
i t s s t r i k e( F i g ,
16),
Intersections.
oftlrisfaultzonewithnortheast-trend.ingfaultsofOligocene
age are probably
of s'econdaryimportance
i n t h e l o c a t i o n of
t h e s es t o c k s ,
Assuming d i p - s l i p movement. on t h e G a l l i n a s
Shrings fa.ult,
i t s estimated.throwbased
o fc o n t a c t s
i s 1200 f e e t .
on strilse s e p a r a t i o n
The t r e n do fO l i g o c e n el o n g i t u d i n a l
f a u l t s (average N 1 W'd) i s g e n e r a l l y p a r a l l e d by o l d e r
(Lararmide)andyounger(basin
and r a n g e ) s t r u c t u r e s ,
Deformation even older than
Laramide a l o n g t h i s t r e n d
suggested by t h e p r e s e n c e
o f bedded sedimentary breccias
an o u t c r o p o f ' t h e Abo F o r m a t i o n l o c a t e d a l o n g t h e e a s t
of t h e u p l i f t
(W.H0 l!lilliinson, oral commun.,
7972)-
is
in
flank
From
99
N 1WYl
theseobservationstheirritersuggeststhatthe
t r e n d may be a m a j o r s t r u c t u r a l t r e n d i n c r y s t a l l i n e
r o c k si nt h e
basement
Ylertz (1968, p. 279) h a s
Hagdalenaregion.
used linear trends of intrusions to define the orientaticn-
o f primary zones of
A r b ona
.
17ealsness i n basement rocks
of s o u t h e a s t e r n
..
t h e east i n a r e l a t i v e l y narrow zone along the east
the Gallinas uplift
f l a n k of
from Abbey S p r i n g s t o Cat Efountain, a
A similar occurrenceofeast-
d i s t a n c e o f about 20 Iniles.
dipping volcanic rocks in
theeastflank
to
strata c o n s i s t e n t l y d i p
Folding.Oligocenevolcanic
the Big Rosa Canyon a r e a and along.
of t h e S a n Hateo Moun'tains (E.G.
Deal,oral
conknun,, 1972) may be a c o n t i n u a t i o n o f t h i s s t r u c t u r e .
Volcanicunits
t o the vest
of this zonehave
variable dips
i n a south to southwesterly directiokienerally within
.degreesofthehorizontal.Oligocene
10
strata a r e notexposed
W e d i a t e l y e a s t of t h i s zone and ' t h e i r a t t i t u d e mustbe
inferred.
of the
A srnall exposure o f a p p r o x i m a t e l yh o r i z o n t a l
by Chapin ( o r a l
A-L PeakFormationhasbeenobserved
commun?, 1972) about 2 m i l e s s o u t h e a s t o f
Thus this zone may b e i n t e r p r e t e d
l i n e nil1 be used herein
as t h e c r e s t
The termhinge
a late-Oligocene age
is
The f l e x u r ec l e a r l yp o s t d a t e s
t h e mid-Oligocene s i n c e i t deformsrocks
1-Eocene, or younger,age
of an
as a synonym f o r monocline.
By a p r o c e s s o f e l i m i n a t i o n ,
i n f e r r e d f o r t h eh i n g el i n e .
Coumcil Rock.
(Fig. 16).
e a s t e r l ys l o p i n gm o n o c l i n a lf o l d
strata
f o r t h eh i n g el i n e
o f t h i s age.
A mid-
i s e l i m i n a t e d by
t h ef o l l o w i n go b s e r v a t i o n s .B a s i na n dr a n g ef a u l t s ,s u c h
as
100
Rock f a u l t , c l o s e l y p a r a l l e l
the Council
a g e n e t i ct i en o u l d ' s e e mr e a s o n a b l e
the h i n g e l i n e a n d
by a s s o c i a t i o n ,
i s a possibie
alongthesedow-to-the-eastnormalfaults
dips i n t h e strata,
mechanism t o c r e a t e e a s t e r l y
Dr8.g
However,
r a p i d c h a n g e s o f d i l ~observed i n t h e s t u d y a r e a a n d . a t t r i b u t a b l e
t o d r a g do not extend
h i n g el i n e
uore t h a n 5 0 0 , f e e t . from f a u l t s .
zoneaveragesabout
gravels just
.The
one m i l e i n vridth,Pediment
east ofCouncil
Rock havebeen
rotated t o a
w e s t e r l y d i p o f 10 t o 1.5 d e g r e e s ,T h i ss t y l eo fr o t a t i o n
of the upper surface
recognized by
is
of f a u l t b l o c k s away fromgrabens
many i n v e s t i g a t o r s as a common trait of b a s i n
a n dr a n g ef a u l t i n g ,
Hence theformation
o f this f l e x u r e i s
e l i n i n a t e d f r o m basin and range deformation
on t h e basis of
t e c t o n i c s t y l e and t h e l a c k of a reasonable mechanism f o r i t s
formation under 'conditions
ofnormal
faulting.
According t o D e S i t t e r (1964, p. 196)
..
I5.t
that all n o n o c l i n e s are r e l a t e dt ou p t h r u s t s
is probable
as apposed t o
normal. f a u l t s !'.since i n a t e n s i o n a l f i e l d i t mould be v e r y
improbableforthesu-rfacelayers
He d e f i n e ru p t h r u s t
fau.ltstl.
anglereversefaultt1
n o t t o be brokenbynormal
(ope c i t , , p. 137) as
and n o t e s t h a t % h e y a r e f r e q u e n t i n
c r y s t a l l i n eb a s e n c n tr o c k sb o r d e r i a gm a j o ru p l i f t s ,
presenceof
high-
Ita
The
this m o n o c l i n e t h e n i m p l i e s t h e e x i s t e n c e
north-trendingbasementfractures.Compression
of
or vertical
uplift are the only plausible
rnechanismsbywhich
axeformed,Nackin
118) s t r e s s e st h ei m p o r t a n c e
(1960, p.
upthrusts
of magmatic i n t r u s i o n s as " s t r u c t u r e m a k e r s t t i n t h e G r e a t
1Basj.n.
A s s t a t e de a r l i e r ,w i d e s p r e a di n t r u s i o n
of f e l s i c
. 101
magma t o o k p l a c e i n t h e
study area during late Oligocene.
i s assumed f o r
Hence, by a s s o c i a t i o n , a l a t e O l i g o c e n e a g e
themonoclinal
Assuming this c o n c l u s i o nt o
fold,
this p a t t e r n ;
several otherobservationsfitin-to
1,
A decrease in the
be c o r r e c t
avera.ge e a s t e r l y d i p from about
60 d e g r e e s t o 30 degrees
n
i t h e area n o r t h o f C o u n c i l
Rock may be a t t r i b u t e d t o w e s t e r l y r o t a t i o n o f t h e
hinge line
west
2.
by t h e b a s i n a n d r a n g e f a u l t s t o t h e
.
The g r e a t e s t i n t e n s i t y o f h y d r o t h e r m a l a l t e r a t i o n
in t h e s t u d y a r e a c l o s e l y
3.
'foilovrs t h e
monocline;
The absence of La Jara P e a k a n d e s i t e
(2.3.8 m.y.,
Ghapin, 1971 a ) on t h e G a l l i n a s u p l i f t
may be
a t t r i b u t e d t o the monocline.,forming
..
a topograp2lic
barrier to westerly flowing 'lavas.
4.
compared t o the
Deep e r o s i o n o f t h e G a l l i n a s u p l i f t
Bear Mountains and Southern Magdalena Mountains
be attributed
may
t o a relatively long and unbroken
e r o s i o n a lh i s t o r ys i n c el a t e - O l i g o c e n e
inferred origin
time,
The
o f t h i s m o n o c l i n a l f o l d by magmatic
i n t r u s i o n i s shoivn i n Figure 18.
correct,thenthis
If t h i s model i s
f o l d is t h es u r f a c ee x p r e s s i o n
of a twenty-mile-long,
deep-skated
pluton
with
cupola-like extensions
-
k h a t a r e now recognized .as
exposed o r i n f e r r e d s t o c k s ,
&lostothervariations
comparison t ot h em o n o c l i n e .
o f s t r u c t u r a l d i p s a r e minor i n
A s shown i n s t r u c t u r e s e c t i o n
E-E', f l a t bottomed synclines may, beformedwhere
doming by
102 .
PRE-VOLCANIC
.
BASEMENT
..
.
MIDDLE MIOCENE
r
WEST
18.
EAST
I
..
D i a g r a m a t i c east-west c r o s s s e c t i o n s a t t h e
.app.roxiimate l a t i t u d e o f Ga1lina.s Canyon
i l l u s t r a t i n g t h e development o f t h e G a l l i n a s
Springs Fault and a'monocline produced by
magmatic i n t r u s i o n .
..
stocks has reversed the regional southerly
dip.
S t r u c t u r a l P a t t e r n s "__
around Stocks,
"
"
accompaniedby
"
Doaing
o f vrall r o c k s
is a n i m p o r t a n t
radial and concentric faults;
Council Rock s t o c k .
't.he i n f e r r e d
Tres Kontosas stock and
characteristic of the
Wall r o c k ss u r r o u n d i n g . t h e
Tres Montosas
s t o c B are modera.tely domed with quaquaversal d i p s r a n g i n g
from 20 t o 40 degrees i n a half-mile concentric zone about
t h ep e r i m e t e r .
Vertical t o overturned d i p s i n a two s q u a r e
mile area e a s i o f t h e o u t c r o p o f t h e s t o c k
are p a r t i a . l l y
a t t r i b u t e d t o foundering of t h e r o o f of the s t o c k (see
s t r u c t u r e s e c t i o n A-A',
Pl. 1 ) and t o e x a g g e r a t i o no f
e a s t e r l y d i p s i n strata c r e a t e d by t h e p r e v i o u s l y d i s c u s s e d
monoclinal fold,
Foundering of roof blocks
come v e r yc l o s et o
i s common
in ,intrusions 'which
.. . .
t h e s u r f a c e( N o b l e ,
s a t e l l . i t i c l a t i t e porphyryplug
195Zj2,p. ItO).
t o t h e east of the s t a c k is
i n f e r r e d t o have f i l l e d the v o i d c r e a t e d b y
a foundered block
vest o f t h e plage .Evidenceforcollapseoft.hese
i s c i r k u s t a n t i a l b u i convincing.
mere t o be
on t h e basis o f b l o c k ' f a u l t i n g , t h e n
5OCO f e e t o f dom-to-the-westdisplacementwould
on t h e n o r t h - t r e n d i n g f a u l t l o c a t e d a l o n g t h e
t h e plug.
roof rocks
If t h e o u t c r o p d i s t r i b u t i o n
and attitude of units around the porphyry plug
explainedentirely
The
F u r t h e rn o r t h ,a l o n gt h e
over
be r e q u i r e d
east s i d e bf
strike o ft h i sf a u l t ,
a displacement of more t h a n a fern h u n d r e d f e e t . c a n n o t be
justified,Therefore,founderingaccompanied
the foundered block about
by r o t a t i o n of
a hinge a t i t s west edge i s here
proposed t o e x p l a i n t h e o u t c r o p p a t t e r n .
Overturned strata a t some d i s t a n c e t o t h e n o r t h and.
s o u t h of the porphyry plug are probably related
faultsassociatedmith
magma.tic u p l i f t .
to' r e v e r s e
%n m o s t c a s e s t h e
stock is n o t revealed
a t t i t u d e of faults associated with the
i n outcrop.Thus,thetypeoffaultcannot
be i n f e r r e d
. .
e v e nt h o u g hs t r a t i g r a p h i co f f s e t sa r e
.However r e v e r s e
known.
f a u l t i n g i s r e q u i r e d t o j u s t i f y an a n o m a l o u s l y t h i c k i n t e r v a l
of upper Spears e p i c l a s t i c r o c k s i
n a d r i l l hole located
about one-halfmilenortheast
s e c t i o n B-Br P1. 1 ) .
theCouncil
of t h e s t o c k ( s e e s t r u c t u r e
Elinor f a u l t s T i l l e d by q u a r t z v e i n s
Rock a r e a havedetermintibledips.
The s t r a t i g r a p h i c
some are r e v e r s e f a u l t s ,
offset umnistakably indicates that
Arcuateconcentricfaults,generally
theapex
downthrown towards
wall
of t h e Tres Piontosas stock, have
broken the
. .~
r o c k s i n t o numerous small blocks.
most. o f t h e s e
The d i p o f
i s n o t knavm.
faultsandtheirclassification
in
Flow foliation
i n a c o n c e n t r i c a l l y o r i e n t e d l a t i t e d i k e , west o f t h e s t o c g ,
dips
\
75 degreestowardthestock.Anderson
would d e s c r i b e t h i s f r a c t u r e
doming ofthe
(1937, p. 36)
as a cone sheet
forruod. d u r i n g
However a. c o n es h e e to r i e n t a t i o n
wa11 rocks.
i s notphysicallycornpatiblefor
toward t h e apex o f t h es t o c k : .
a r e assumed t o be v e r t i c a l .
a l l ofthefaultsdomthroxn
Most o f t h e c o n c e n t r i c f a u l t s
T h i s dom.-to-the-stoclrpattern
o f c o n c e n t r i c f a u l t s i s most l i k e l y r e l a t e 6 t o t h e s u b s i d e n c e
of magma p r e s s u r e .
The' r ; " u u c t u r a l p a t t e r n
for the inferred Council
of t h e s t o c k
o f doming, v e i n s , d i k e s
and f a u l t s
Rock s t o c k i n d i c a t e s t h a t t h e a p e x
i s ea.st o f theCouncil.
Rock f a u l t ,
T h i s basin
a t depth,
and r m g e f a u l t p r e s u m a b l y t r u n c a t e s t h e s t o c k
S t r u c t u r e s e c t i o n C-'Ct i l l u s t r a t e s t h e r e l a t i o n s h i p
and dikes
t o t h e ' i n f e r r e d Cou.nci1 Bock stock.
Basin and
Ranga Tectonics
Normal f a u l t i n g a l o n g a n o r t h tcr
L o n g i t u d i n a lF a u l t s .
of t h e N u l l i g a n
north-northwesttrendrelatedtotheformation
Gulchgraben
Grande r i f t ) began i n l a t e Oligocene
(a.nd t h e R i o
o r earliestNiocenetime(ChrisLiansen
Chapin, oral comun.,
1974).
arid Lipman, p. 259;
L o n g i t u d i n a lb l o c kf a u l t i n g
s i n c e t h e Miocene has been the dominant
mechanism i n t h e
formation o f t h e s e v e r a l h o r s t s a n d g r a b e n s
area.
of veins
i n the. Nagdalena
Longitudinal b a s h a n dr a n g ef a u l t sa r e
well expressed
as proroinent f a u l t s c a r p s i n t h e MagdalenaandBear
Mountains.Figure
16 shosrs t h e r e l a t i o n s h i p o f t h e s e
faults to the major structural blocks
i n theMagdaleca
region.
In the study area,
a narrow zone of' down-to-the-east
normal f a u l t s f o r m s t h e e a s t .
bounda&.y of. t h e G a l l i n a s h o r s t ,
A s e s t m a r d r o t a t i o n o f ' t h e o l d e r pnd.iment g r a v e l s away .from
the Nulligan Gulch graben
i s t y p i c a l o f ' t h e basin and range
s t y l e of defornation.Contrary
t o o t h e rl o c a l i t i e sm e n t i o n e d
above, this f a u l t zone i s t o p o g r a p h i c a l l y e x p r e s s o d o n l y
minorescarpment.
The Council Rock f a u l t ,l o n g e s to f
typn, extends. continously in
along the entire length
by a
this
a north-northwest direction
of thestudyarea(about
6 miles),
South of Arroyo PIontosas this- fault disappears under pediment
gravels and north of Council
Rock Arroyo i t . a p p a r e n t l y
'
106
b i f u r c a t e s i n t o a conjugate set of f a u l t s w i t h a g e n e r a l
n o r t ht r e n d .
The f a u l t p r o b a b l yd i e so u t
>hestudyarea
sornevhere between
and t h e P u e r t e c i t o . q u a d r a n g l e ( g e o l o g i c
Tonl-;ing, 1957, pl. I ) .
map of
A t h r o v o f 800 t o 1500 f e e t f o r theCouncil
Rock f a u l t is e s t i m a t e d from s t r a t i g r a p h i c r e l a t i a n s h i p s .
Displacenent on t h e f a u l t p r o b a b l y i n c r e a s e s
toward the.
f a u l t s which place
south,Short,arcuatedown-to-the-east
H e l l s Piesa t u f f s i n j u s t a p o s i t i o n v r i t h t h e t u f f
o f Council Rock a r e i n t e r p r e t e d
Hountain i n t h e a r e a s o u t h
as l a n d s l i d e b l o c k s s e c o n d a r y i n o r i g i n
f a u l t .N o r t h
of Nipple
t o northwest-trendingbasin
faults located t o the east
t o the Council Rock
and rangenormal
o f t h e C o u n c i l Rock f a u l t a r e
recognized only because they are intruded
by o l i v i n e basalt,
A few down-to-the-east north-trending faults which' occur
. ..
neartheGallinasSpringsintrusivecenter
are probably
Oligocene i n age.
f o r down-to-the-
A basinandrangeage
westlongltudinalfaultscannot
beproved
o r disproved.
Sone movement re1a.ted t o f o r ~ n a t i o n o f t h e N o r t h
i s assmed, honever, 'most
o f $his movement max havetaken
place i n -the Xorth Ida? f a u l t zonedescribed.
(1957, p. 2 2 ) ,
Lake graben
by Givens
Q u a t e r n a r y movement a l o n gn o r t h - t r e n d i n gb a s i n
and r a n g e f a u l t s a r e i n d i c a t e d
by f a u l t s c a r p s d i s p l a c i n g ,
alluvial sediments along I4ulligan Gulch and east
TransverseFaults.
Very l i t t l e physiographic o r geologic
expression o f late-Cenozoictransversefaulting
t h es t u d ya r e a .T h c r e f o r e ,t h ei n t e r g r e t a t i o n
s t r u c t u r e s made h e r e r e l i e s h e a v i l y
o t h e ri n v e s t i g a t 0 r s . i . nt h e
o f Magdalena,
is evident in
of t h e s e
on the observations of
&Ta.gdalena area.Important
107
characteristics. of these faults
as d e s c r i b e d by Brown (1972,
p. 86-90) are:
1,
A general northeast trend and
of oblique-slip
the area east
2.
a left-lateral sense
as deterininedby
offset of d i k e s i n
'of Ka.gdalena,
A s t r u c t u r a l and topographlc l o w between the Bear
Mountains and the llagdalena Mountains attributed to
a northeast-trending graben
ofthe
30
formedby
bifurcation
B i o Grande R i f t . ( C h a p i n , 9971b),
Complex m u t u a l l y c r o s s - c u t t i n g r e l a t i o n s h i p s b e t w e e n
the longitudinal and transverse basin and range
f a u l t systems.
4.
Cessation o f movement on n o r t h e a s t - t r e n d i n g t r a n s v e r s e
faults but continued activity along longitudinal faults.
..
a maxirnum
Chapinandothers(inpreparation)haveinferred
age of l a t e Piiiocene f o r development. of the San Augustin
graben.
T h i s conclusion i s based on f i e l d r e l a t i o n s h i p s o f
r h y o l i t e l a v a fLours extruded from a . v e n t on Magdalena
which a r e d a t e d a t 1 4 n.y.(Weber,
'1971 )o . T h e s e r h y o l i t e
l a v a s must be al-der than t h e g r a b e n s i n c e t h e y
n o r t h l n t o its structural topographic
d.0 n o t f l o w
low.
Some n o r t h e a s t - t o e a s t - n o r t h e a s t - t r e n d i n g f a u l t s i n
study area hme features generally silnilar to those
describedabove.
A l a t e Cenozoicage
consistent association
is i m p l i e d by t h e .
o f pediment gravels deposited
p r e s e r v e d on t h e dovmthro-gin blocks.
on or
Nearly a l l o ft h e
t r a n s v e r s e f a u l t s a r e doumtlrrown t o t h en o r t h .
r e l a t i o n s h i p has causedprogressivelyolder
This
strata t o be
the
. .
. 1 OS
exposed a t t h e s u r f a c e from n o r t h t o s o u t h i n s p i t e of a
r e g i o n a ls o u t h e r l yd i p ,
A s t r i k es e p a r a t i o n
of t h e G.al1ina.s
by a b r a n s v e r s e f a u l t n o r t h w e s t
S p r i n g fault uhere cut
of the
c e n t e r of t h e s t u d y a r e a i s i n a.greement s n t h o b l i q u e - s l i p
I
movement b u t n o t i n
a l e f t - l a t e r a i sense.
a t t i t u d e of the Gallinas Springs rault
s u g g e s t e ds t r i k e - s l i p
true
S i n c et h e
is n o t lmoum, t h e
component may 'not be real, A l t e r n a i i v e l y ,
theapparenthorizontalseparationcouldbe,theexpression
of u p - t o - t h e - s o u t h d i p - s l i p
movementon
if t h e d i p Of t h e G a l l i n a s S p r i n g s
the t r a n s v e r s e f a u l t
Fau.lt is eastward.
A lrajor zone of t r a n s v e r s e f a u l t i n g e x p r e s s e d
pediEent gravels and
by f a u l t e d
a northeast"krenc1ing basalt d.ike p a s s e s
t h r o u g ht h en o r t h e r nh a l f
of t h e T r e s Montosas.stock.
As
s u g g e s t e d i n Figure 16., this zone may. be a n . e x t e n s i o n of the
.
Bear Springs-Deer Springs fault cone dkscribed
('ig7.2, p. 88).
Evidence f o r . t h i s . c o r r e l a t i o n i s based on t h e
repeated occurrence
both fault zones
by B r o m
o f narrow'I,northeast-trending-grabens
and t h e f a c t t h a t
a straight-line projection
from t h e Bear X o u n t a i n s i n t e r s e c t s t h e G d . 1 i n a . s u p l i f t
hovrever, B r o m s u g g e s t s t h a t t h e s e f a u l t s c h a n g e , t r e n d
u p l i f t s o u t h o f U.S,
23
t o t h i si n t e r p r e t a t i o n ,
a b o u tt h ep r o p a rl a t i t u d e .C o n t r a r y
more s o u t h - s o u t l x v e s t d i r e c t i o n
in
to a
t o intersecttheGallinzs
Highway 60.
It i s suggested here t h a t
' e i t h e r i n t e r p r e t a t i o n i s r e a s o n a b l e and a s o l u t i o n t o this
problem must a w a i t f u r t h e r data.
.
...
ECO'XOI~IIC GEOLOGY
The s t u d y a r e a i s centered about the Council
were 'mined i n t h e '
d i s t r i c t where v e i n s o f l e a d c a r b o n a t e
"1932, p. 55).
18801s for t h e i rs i l v e rc o n t e n t( L a s k y ,
history ofthedistrict
district, originally
b r i e f l y by &'.A,
i s s h o r t l i v e d and obscure.
ore p i p e s o f " h a r d - c a r b o n a t e l e a d - s i l v e r
hmeaveraged
The
He s t a t e s t h a t
orell were mined as
Ore from t h e Old Boss mine is r e p o r t e d t o
$250 perton.Jones
bearingveinwhich
The
hovm as I r o n t,iountain, i s d i s c u s s e d
Jones (TgOlt, p. 723-125),
deep as 300 f e e t .
Rock mining
a l s o mentions an i r o n -
was prospected t o . a depthof
opiginal. name o f t h e d i s t r i c t
1712s
80 f e e t ,
The
d e r i v e d by i t s a s s o c i a t i o n
w i t h a.bv.ndant m a g n e t i t e f l o a t i n p e d i m e n t g r a v e l s t o t h e
s s u t h e a s t of t h e vein workings.
The. name o f t h e d i s t r i c t
was l a t e r changed t o Council Rock f o r a t o w n s i t e e r e c t e d i n
t h e 1880;s t o t h e n o r t h of t h e mine worlringsnearCouncil
Rock.
The d i s t r i c t i s d e f i n e d h e r e
as a
4 n i l e by 2 mile
nortli-nortki~~est,-tsendjn~f
zone of abundant quartz-carbonate
e p i t h e r n a lv e i n s
(Pl. 1 ) .
Some v e i n sa r ea s s o c i a t e dw i t h
n o t i c e a b l e d i s p l a c e m e n t .of s t r a t i g r a p h i c c o n t a c t s and appear
t o be r e l a t e d t o b o t h n o r i a l f a u l t s
and reverse f a u l t s .
14a.ny a p p e a r t o have minor displacements
as t h e y commonly
die out along strike in silicified shear zones.
'
The g r e a t e s t
d e n s i t y o f v e i n i n g and o l d vrorlrings are c o n c e n t r a t e d i n t h e
s o u t hp a r t
?I 10" N t o I?
of t h e d i s t r i c t .
Tvro vein t r e n d s are a p p a r e n t ,
Lt5"Yi and N 3O"E t o N 60"E, ,Most o f t h e
cld
morlring a r e s i t u a t e d at i n t e r s e c t i o n s of t h e s e t v o ' t r e n d s ,
hencethehistoricaldescriptionof"pipeveins"appears
accurate.
The d i s t r i b u t i o n of v e i n s ,r h y o l i t ed - i k e s
hydrothermal alteration
as shoxn on t h e g e o l o g i c
and
map i n f e r s '
.
~
t h e prsaence of an unexposed stock centered
.
t o the east of
thedistrict.
Veins range i n t h i c k n e s s from 2 i n c h e s t o 5 f e e t ,
white q u a r t z m i t h minor amounts
wider v e i n s a r e p r e d o m i n a n t l y
of carbonates.
The
Smaller v e i n s o f t e n e x h i b i t a l t e r n a t i n g
bznds
o fb a r i t e - s i d e r i t ea n dc a l c i t e - h e m a t i t e ,B a r i t e - s i d e r i t e
bands a r e g e n e r a l l y c o a r s e r g r a i n e d
crystals up to
with white bladed barite
1 i n c h i n l e n g t h s e p a r a t e d by a matrix o f
rhombohehralgray-brown
bands i s b l a c k , f i n e
?
siderite,
Hematite i n t h e a d j a c e n t
.LLrained and disseminated through the
c a l c i t e g r a b s , thus imparting ,an overall black color
. ,.
to
t h e band.
the t u f f of Nipple
One s h o r t v e i n o b s e r v e d c u t t i . n g
Mountainabout
a half mile southriest o f t h e C o u n c i l Rock
mineralogy compared
r a n c hh a s a c o n t r a s t i n g , r a t h e r u n u s u a l
A s shovm i n F i g u r e '19, i t
t o other v e i n s i n t h e d i s t r i c t ,
i s a breccia-vein containing
partially to coiupletely chloritized
a matrix o f f i n e
clasts o f t h e t u f f o f N i p p l e N o u n t a i n i n
g r a i n e dq u a r t z ,h e m a t i t ea n d
barite.
black o u t l i n e s a r o u n d t h e c l a s t s ,
Rematite commonly f o m s
A c r u s to fl i g h t - g r a y
late-
stage f l u o r i t e i s v i s i b l e a l o n g t h e t o p o f t h e s p e c i m e n .
C a l c i t e and s i d e r i t e a r e n o t a b l y a b s e n t
S u l f i d e rxinerals were not observed
material i n o u t c r o p o r as f l o a t ,
was o b s e r v e db u to n l yI n
fron t h i s v e i n .
i n anyofthe
vein
A t r a c eo fm a l a c h i t e
dump m a t e r i a l .
A few cube shaped
111
Figure 19.
Vein b r e c c i a c o l l e c t e d n e a r t h e n o r t h
end of t h e
Clasts of t h e Tuff
Council Rock m i n i n g d i s t r i c t .
of Nipple IjIountain are completely altered
to
c h l o r i t e a n d q u a r t z where a g r a y c o l o r i s found.
Note t h e c o n c e n t r a t i o n o f black hematite which
o u t l i n e s many o f t h eb r e c c i af r a g m e n t s .Q u a r t z ,
b a r i t e and f l u o r i t e make up the remainder
of vein
material.
vugs were observed i n quartz v e i n l e t s i n a n o u t c r o p o f
s i l i c i f i e d t u f f of Uipple14ountain
c r o s s e d byCouncil
componentof
and some a s s o c i a t e d
2oclr Arroyo,This,
limonite staining, suggest pyrite
it is
just north vhere
may have b e e n ' a s p a r s e .
the veinlets.
The v e i n n i n e r a l p a r a g e n e s i s
from first c r y s t a l l i z e d
t o I.ast is: m L c r o c r y s t a l l i n eq u a r t z , .c a l c i t ea n dh e m a t i t e ,
. .
This o r d e r 'of
b a r i t e and s i d e r i t e ,f l v . o r i t e ,d r u s yq u a r t z .
c r y s t a l l i z a t i o n is g e n e r a l l i z e d and s e v e r a l e x c e p t i o n s mere
noted i n t h i n s e c t i o n .
I n some s p e c i m e n s ,b a r i t ea p p e a r s
to
be o l d e r t h a n h e m a t i t e a n d i n o t h e r s c a l c i t e o l d e r t h a n
microcrystallinequartz.
C i r c u l a t i o n o f h y d r o t h e r m a l s o l u t i o n s vcithin t h e d i s t r z c t
a p p e a r s t o haye had a l o n g h i s t o r y . . V e i z i n g y o s t d a t e s a 5 h y o l i t e
d i k e t o the southwest
.of Council Rock,. Nhere i n t e r s e c t e d by
quartzveins,thedikehas
a light green color
c h l o r i t i z a t i o n of thegroundnassminerals.
du.e t o
Most v e i n
time
formation apparently took place in late Oligocene
associated xdth crystallization
t o the east
of t h e i n f e r r e d s t o c k c e n t e r e d
of the d i s t r i c t and l a t e r d o y m f a u l t e d by b a s i n
a n dr a n g ef a u l t s .
A n o r t h e a s t - t r e n d i n g ,t h r e e - f o o t - t h i c k
q u a r t z v e i n one mile south-southwest
of Council Rock i s
a b r u p t l y t e r a b a t e d Sy a f a u l t i n t e r p r e t e d t o be secondary .
t o t h e major Council Rock f a u l t .
. a l s o e x h i b i t s -or
quartz-calcite veining in
about 2000 f e e tf w t h e r
and range faulting
Hovrever this 6econdary fault
an open cut.
,
t o t h es o u t h T
, h i os u g g e s t sb a s i n
may have r e a c t i v a t e d m i n o r c i r c u l a t i o n o f
h y d r o t h e r m a ls o l u t i o n s .A l t h o u g ha p p a r e n t l ys e p a r a t e d
by a
113
d i s t i n c t i n t e r v a l o f time, i t does n o t seem r e a s o n a b l e t o
separate these mineralogically
similar v e i n s i n t o tvro e p i s o d e s
of nineralization.
Hydrotherial alteration in the Council
Rock d i s t r i c t b a s
no r e c . o g n i z a b l eo v e r a l l . zonal c h a r a c t e r i s t i c s ' .G r e a t e r
].atera1
than vertical'permeablilty through the stratigraphic units
created shap,discontinuities i n the alteration pattern.
..
Some minor , q u a r t z v e i n s c u t t i n g p e r m e a b l e r o c k s
a r g i l l i c and p r o p y l i t i c a l t e r a t i o n . v h i c h c a n
have a s s o c i a t e d
be d i r e c t l y
I n agreementwith
a t t r i b u t e d t o t h ev e i nm i n e r a l i z a t i o n ,
thegenerallyrecognizedzonalpatternselsewhere,argill.ic
a l t e r a t i o n vas c l o s e s t t o t h e v e i n s and surrounded by p r o p y l i t i c
E l t e r a t i o n( C r e a s e y ,
1966, p. 73)
I nd e c r e a s i n go r d e ro f
and
a r e a le x t e n t ,p r o p y l i t i z a t i o n ,a r g i l l i z a t i o n ,p y r i t i z a t i o n
.. ...
silicification are the recognized types
d i s t r i c t .P y r i t i z a t i o n
of a l t e r a t i o n i n t h e
i s g e n e r a l l yc o n f i n e d
t o outcropsof
to
t h e - t u f f o f Nippletlountain.Disseminatedpyriterelatec?
faultzonesseems
hinge-linezone
t o be c o n s i d e r a b l y more abundant i n t h e
t o t h e s o u t h and t o t h e n o r t h o f t h e d i s t r i c t *
The s c a r c i t y o f s u l f i d e m i n e r a l s i n v e i n s ' o f t h e d i s t r i c t
does not
rule o u t t h e p o s s i b i l i t y
d e p t h .K i n e r a l i z a t i o no ft h e
zinc,lead,silverdeposits
o f base-metalsulfides
same agehasformedvaluable
i n theKellydistrictonly'l2
m i l e s t o theeast..Inasmuch
as theCouncil
d i s t r i c t i s l o c a t e d on the buried flanlr
i t is quite difficult
KellyLimestone.
6800 f e e t
at
Rock m i n i n g .
o f a Laramide u p l i f t ,
t o estimatethedepthtothefavorable
A minimum d r i l l i n g d e p t h t o t h e
a n d m a z i m m depthof
'
Kelly o f
12,300 fee-t i s b a s e d , o n t h e
114 .
follow5ngestimatedstratigrapkicsection:
Lower Spears
Formation 1200 f e e t , Baca Formation 0-2000 feet,Nesozoic
r o c k s 0-3500 f e e t , Permianrocks
2000 f e e t ,S a a d i aF o r m a t i o n
moderatedoxing,
j000 f e e t ; MaderaForai,ation
GOO f e s t .
The e x i s t e n c eo f
of emplacement
and c o m p a r i s o n w i t h t h e l e v e l
of t h e Tres t f o n t o s a s s t o c k , s u g g e s t s t h a t t h e
inferred s t o c k may be within 2000 feet
r o o f of t h e
of . t h e s u r f a c e ,
its
NagnetitefloatvhichgavetheCouncilRock'district
o r i g i n a l name " I r o n H o u n t a i n d i s t r i c t "
metasomatic iron deposits
TresMontosasstock.
was d e r i v e d from
along t h e n o r t h m a r g i n
of t h e
T h i s conclusion i s supported bymagnetite
m i n e r a l i z a t i o n at t h e B i g John mine on ,the northwest perimeter
of thestock,Informationprovided
comun.,1972),
by WeBe WalLsh ( m i t t e n
co-owner of t h e mine,. .i.n d i c a t e s t h a t p r o d u c t i o n
exceeded 200 t o n s i n 1966.'
Elost o f t h e o r e mas used i n t h e
manufacture of h i g h - d e n s i t y c o n c r e t e f o r n u c l e a r s h i e l d i n g .
tla&etonetersurveysrindicatethatthegreatestconcentration
o f magnetite i s t o t h e west o f t h ep r o s p e c ts h a f t .S a m p l e s
from t h e dump i n d i c a t e a c o n t a c t m e t a s o f l a t i c o r i g i n f o r
magne.tite.
The h o s tr o c k
the
i s a medium-grained hed.enbergite
s y e n i t e formedby
p o t a s s i c metasomatism of granodiorite
porphyry.Traces
of p y r i t eo c c u ra l o n gt h ee d g e so fm a g n e t i t e
v e i n l e t s which v a r y i n t h i c k a e s s
green hedenbergite occurs
from 10 cm t o 1
m
m
.
as m a s s i v e c l o t s a d j a c e n t
Dark-
t o veins
abd as s n a l l g r a i n s d i s s e m i n a t e d t h r o u g h t h e h o s t r o c k .
Some c a l c i t e 1-7a.s a l s o found i n a s s o c i a L i o n w i t h l a r g e r
masses
of pyroxene.
$finor pyritiza-tion and silicification
of t h e A-L peak
'
115
Formation are t h e o n l y e x p r e s s i o n
of h y d r o t h e r a a l a c t i v i t y
x e l a t e d t o t h ep r o p o s e dG a l l i n a sS p r i n g ss t o c k .
o f domingand
The absence
veining in the area suggests that the
roof of
t o favorable
t h e s t o c k i s q u i t ed e e p ,D r i l l i n gd e p t h s
and ( o r ) t h e s t o c k are v e r y l i k e l y
replacement horizons
uneconomic,
A wide a r e a o f p r o p y l i t i c , a r g i l l i c
o c c u r st ot h ee a s t
and p y r i t i c a l t e r a t i o n
o f t h eT r e sM o n t o s a ss t o c k ,T h r e ep o s s i b l e
sourcesare:
1e
t h eT r e s
Montosas s t o c k
2.
a deep magmatic sourceconnected
a ulajor uptlxcust which
3.
-
tothesurface
fornod the monoclinal
by
fold
(hingeline)borderingthe
east f l a n k o f t h e Gallinas
uplifte
.. ...
an unexposed. p l u t o n ( s ) t o t h e east i n t h eN u l l i g a n
Gulch graben(Chapin
The Tres Montosas stock can
and o t h e r s , i n p r e p a r a t i o n ) .
be e l i m i n a t e d s i n c e t h e
a l t e r a t i o np a t t e r n sa r en o ts p a . t i a l l gr e l a t e d
between the second
t o it.
Choosing
and t h i r d s o u r c e s would r e q u i . r e s u b s u r f a c e
d a t a n o tp r e s e n t i ya v a i l a b l e . .
However thesemi-continous
occurrence o f a l t e r a t i o n a n d m i n e r a l i z a t i o n a l o n g t h e h i n g e
l i n e z o n e fromCouncil
second source.
Rock s o u t h t o C a t Mountain favors the
116
coNcLusIoI?s
Volcanic strata exposed i n t h e C o u n c i l Rock d i s t r i c t
consist chiefly
o f welded ash-flow t u f f s with minor intwrljedded
l a v a f l o w s and v o l c a n i c l a s t i c s e d i m e n t s .
t u f f s of the upper Spears,
’
Welded a s h - f l o g
’
Hells Mesa and A-L Peak Formations
were produced during a m a j o r p e r i o d o f l a t i t i c t o r h y o l i t i c
p y r o c l a s t i ce r u p t i o n si nt h e
mid-Oligocene.
F i e l d and
rocks i n
petrographic observations of the Oligocene volcanic
t h e s t u d y area i n d i c a t e t h e y a r e v e r y
similar t o an e q u i v a l e n t
(Brown, 1972).
s e c t i o n i n theSouthernBearMountains
Notable
d i f f e r e n c e s , o c c u r i n c o o l i n g breaks o f t h e a s h - f l o w s h e e t s
which may, o r may n o t , be. occupied by andesite
ash f l o w i n t e r b e d d e d i n
a n d e s i t el a v a s .T h i nc r y s t a l - r i c h
t h e A-L Peak Formation ‘in the Southern
notobserved
or basaltic
Bear Hountains vfere
F i e l d vrork by several
inthestudyarea.
i n v e s t i g a t o r s i n t h e v i c i n i t y of the s t u d y area i n d i c a t e t h e
L a Sara Peak Andesite
is not present
on t h e G a l l i n a s u p l i f t
(Tonking, 1957; Givens, 1957; \Yilldnson, i n preparation) e
High-angle- f a u l t s developed during the Oligocene
attributed to crustal adjustments induced
i n t r u s i o n of l a r g e volumesof
magma.
are
by e x t r u s i o x o r
R e a c t i v a t i o no f
g e n e r a l l y north-northrrest-trelldinlS
basement f a u l t s begm
p r i o r to, o r d u r i n g , e r u p t i o n o f t h e
H e l l s Mesa and A-L Peak
a s h flours.Topography.developedalongtheGallinas
Springs
f a u l t zone i s i n t e r p r e t e d t o be t h e c a u s e o f a n i n c r e a s e
thickness fron
east t o viestof
in
t h e Bells, Mesa a n d a l s o f o r
l a v a flows i n t e r b e d d e d i n t h e A-L Peak Formation.
A %elded”
117
i n $he lower
fault contact between ash-flow units
A-L Peak
F o r m a t i o nh e l p sc o n f i r mt h i si n t e r p r e t a t i o n .S e v e r a le x p o s e d
and inferred stocks of late Oligocene age occur along the
i t was a m a j o r c o n t r o l
Gallinas Springs fault zone indicating
i n theLr emplacement.
\
I n t e r s e c t i o n s of t h e G a l l i n a s S p r i n g s f a u l t z o n e v r i t h
o f secondaryimportance
n o r t h e a s t - o r i e n t e dO l i g o c e n ef a u l t sa r e
i n t h e emplacement of t h e s es t o c k s .
northeast-trending faults
a l s o intersect these stocks
t h eO l i g o c e n ef a u l t sw i t ht h e
stocks are inferred to
same t r e n d .
be e x t e n s i o n s o f
which has been intruded into
The n o r t h t r e n d i n g
a 20-mile-long pluton
a m a j o r basementzone
i s egpressed a t t h e s u r f a c e
..
follows thewestsideof
Gulchgraben.Topographydevelopedalong
mon-oclinzl f o l d nayhaveformed
'
a b a r r i e r t o westerlyflowing
T h i s would e x p l a i n t h e ' a b s e n c e o f t h e
on t h e Gallinas u p l i f t .
Jara PeakAndesit.e
aqd r a n g e f a u l t s
whichfbllovlthe
down f a u l t e d t h e e a s t
Stocks extending
La
Ycungerbasin
axis o f the monocline have
limb o f t h e f o l d .
from t h i s e l o n g a t e p l u t o n r o s e
1000 f e e t o f t h eO l i g o c e n es u r f a c e .E r o s i o nd u r i n gt h e
t h eT r e s
this
L a Jara Pe&. F o r m a t i o n d u r i n g t h e i r e r u p t i o n i n
mid-Miocene time.
andPliocene
of weakness
by a n e a s t e r l y s l o p i n g m o n o c l i n a l
f o l d whichborderstheupliftand
l a v a s of the
masking
o f t h eG a l l i n a su p l i f t .T h i si n t r u s i o n
a l o n gt h ee a s ts i d e
t h ei % l l i g a n
b f a j o r basin andrange
h a s e x p o s e dt h es h a l l o w e s ts t o c k ,h e r e
Montosasstock.
Miocene
named
Unexposed i n f e r r e d s t o c k s a t Council
Rock a n d t h e a r e a one mile southwest
i n d i c a t e d by f e l s i c d i k e
t o mithin
of Gallinas Springs are
swarm, epithermalveins,hydrothermal
.
118
and at t h e l o c a t i o n
alteration, local structural features
southwestof
Gallinzs S p r i n g s by a c e n t r a l v e n t f o r b a s a l t i c -
.
andesitelavas.
Geologicevents
i nt h e
developmentoftheDatil-Mogollon
.
.
similar i n p e t r o l o g i c c h a r a c t e r ,
volcanic field appear to.be
..
t i m i n g and a b s o l u t e a g e t o t h a t o f t h e , S a n J u a n v ' o l c a n i c f i e l d
(cf. Elston
a n d o t h e r s , 1973; and Lipman 'and others, 1970).
i n mafic l a v a s a n d .
The composition of ferromagnesian minerals
a few c h e m i c a l a n a l y s e s s u g g e s t s d i f f e r e n t i a t i o n f r o m
o l i v i n e basalt.
alkali-
C o m p o s i t i o n a lv a r i a t i o n si nt h eB e l l s
Nesa
Formation i n d i c a t e . t h a t i t i s an f l u p s i d e down1l c o m p o s i t i o n a l l y
.
.
A model o f a s h - f l o we r u p t i o np r e s e n t e d
zonedash-flowsheet.
by Brovm (1972) is expanded,upon the basis' o f t h e p e t r o l o g i c .
.
~
c h a r a c t e r o f . t ' n e . T r e s Montosas s t o c k and data f r o m t h e l i t e r a t u r e .
.. ... ,
Lipnan and F'riedman (1974) r e p o r t t h a t l a r g e q u a u i t i e s
meteoric water
.
of
vrere i n c o r p o r a t e d i n s i l i c i c magmas b e f o r e
,.
-
a compositionally zoned ash-'flow
their eruption to produce
Kennedy ( 1 955), -Boone (19621, O r v i l l e
s h e e t i n s o u t h e r n Wevada.
a1 s t r e s s e d ' t h e '
a b i l i t y of an
aqueousvapor-phase
t o fractionate an alkaline
magma i n t o a
p o t a s s i c - r i c hp h a s e
at t h e t o p o f a magma chamber.
(1963) ' a d Cater (1969)have
'
A model
t h a t i n c o r p o r a t i o n o f abundant meteoric
proposedheresuggests
water by a l a r g e b o d y of a l k a l i - a n d e s i t e magma vras t h e
.essential factor in the production
' o f magmas which were l a t e r
e r u p t e d t o form t h e H e l l s Nesa and A-L PeakFormation.
waterentersthe
i s r a p i d l y and
magma s y s t e m , t h e m e l t
e f f e c t i v e l yf r a c t i o n a t e d ,
.
.
by a p r o c e s s o f .llaqueo-alkalill
diffusion,intostronglyzoned,gas-chargedsilicic
.
Once
.
.
.
magma.
.
,
The p r o c e s s may be d e s c r i b e d by two r e l a t e d c h a i n s o f
eventswhich'takeplace
f.
at different l e v e l s i n t h e chamber:
Because of pressure gradient the aiueous vapor
d i f f u s e st o w a r d
the t o p o f t h e
c o n c e n t r a t e d there.
phase
magma chamberand
is
..
,
.
Components o f . a l k a l if e l d s p a r
and t o a lesser e x t e n t q u a r t z a r e s t r o n g l y p a r t i o n e d
to'
into the vapor phase during the diffusion process
form a l o w - s i l i c a ' , a l k a l i - r h y o l i t e
of the chamber,
magma at. t h e t o p
Normal t h e r m a l g r a d i e n t s t e n d t o
late
c o o l the magma b u t c r y s t a l l i z a t i o n o c c u r s v e r y
because o f the e f f e c t o f h i g h m a t e r c o n t e n t
on the
liquidustemperature.
2.
An a p p r o x i m a t e l y o p p o s i t e c h a i n o f e v e n t s o c c u r s
some l o w el er v ei ni
the magma..chamber.
at.
Although.
~.
.-
e n r i c h e d i n . p o t a s s i u m a n d s i l i c a ' r e l a t i v e t o its
original composition,. the
. .
with respect to
much e a r l i e r .
magma is , u n d e r s a t u r a t e d '
water a n d c r y s t a l l i z a t i o n b e g i n s
A s c r y s t a l l i z a t i o n becomesdominant,
s o do t h e l i q u i d - c r y s t a l f r a c t i o n a t i o n p r o c e s s e s .
Together these p r o c e s s e s f0rm.a c r y s t a l - r i c h q u a r t z
l a t i t e magma which c r y s t a l l i z e s tit a m u c h h i g h e r
'
temperature than the rhyolite because of significantly
l o w e rw a t e rc o n t e n t
and more maficcomposition.
Petrologic observations of the Tres Montosas stock
t oc o n f i r m
this model,
help
The v a r i a t i o n i n c o m p o s i t i o n o f
feldspars and their structural state indicate
the stock crystallized rapidly
relatively slow Process
'
t h a t much o f
with l i t t l e t i m e f o r t h e
of f r a c t i o n a l c r y s t a l l i z a t i o n t o
be
. .
-.
120
effective.
However, t h e s t o c k e x h i b i t s
a wide r a n g e
in
composition f r o m a n d e s i t e t o . g r a n o d i o r i t e , t o m o n z o n i t e
( q u a r t z aqd pyroxene), t o l e u c o g r a n i t e .R a p i dc r y s t a l l i z a t i o n
in
has I1frozenf1 the process of aqueous-potassic enrichment
s o t h a t i t s e f f e c t may still be seen.
the border facies rocks
P h e n o c r y s t s o f Wgh-temperature andesine enveloped
an e x p r e s s i o n o f
temperatureorthoclaseare
by low-
this process.
'
The c o n c e n t r a t i o n o f a potash-rich aqueous phase continued
t h r o u g h o u tt h ee n t i r ec r y s t a l l i z a t i o n - o ft h es t o c k .T h i s
r e s u l t e d i n t h e formation of a 1 e u c o g r . d i t e c o r e . A l t h o u g h
due t o a n o t h e r i n t r u s i v e e v e n t
the g r a n i t e c o r e c o u l d b e
:
t h e r e i s no i n d i c a t i o n t h a t i t i s a %ormaltl d i f f e r e n t i a t e
o ft h eo r i g i n a la l k a l i - a n d e s i t ep a r e n t
i n t h e g r a n i t e i s approximately the
magma..
same composition as t h a t
..
of monzonites
Plagioclase
._
.I.
i n theborderfaciesa1though:ofthe
l o w
temperature structural variety.
Unexposed s t o c k s a t Council Rock and near Gallinas
Springsarefavorabletargets
reineralization.
f o r base-metal s u l f i d e
The s c a r c i t y o f s u l f i d e s i n t h e . C o u n c i l
o f replacement o r
veins does not eliminate &he possibility
p o r p h y r y - t y p es u l f i d em i n e r a l i z a t i o n
the Council
Rock
a t depth.Because
.
.
Rock x i n i n g d i s t r i c t i s l o c a t e d on the b u r i e d
flank of a Larmide uplift, the estimated .depth to
f a v o r a b l eK e l l yL i m e s t o n e
i s betweenthe
6800 f e e t t o 12,300 f e e t .
l e v e l ofemplacementof
roof of the Council
t h es u r f a c e .H e m a t i t e
the
wide limits of
'
Doming and c o m p a r i s o nw i t ht h e
'
the Tres Montosas stock suggests the
Rock s t o c k may be within 2000 feet o f
and s i d e r i t e are a b u n d a n tc o n s t i t u e n t s
'
._
Of
t h ee p i t h e r m a lq u a r t zv e i n s
t h e y may b e r e l a t e d
121
T h i s suggests
a t Council Rock.
t o a hydrothermal system a t d e p t h s i m i l a r
t o t h a t whichproducedcontact
( ? ) metasomaticmagnetite
m i n e r a l i z a t i o n a t t h eT r e sM o n t o s a ss t o c k .F i e l dr e l a t i o n s h i p s
.
around the inferred GalXinas Springs stock indicate the depth.
to possible mineralization
which i s
i s much g r e a t e r t h a n t h a t
p r e s e n t l ye c o n o m i c a l l yf e a s i b l e
t o develop.. The occurrence::
o f s e m i c o n t i n o u s h y d r o t h e r m a l a l t e r a t i o n 'and , m i n e r a l i z a t i o n
. . .
in the hinge-line
'zone'. (monoclinal fold)
from Council Rock'to
..
C a t Hountainfavors
a deepmagmatic
origin for the
.
hypogene
s o l u t i o n s which produced these' features..
Basin and raxige f a u l t s , b o t h l o n g i t u d i n a l
foilow trends
and t r a n s v e r s e ,
.
similar t o t h e s e o f e a r l i e r O l i i o c e n e f a u l t s . '
. .
The c l o s e a s s o c i a t i o n o f t h e C o u n c i l
m o n o c l i n a lf o l d
.
Rock f a u l t and t h e
..
i s a t t r i b u t e d t o r e a c t i v a t i o no f
the same
.
zone of weahess which controllei formation 'of the monocline.
Overlapping of the longitudi&.l
and t r a n s v e r s e f a u l t s
i n -space
and t i n e has produced a complex f a u l t p a t t e r n ~ h i c hi s o v e r p r i n t e d
on t h eO l i g o c e n es t r u c t u r e s .B l o c kf a u l t i n go f
rangetype
first produced a s e r i e s ofnorth-northwest-trending
g r a b e n sa n dh o r s t s .
form the
the b a s i na n d
Later branching o f t h e E o G r a n d e ' r i f t t o
..
S m hugustinplainproduced
an o v e r l a p p i n g s e t o f
n o r t h e a s t - o r i e n t e dg r a b e n sa n dh o r s t s .E r o s i o no ft h e s eh o r s t s
sinceearlyKiocenetime,withconcurrentdeposition
i n the
g r a b e n s ,h a sp r o d u c e da b u n d a n ts u r f i c i a lc o v e r .P e d i m e n t
g r a v e l s i n the Hulligan Gulch graben
from t h e G a l l i n a s u p l i f t .
is expressedbyolivine
a t Council Rock.
are d o m i n a n t l y d e r i v e d
Late P l i o c e n e o r Q u a t e r n a r yf a u l t i n g
basalt d i k e s w h i c h c u t p e d i m e n t g r a v e l s
. .
...
APPENDIX
Optical Determination Hethods
The composition and s t r u c t u r a l state o f p l a g i o c l a s e yere
determined with
a 5-axis u n i v e r s a l s t a g e u s i n g t h e R i t t m a n n -
zonemethod(Rittmann,1929)
i n combination with 2 V . & g l e s
measured by s t a n d a r d e x t i n c t i o n t e c h n i q u e s on t h e u r c i v e r s d l
s t a g e (Emmons, 1943, p. 23-41).
Smith (1968, p. 1191)
J.R.
indicates that the differing nature'
of v a r i a t i o n s i n 2 V 1 6 t h
composition f o r high- and l o w - t e m p e r a t u r e p l a g i o c l a s e c a n
used t o i d e n t i f y t h e c o r r e c t . s t r u c t u r a l
o ft h ep l a g i o c l a s e
because o f n e a r l y e q u a l
An
s t a t e i f . t h e composition
However, t h i s method fails i n
i s knom.
t h ec o m p o s i t i o nr a g e s
be
40 t o An6*
and f r o m Ang0 t o
Anloo . .
2V valuesintheseintervals.Other
methods t o distinguish high- and lor:!-temperature plagioclase
are d e s c r i b e d by Slerimons (1962), and
(1961
). Thesemethodsemploy
RittmaG andEl-Hinnawi
the
o p t i c i n d i c a t r i x which
and i s complicated by many
i n v o l v e s complex procedures
ambiguities.
The combinedRittmannzone
determine the structural state
by J.R.
.
t h e measurementofangles
between c r y s t a l l o g r a p h i c a x e s a n d
o ft h a ts u g g e s t e d
.
- 2Vz
method u s e d h e r e t o
of p l a g i o c l a s e . i s a m o d i f i c a t i o n
Smith (1958).
The p r o c e d u r e i s as
follows :
I.
D e t e r ~ n e t h e2 V of a s u i t a b l e p l a g i o c l a s e i n d i v i d u a l
using standard extinction techniques.
..
2.
I n t h e same i n d i v i d u a l ,d e t e r m i n et h e
maximum
' e x t i n c t i o n a n g l e X ' A (01 0) i n t h e z o n e p e r p e n d i c u l a r
tothe
lO101.
.
'
3.
X'
EnterRittmannzone,
Using extinction
A
(OIO),
and ZV, c u r v e s
a n d 2 V a n g l e s d e t e r m i n e d above
and r e c o r d all t hpeo s s i b laen o r t h i tpee r c e n t a g e
values compatible
.
of 4 v a l u e s ) .
with t h e o p t i c a l
The v a l u e ss h o u l d
with respect to high-
4.
d a t a (minimum
be s e p a r a t e d
. .
and l o w t e m p e r a t u r e s t a t e s .
Compare thecompositionsdetermined,
The t r u e
values
s t r u c t u r a l s t a t e musthavecomposition
thtitagree
..
within 4 percent anorthite.
VariationcurvesfortheRittmannzone
method a n d
2Vz which d i f f e r e n t i a t e v o l c a n i c a n d p l u t o n i c s t r u c t u r a l
s t a t e s that wereused
.
in the determinations are presented
i n F i g u r e s 1 8 and 1B r e s p e c t i v e l y .
Over 30 trials o f t h i s
.
method i n d i c a t e i t is a v a l i d means t o d i s c r i m i n a t e h e t w e e n
50
Smith, 1958)'.
( c f . J.R.
As. i n d i c a t e d by
2Vz f o r v o l c a n i c and pluton-ic
Figure l B , t h e d i f f e r e n c e i n
t o An5o v a r i e s f r o m 2 t o
p l a g i o c l a s e i n t h er a n g e
An
3 d e g r e e s , a value which
is t o o near the error involved
i n a 2 V determination.'
can not
be used
4Q
.
This i s why S m i t h i n d i c a t e s 2V a l o n e
to distinguish the different. structural
s t a t e s i n c a l c i ca n d e s i n e s .
However, i n s p e c t f o no fF i g u r e
1A
i n d i c a t e s a d i f f e r e n c e i n e x t i n c t i o n angles o f 6 t o 8 degrees
f o rt h e
two s t r u c t u r a l s t a t e s i nt h er a n g e
which i s much g r e a t e r t h a n t h e
the
measurement.
- 0.5
I.
.
.
An
40
t o An5o,
degree e r r o r i n v o l v e d i n
Thus i n t h e . c o m p o s i t i o nr a n g e
t h e method i s reversed and
.
'
-
v o l c a n i c and p l u t opnl ai cg ci ot hcmeil nap soes i t i o n
range And t o An
'
An
40
to
2 V i s used t o estimate t h e
composition with the structur!al distinction
made on t h e basis
124
,
RITTiLNN ZO€Z METHOD
1. A.
2Vz FOR VOLCANIC AiTD PLUTONICPLAGIOCLASE
..
-
.
1. B.
Figure 1..
A.
B.
Rittmannzonecurves
f o r X' (010) v e r s u s
composition of volc+ic
and p l u t o n i c
p l a g i o c l a s e( a f t e rT r o g e r ,
1959, p. 111).
2V, versuscompositionofvol.canic
and p l u t o n i c
p l a g i o c l a s e ( a f t e r S l e m o n s , 1962, p l , 111-
125
..
.
Examples of some
O f t h ea s s o c i a t e de x t i n c t i o na n g l e .
are l i s t e d i n Table 1.
d e t e r m i n a t i o n s made b y t h e w r i t e r
Samples TH-12 and 1-13 i l l u s t r a t e t h e d i s c r i m i n a t i v e
c a p a b i l i t i e s of t h e method w i t hC a l c i ca n d e s i n e s .
As
1-69 and 1-67, d e t e r m i n a t i o n of 2V a l s o
i n d i c a t e d bysamples
of t h e Ritmann zone
eliminates ambiguous extinction angles
Ano
method f o r thecompositionrange
to
Anio. Compositions
determined from e x t i n c t i o n a n g l e s ( R i t t n a n n z o n e . m e t h o d )
a c c e p t e d as b e i n g more a c c u r a t e t h a n t h o s e
are
determined from 2V.
The R i t t m a n n zone method i s r e p o r t e d by Emmons,(1943, p. 133)
t o beaccurateto
Rittmann zone
-+ 3 p e r c e n t a n o r t h i t e . '
- 2V,
.
Thecombined
method will. p r o b a b l y n o t d i s t i n g u i s h
plagioclases with a transitional structural state.
The a l k a l i f e l d s p a r s :s a n i d i n e ,a n o r t h o c l a s e
'
'
and
-
o r t h o c l a s e were d i f f e r e n t i a t e d on t h e basis of2Vmeasurements
..
..
and a common p h y s i c a l t r a i t .
A l t h o u g hn o td i a g n o s t i c ,
i n v e s t i g a t o r s have noted that
i n unaltered rocks 'sanidine
many
. a n d a n o r t h o c l a s e are c h a r a c t e r i s t i c a l l y c l e a r w h i l e o r t h o c l a s e
T h i s r e l a t i o n s h i p is well i l l u s t r a t e d by
is u s u a l l yc l o u d e d .
thephotomicrograph
The 2V o f alkali f e l d s p a r s
on page 70.
varieswiththeircomposition,
however,the-degree
of exchangablecationsandthedegree
t h e 2V o f f e l d s p a r s .
beused
t oe s t i m a t e
of u n m i x i n g a l s o c o n t r o l
The l a t t e r t v m v a r i a b l e s are primarily
.
,
of t h er o c k .
r e g u l a t e db yt h ec o o l i n gh i s t o r y
MacKenzie axd J.V.
of ordering
S m i t h( 1 9 5 5 1
, 9 5 6 )i n d i c a t et h a t
Or c o n t e n t t o
plu-konicandhighgrademetamorphic
i n extrusivevolc:.;licrocks.
Studies by
2V may
'
10 p e r c e n t i n . mesozonal
r o c k s , a n d t o 5 20 p e r c e n t
A laterinvestigationby
> .
Emeleus
Examples o f determinations for structural state
of plagioclase made by the
combined "Rittmanmr, Zone2V," method.
Table 1.
.
Rittmann z o m
X'
rock .' sample
number
type
A
(010)
( 2 0.5)
rhyolite
ash-flow tuff
1-69
5.0"
rhyolite
vitrophyre
(dike)
1-67
8.0"
2V
Q*
&
14
10
E
12-5O:?::1"
23
I
structural
$ anorthite
2vz
17
21
none*
'
12
An10
....
'
29.5"
andesite
T"12
.
..
1-13
pyroxene
diorite
!&
.
'
quartz
monzonite
'55
,
"
.88"+2" &O&
''
'
,. ,
,
..
,' 29.0"
'
.
1-80
volczlnic
none*
volcanic
..
'
%
3
128' 2 1"
25
22
Tres Montosas stock
border facies
inferred
% anorthite
volc.
plut.
volc.
plut.
state
24.OV
54
.
,
89'2
,
..
2 ;40
.
'
2
44
.
.
97OL 2*
1.5
'
37
69
.
'
92
' 4 . .volcanic
68 . . An44
38
'
24
28
80
volcanic
.
.
,
...
t43
volc&ic
An37
core
_
I
*compositions in agreementa r e underlined
.
* not Sossible according to
Rig. 1B
&-I
and J.V.
Smith' (1959) i n d i c a t e s t h e e r r o r , i n v o l v e d i n
e s t i m a t i n g Or c o n t e n t u s i n g 2 V measurements i n hypabyssal
i g n e o u s r o c k s lnay be as much as 2 100 p e r c e n t . T h u s t h e
writer has only used
2 V measurements o f a l k a l i . f e l d s p a r s i n
Mesa and A-L Peak Formations t o
ash-flow t u f f s o f t h e H e l l s
5 20 percent.
estimate t h e i r c o m p o s i t i o n t o
The'diagram
shown as E g u r e 2 mas t h e basis f o r t h e s e estjmates of
composition.
The approximate composition of clinopyroxene
from a combination of
2V a n d e x t i n c t i o n a n g l e
was d e t e r n i n e d
(ZAC) measurements.
E x t i n c t i o n a n g l e s were determined from s t e r o g r a p l e c p l o t s '
axes and t h e i r r e l a t i o n s h i p t o t h e
of the optic indicatrix
.
o r i e n t a i i o n o f t h e ( 1 10) prismaticcleavages.
, .
o f t h e s ec l e a v a g e s
i s p a r a l l e l t o t h e C axis.
used t o e n t e r t h e d i a g r m
I
the modal composition of the rock
trend is t o i n c r e a s e t h e
v r i t h . i n c r e a s i n gd e g r e e
The i n t e r s e c t i o n
T h i s d a t a was
-
o f Figure 3Ai' Hhen ambiguity i n
.
the extinction angle occurred in the use
d e g r e eo fd i f f e r e n t i a t i o n .
.
o f t h i s diagram,
was used t o estimate i t s
The normalmagmatic
differentiation
Fe c o n t e n t o? a s s o c i a t e d c l i n o p y r o x e n e
o f d i f f e r e n t i a t i o n ,T h u s
..
a choice
between a n iron-poor and a n i r o n - r i c h . p y r o x e n e c o u l d b e
made
w i t h some degree of v a l i d i t y . . The .nomenclature of cllnopyroxenes .
as used i n this r e p o r t i s i l l u s t r a t e d i n F i g u r e 3B.
I
..
..
.
'128
.
.
-
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