Mineralogy and geochemistry of Eocene intrusive rocks and their

American Mineralogist, Volume 76, pages 1306-1318, I99I
Mineralogy and geochemistryof Eoceneintrusive rocks and their enclaves,
El Paso area, Texas and New Mexico
Cu,vrN G. B.q.RNns,SusnN E. ENsnN,c.rt
Department of Geosciences,Texas Tech University, Lubbock, Texas 79409, U.S.A.
Jlvrrs D. Hoovnn
WestinghouseHanford Company, Richland, Washington 99352, U.S.A.
Ansrru,cr
Trans-Pecosmagmatism in the El Paso, Texas, area was characterizedby numerous
shallow trachyandesiticintrusions of Eoceneage.Cognateenclavesin the intrusions consist
of monzodiorite and porphyritic trachyandesite.The compositions of these enclavesindicate that they are cumulates formed near the margins of a crustal (possibly midcrustal)
magma chamber. The mineral assemblagesof the trachyandesitic and monzodioritic enclaves (tschermakitic amphibole + biotite + oxides + plagioclase + titanite + quartz)
constrain preemplacementtemperature to less than 850 "C and .fo, to I or 2 log units
above NNO. HrO contents were probably in the range of 3.0 to 3.5 wto/oand the confining
pressureof the crustal chamber as much as 5.5 kbar (basedon Al content of amphibole).
Xenolithic enclavesinclude dioritic and anorthositic compositions. These are interpreted
to be the wall rocks of the crustal chamber and are probably Precambrian in age.Incompatible element contents of the trachyandesiticmagmas are interpreted to be the product
of fractional crystallization of a subduction-relatedbasaltic magma. Disequilibrium among
phenocrystsin the trachyandesitic magmas and local enrichment of compatible elements
can be explained by minor contamination by anorthositic and dioritic xenoliths.
INrnooucrrou
Eocenehypabyssalintrusive rocks that crop out in the
vicinity of El Paso, Texas, represent some of the oldest
magmatic activity in the Trans-Pecosprovince (Hoover
et al., 1988). These intrusions consist of porphyritic
trachyandesite(classificationofk Bas et al., 1986) that
is considerablymore uniform in major and trace element
compositions than other igneous suites in the province
(e.g.,Nelson et al., 1987; Henry and Price, 1986).Many
ofthe intrusions contain enclaves(terminology of Didier,
1913) that range from trachyandesite nearly identical to
the host rocks, through monzodiorite and diorite to anorthosite.
We present a description of the mineralogy of the intrusions and their enclaves.Thesedata serveas indicators
of the magmatic temperature and fo, the degree of magma and enclaveinteraction, and in some cases,the depth
of enclave crystallization. In addition, new major and
trace element analysesof the samplesprovide constraints
on the compositions of possible sourceregions.
Surrlrlny
oF FrELD RELATToNS
The intrusions crop out in a north-trending belt l0 km
wide that extends from northern Chihuahua, Mexico, to
north of El Paso, Texas (Fig. 1). They were emplaced in
* Present address: 1604 W. Dallas St.. Artesia. New Mexico
8 8 2 1 0U
, .S.A.
0003-004x/9l /0708-I 306$02.00
Lower Cretaceous sedimentary strata (Lovejoy, 1976;
Hoover et al., 1988;Hoffer, 1970;Garcia,1970;Chavez,
1986;Ensenat,1989)during Eocenetime (-47 Ma, three
K-Ar dateson biotite; Hoover et al., 1988;Hoffer, 1970).
In the southern part ofthe area,trachyandesitecrops out
as dikes and sills in the Sierra de Sapello and Sierra de
Juarezranges(Fig. l; Hoover et al., I 988; Chavez, I 986).
The central part of the area contains the largest intrusions; these include the thick (> 100 m) sill-like Campus
andesite (Hoffer, 1970) and smaller related bodies and
the Sierra de Cristo Rey intrusion (Lovejoy, 1976; Ensenat, 1989).The Cristo Rey intrusion was interpreted to
be a laccolith by Lovejoy (1976), but detailed study of
enclavedistributions within the pluton suggestsa pluglike
morphology (Ensenat, 1989; see below). A rhyolitic sill
that crops out south and southwestof Cristo Rey (Fig. 2)
dips radially away from Cristo Rey, indicating that it was
domed by later intrusion of the Cristo Rey magma. The
northern intrusions (herein referred to as the Three Sisters, Coronado, and Thunderbird sills; seeFig. l) are also
sill-like.
The magmasrose through a thick Precambrian section
overlain by Paleozoicsedimentaryrocks (principally carbonate). The upper part of the Precambrian section is
exposedin adjacent fault blocks of the Franklin Mountains. It consistsof siliciclastic and calc-silicaterocks and
basaltic breccia overlain by weakly metamorphosedtrachytic to rhyolitic lavas and ash-flow tuffs (Harbour, 1960,
l 306
BARNES ET AL.:INTRUSIVE
ROCKS AND THEIR ENCLAVES
1307
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MEDIAN
ENCLAVE
slzE
(cM)
,'-'
(!,
CIUDAD JUAREZ
.,,
u./,,_
.S1..u",",,,.",,,,,,,...
TBACIIYANDESITE
;;r;';,.gp,I
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F:I::::-Il
rJ
l.:.1:1.:r.
RHYOLITE
JUAREZ'r.
Fig.2. Simplified geologicmap of Sierrade Cristo Rey showing median diameter of monzodioritic enclaves and outcrop pattern of older rhyolitic sitl. After Ensenat (1989) and Lovejoy
(r976).
NEW MEXICO
The remaining enclave types are anorthositic and dioritic in composition. They are angular and are typically
in sharp contact with trachyandesitic host rocks, but some
\TE XAS
'',',:i
occur as enclavesin monzodioritic enclaves.
Anorthositic enclavesrange up to 22 cm in diameter
C
H
I
H
U
A
H
U
A
EE-I
lokn
0
5
and are the most abundant ofthe angular enclaves.They
Fig. 1. Locationmap.NorthemsillsareThreeSisters(THR), are weakly foliated to massive and rangein composition
Coronado(COR),and Thunderbird(TBD). Centralintrusions from anorthosite to anorthositic gabbro and rarely gabareSierrade CristoRey(CR),Campusandesite(CA),and Uni- bro. They are present in all of the northern and central
versityPlaza(not shown)justnortheastofCA. Southernintru- intrusions and are particularly abundant in the Cristo Rey,
sionscrop out in Sierrade Juarezand Sierrade Sapello.Inset Three Sisters,and Coronado bodies. They are absent in
showslocationof KilbourneHoleandPotrillomaar.After Hoo- the southern intrusions. Clots of coarse-to very coarseveret al.(1988).
grained amphibole (l-5 cm in diameter) in the Cristo
Rey and Three Sisters intrusions are interpreted to be
from mafic zones or layers of the anorthositic enclaves.
Angular dioritic enclaves range from fine to coarse
1972; Thomann, 1980). Alkali granites intruded all of
these rocks at approximately l.l Ga (Shannon, 1989; grained. In rare cases,host trachyandesiteappearsto be
Shannon and Barnes, 1989; Wasserburg et al., 1962; chilled against the diorite. Dioritic enclavesare particularly abundant in the Coronado and Three SistersintruCopeland and Borvring, 1988).
sions, where they range from l0 cm to lessthan I cm in
Enclaves
diameter. Petrographicallyidentical diorite occursas thin
A variety ofenclaves occur in the central and northern dikes cutting anorthositic enclaves.
intrusions. They rangein diameter from lessthan 0.5 cm
Ensenat (1989) interpreted the monzodiorite and rare
to at least 1.2 m, with median diameters lessthan 10 cm. trachyandesitic enclaves to be cognate (i.e., genetically
Rounded, spherical to oblate, medium-grained monzo- related to the host trachyandesitic magma) on the basis
diorite and quartz monzodiorite (Ensenat, 1989) is the of the lack of chilled host rock at contacts with these
most common enclave type, particularly in the Cristo Rey enclavesand the rounded nature ofthe enclaves.Anorand Campus andesiteintrusions (Fig.2; Ensenat, 1989). thositic and dioritic enclaves(and amphibole clots) were
This is the only enclave type with diameters larger than interpreted to be xenoliths on the basis of their angular
22 cm. Some have thin (<5 mm) rims of biotite. Por- (broken) shapesand becausethe host trachyandesiteis
phyritic trachyandesite forms a second but very sparse locally very fine grained against these enclaves.
Xenoliths of the host Cretaceoussedimentaryrocks are
type ofrounded, oblate enclave. These enclavescontain
fewer phenocrysts than the host trachyandesite and were rare. They consist offine-grained hornfels and skarn, and
recognizedonly in the Cristo Rey intrusion.
they only occur near pluton maryins.
SAPELLO
)1"""",.
-'":jj=&d
l 308
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
TABLE1. Summary of locations and petrographic features
Mineral
assemblage
Average
Maximum abundimension dance
(mm)
(vol%)
Notes
ter the shape and style of intrusion of the Cristo Rey
body. Only monzodioritic and anorthositic enclaveswere
abundant enough (40-400 enclavesper site) to yield useful population data. For theseequidimensionalto slightly
oblate enclave types, the median length of the long axis
decreasesfrom the margin to the central part of the pluton. Enclaves with the smallest diameters occur northwest of the center of the pluton (Fig. 2). Inward decrease
of enclave size is consistent with intrusion as a diapir
(e.g., Castro, 1987) with in situ ballooning toward the
southeast.Diapiric emplacementfollowed by ballooning
is also consistent with steeply dipping flow foliation and
banding.
Trachyandesitic host rocks
Phenocrysts
Fine-grainedintergranularto
Calcic amphibole
5
12
subtrachyticgroundmass.
Biotite
3
2
Ca amph ranges from
Plagioclase
3
28
equant to elongate (Uw :
Quartz
<1
1
14). Resorbedcores in
Groundmass
56
northern intrusions.Accessory Fe-Ti oxides, apatite, zircon.
Porphyritic trachyandesitic enclaves
Phenocrysts
In Cristo Rev intrusron.
Calcic amphibole
3
15
Rounded,oblate shape.
Pnrnocru.prrv
g
Biotite
2
Fine_grainedintergranular
Pfagioclase
Detailed petrographic descriptions are presented in
2
17
groundmass.Ca amph
Groundmass
65
typicaily elongate (l/w :
Hoover et al. (1988),Ensenat(1989),Garcia (1970),and
9). Accessory Fe-Ti oxHoffer (1970). The main petrographic features are sumides, apatite, zircon. Accessory apatite and zircon marized in Table l.
typically etongate(l/w :
The trachyandesitichost rocks contain phenocrystsof
20); apatite commonly
tschermakitic amphibole, biotite, plagioclase,and quartz
hollow; rare acicular apatite and zircon.
with microphenocrystsof apatite, ilmenite, altered magMonzodioriticenclaves
netite, and rare zircon. The groundmass consists of inCalcic amphibole
5
In central and northern intsiotite
5
trusions.Rounded,oblate, tergranular to subtrachytic arrangementsof plagioclase,
Plagioclase
3
lack cuspate margins.
alkali feldspar, quartz, biotite, oxide minerals, and zirSanidine
1
Medium-grainedhypidicon. In the Three Sisterssill, amphibole and clinopyroxQuartz
1
omorphic granular ave.
grain size 1.5 mm. Ca
ene are rare groundmass phases. Porphyritic trachyanamph ranges from equant
desite
enclavesare similar to the host rocks (Table l).
to elongate (l/w : 14). AcThe rhyolitic sill at Cristo Rey consists of very finecessory Fe-Ti oxides, apatite, zircon, titanite. Congrained quartz and altered feldspars. No mafic minerals
tains 0.5 mm stubby
are present in this rock becauseof the effectsof weathapatite and acicular apaering and hydrothermal(?) alteration.
tite to 1.2 mm.
Anorthositic xenoliths
Monzodioritic enclaves are medium-grained hypidiCalcicamphibole
10
In Three Sisters, Coronado,
granular (averagegrain size - I mm) and conomorphic
Clinopyroxene
5
Cristo Rey, and Campus
Plagioclase
> 100
Andesite intrusions.
sist ofblocky plagioclaseand sanidine,equant to elongate
ffmenite
2
Sharp, angular contacts
subhedral tschermakitic amphibole, subhedral biotite, and
with host. Highly variable
modal proportions. Acces- anhedral qtarlz, and rars clinopyroxene (Table l). Plasory titanite. Commonly
gioclase-richmonzodioritic samplescontain fine-grained
deformed.
interstitial material similar to the groundmassof the host
Dioritic xenoliths
Calcicamphibole
trachyandesite.K-rich members of this group contain re2
In Three Sisters, Coronado.
Biotite
0.5
Cristo Rey, and Campus
sorbed plagioclase replaced by sanidine, indicative of
Plagioclase
2
Andesite intrusions; as
low-Z
reaction of plagioclasewith rrcsidualfelsic liquid
rare dikes in anorthositic
(e.g.,Carmichaelet al., 1974;Nekvasil,1990).Interstitial
xenoliths.Sharp,angular
contacts. Variable modal
orthoclase and granophyre are locally present in these
proportions and grain size
samples.
(fine to medium).Foliated
to massive. Accessory
Anorthositic xenoliths consist of varying proportions
Fe-Ti oxides, acicular aoaof
sutured plagioclase(-Anor, with bent or broken twin
tite.
Enclave distribution in the Cristo Rey intrusion
Lovejoy (1976) interpreted the Cristo Rey intrusion to
be a laccolith. However, faint flow foliation and flow
banding in the intrusion are steeply dipping or vertical.
This orientation is not compatible with laccolithic style
of intrusion (Corry, 1988). The distribution and size of
enclaveswere measuredin an attempt to understandbet-
lamellae) and interstitial tschermakitic amphibole, clinopyroxene, and ilmenite. Trails of clinopyroxene mark
healed fractures in plagioclasegrains in some samples.
These textures are typical of high-temperatureprotoclastic deformation and recrystallization. The effectsof immersion of anorthositic xenoliths in host trachyandesite
are locally apparentas intergranular growth ofbiotite and
amphibole, as titanite rims on ilmenite, and at enclave
margins, as rims of plagioclasetypical of the host (-Anr,)
around plagioclasetypical of the anorthosite (-Anor).
BARNES ET AL.:INTRUSIVE
analyses
Tlaue3. Clinopyroxene
TmLe2. Summary
of plagioclase
compositions
Enclaveplag comp.
(4n74
Sample
Enclave
type
Range"
Host plag comp.
(An%)
Zoning
Range'
Zoning
Cerro de Cristo Rey
CRSTR4
CR79B
CR2B
CR81EN
CRsE
CRTF
THR3
THRs
THRT
THR17
C5
co18
co21
co22
diorite
diorite
porph.tr.
anorthosite
35-24
42--38
23 (ave)
41-36
Three Sisters
anorthosite 44 (ave)
anorthosite 41 (ave)
anorthosite 48 (ave)
anorthosite 47 (ave)
anorthosite 53 (ave)
Coronado
anorthosite 43-19
anorthosite
,
33-28
41-23
N
N
35-28
N
48-31
36-28
R
N
31-28
37-26
N
N
29-1I
N
36-26
n
N
N
sio,
Tio,
Al203
FeO
MnO
Mgo
CaO
Naro
Total
co20
co20
cpxl
cpxS
52.2
0.29
1.32
7.53
0.43
13.8
23.1
0.54
99.28
THR17
cpxl
THR17
cpx2
Oxide wtc6
529
51.3
0.18
0.85
9.09
0.64
13.0
23.0
0.59
100.16
0.59
2.13
10.6
0.39
12.8
21.6
0.48
99.83
51.7
0.48
2.09
10.3
0.32
12.8
21.7
0.47
99.89
Cations per six O atomg
48 (ave) large xtal
39-49, small xtal
University Plaza
UNIPA
l 309
ROCKS AND THEIR ENCLAVES
Si
Ti
AI
Fe
Mn
Mg
Ca
Na
Sum
1.960
0.008
0.058
o.237
0.014
0.775
0.931
0.039
4.022
1.981
0,005
0.038
0.285
0.020
0.724
0.921
0.043
4.017
1.934
0.017
0.095
0.334
0.012
o.717
0.874
0.035
4.019
1.945
0.014
0.093
0.325
0.010
0.718
o.874
0.034
4.013
Sierra de Juarez
SDJl
Note.'Zoning classifiedas normal (N), reversed(R).
Amphibole
Most amphiboles are tschermakite or tschermakitic
hornblende (Table 4; Fig. 3A). Structural formulas (calAN,ql,YTrcAL METHoDs
culated according to the algorithm of Spear and Kimball,
Mineral compositions were determined with a JEOL
1984) indicate that the Fe3t content of these amphiboles
JXA-733 electron microprobe at Southern Methodist is low. An excellentcorrelation exists among rotAland the
University, using natural and synthetic standards and a sum of A-site cations,t6lAl,Fe3+,and (2 x Ti) (not shown).
modified Bence-Albeedata reduction scheme.Whole-rock This indicates that cation substitution is dominated by
major element and Rb contents were determined by the Tschermak component. No correlation is presentbeatomic absorption spectroscopyat Texas Tech. The re- tween talAland the acmite component (sum of A-site catmaining trace elements were analyzed by INAA at the ions minus IM4lNa).
University of Texas at El Paso (seeHoover et al., 1988,
Two broad goups of amphibole compositions can be
for analytical procedure).
distinguished (Fig. 3). The first group has Mg/(Mg + Fe)
greaterthan 0.6 and Na/(Na + K) greaterthan 0.78, and
MrNnnu,ocv
it consists of amphibole phenocrystsfrom the trachyanPlagioclase
desitic host rocks and amphiboles from monzodioritic
Phenocrystsin the trachyandesitetypically display os- enclaves. In detail, phenocrysts from the Three Sisters
cillatory-normal zoning in the rangeAno,-An,, (Table 2); sill are distinctly bimodal and plot in discrete groups,
however, most crystalsrange from Anr. to Anrr. Reverse whereas samples from the Coronado sill plot in a broad
zoning is common only in plagioclasefrom the Three zone (Fig. 3A). Amphiboles from Cristo Rey and most
Sisters sill (Anr,-Anor). Plagioclasefrom monzodioritic amphiboles from the Three Sisters trachyandesite display
enclavesand dioritic xenoliths are Anrr-Anro; however, good correlation between M/(Mg + Fe) and Si per forindividual crystals lack appreciable zoning.
mula unit (Fig. 3A).
Plagioclasein anorthositic xenoliths is unzoned or norThe second amphibole group has Mg/(Mg * Fe) less
mally zoned. Compositions range from An' to An,n but than 0.6 and Na/(Na + K) lessthan 0.78 (owing to larger
are typically )Anoo. Anorthite contentslessthan Anooare K contents). This group consistsof amphibole from angenerally from crystals at or near the contact between the orthositic xenoliths and encompassesthe compositions
anorthositic enclave and the host.
of amphiboles from some dioritic xenoliths, a porphyritic
trachyandesiteenclave, and some of the amphibole pheClinopyroxene
nocrysts in the Three Sistersand Coronodo sills.
Clinopyroxene in anorthositic xenoliths from Three
Rims of actinolitic hornblende are present in some
Sistersand Coronado are salitic to diopsidic. The pyrox- monzodioritic and dioritic enclaves(Fig. 3A). Theserims
enes are Al and Na poor, with less than 0.1 and 0.05 are interpreted to represent subsolidus or near solidus
cations per formula unit, respectively(Table 3).
$owth during slow cooling.
l310
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
TABLE
4. Representativeamphibolecompositions
Phenocrystsfrom trachyandesite
Sample
sio,
Al,o3
FeO
Mgo
Tio,
MnO
BaO
CaO
Naro
KrO
cl
F
Total
Si
AI
FE
M9
Ti
Mn
Ba
Ca
Na
K
cl
F
Total
Mg/(Mg + Fe)
CR79b
core
CRSTR4
rim
CRSTR4
mantle
42.3
12.0
9.36
15.3
1.87
0.09
0.07
14.0
2.61
0.74
o.o2
0.13
98.43
42.9
11.8
8.78
16.0
2.57
0.12
0.06
11.8
2.48
0.75
0.00
0.31
97.56
42.4
12.4
10.8
13.8
2.37
0.10
0.10
11.6
2.50
0.86
0.05
0.06
97.1
6
6:t72
2.066
1.143
3.332
0.205
0.011
0.004
2.186
0.739
0.138
0.005
0.060
16.061
74.5
6.257
2.026
1.070
3.470
0.282
0.015
0.003
1.834
0.701
0.139
0.000
0.143
15.940
76.4
6.267
2.160
1.333
3.046
0.263
0.013
0.006
1.842
0 716
0.162
0.013
0.028
1s.849
69.6
CO21
manfle
THR17
rim
Oxidewtc6
42.6
43.1
12.0
13.0
11 . 1
8.94
13.1
14.9
2.85
2.80
0.23
0.10
0.05
0.06
1 1. 4
11.4
2.66
2.69
0.76
0.75
0.04
0.02
0.34
0.53
97.03
98.26
Cationsper 23 O aloms
6.298
6.222
2.086
2.209
1.375
1.080
2.892
3.208
0.317
0.304
0.029
0.012
0.003
0.003
1.803
1.762
0.763
0.753
0.144
0.138
0.010
0.005
0.159
0.242
15.880
15.939
67.8
74.8
Porph.
THR17
rim
c5
c5
core
core
40.3
15.0
13.9
10.8
2.41
0.21
0.05
10.8
2.66
1.O2
0.10
0.26
97.55
44.9
10.5
8.82
15.7
2.51
0.14
0.04
10.9
2.47
0.83
0.04
0.20
97.1
6.014
2.648
'1.741
2.402
o.271
0.027
0.003
1.730
0.770
0.194
0.025
0.123
15.948
58.0
6.532
1.804
1.073
3.405
0.275
0.017
0.002
1.706
0.697
0.154
0.010
0.092
15.767
76.0
CRse
core
CRSe
nm
42.9
'12.1
10.3
14.3
2.66
0.2',1
0.07
11. 5
2.65
0.98
0.04
0.09
97.84
42.2
11.9
12.8
12.8
1.43
0.42
0.07
11.1
2.42
1.16
0.06
0.39
96.75
40.1
13.8
13.5
11.4
2.06
0.23
0.07
11.5
2.57
0.79
0.07
0.09
96.19
6.286
2.092
1.266
3.118
0.293
0.026
0.004
1.798
0.752
0.183
0.010
0.042
15.869
71.1
6.331
2.101
1.607
2.860
0.161
0.053
0.004
1.777
0.704
0.222
0.01
5
0.185
16.020
64.0
6.083
2.468
1.707
2.577
0.235
0.030
0.004
1.859
0.755
0.153
0.018
0.043
15.932
60.2
/Vote:Porph. is porphyritictrachyandesiteenclave,gb. anorth. is gabbroic anorthosite enclave.
Biotite
Most biotite crystals were nonstoichiometric due to loss
of K. The stoichiometric analyses(Table 5) indicate that
biotite Me/Mg * Fe) rangesfrom 0.56 to 0.64 and avTABLE
5. Biotitecompositions
co18
sio,
Tio,
Alro3
FeO
MnO
Mgo
CaO
BaO
Naro
KrO
cl
F
Total
si
Ti
AI
FE
Mn
Mg
Ca
Ba
Na
K
F
Total
36.0
4.35
14.6
16.2
0.23
13.9
0.01
0.91
0.50
8.39
0.09
0.22
95.32
5.465
0.495
2.603
2.060
0.026
3.134
0.000
0.053
0.147
1.623
0.022
0.102
15.729
co21
CO22
Oride wtc6
36.5
35.7
36.3
4.24
4.00
3.54
14.8
15.6
15.8
14.5
15.6
15.4
0.18
0.22
0.18
14.6
13.3
13.5
0.02
0.00
0.00
0.97
0 73
1.05
0.79
0.65
0.89
8.31
8.29
8.23
0.08
0.09
0.06
0.13
0.00
0.12
95.14
94.15
94.97
Cationsper 22 O atoms
5.502
5.446
5.485
0.477
0.456
0.399
2.625
2.801
2.818
1.816
1.982
1.939
0.022
0.026
0.022
3.279
3.021
3.031
0.000
0.000
0.000
0.053
0.040
0.062
0.229
0.188
0.262
1.595
1.611
8.230
0.018
0.018
0.060
0.062
0.000
0.120
15.677
15.589
15.669
36.3
3.33
15.9
13.6
0.20
15.0
0.14
1.02
0.51
8.16
0.07
0.32
94.51
5.472
0.376
2.818
1.716
o.o22
3.357
0.022
0.057
0.146
1.566
0.013
0.150
15.716
erages0.60. Ba contents are relatively large (up to 1.5
wto,6BaO), and F and Cl contents are negligible.
Gnocrrnnrrsrny
The trachyandesitic intrusions are, in general, typical
of high-K calc-alkaline magmas (Table 6; Gill and Whelan, 1989), although a few samples are potassic enough
to plot in the extensionof the low-K shoshonitefield (Fig.
4). With the exception of the Cristo Rey rhyolite, the
intrusive suite and cognateenclavesdisplay a relatively
small range of SiO, contents (Fig. 5). They are AlrO, rich
(Fig. 5C), as is t1'pical of calc-alkaline suites. In plots of
SiO, vs. TiOr, MgO, and NarO, the compositions of most
dioritic and anorthositic xenoliths do not plot on the trend
defined by the trachyandesiteand cognateenclaves(Fig.
)).
Trace element compositions (Table 7; Figs. 6-8; also
see Hoover et al., 1988) are characterizedby the relatively high concentrations of Sr, Ba, Rb, and LREE and
low concentrations of Y, Nb, and Ta that are tlpical of
high-K calc-alkaline suites (e.g.,Gill, 198l). The Ba content of anorthositic and dioritic xenoliths is much lower
than that of the trachyandesitic host and the cogrrate
monzodioritic enclaves(Fig. 6). The two groups also plot
on distinct trends. In the Sr-Ba plot, two dioritic samples
plot with the trachyandesitic rocks; these diorites (samples CR2B and CRMA) are the samplesthat contain amphibole similar to amphibole phenocrystsin the trachyandesite.
l3l I
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
TABLE4-Continued
Diorite
Monzodiorite
cR69
mantle
cR69
nm
cR49
core
CR49
mantle
cR2b
nm
42.6
11.5
8.51
16.1
2.16
0.11
0.07
10.6
2.61
0.86
0.02
0.00
96.15
41.5
13.1
12.2
'13.2
2.52
0.17
0.04
11. 1
2.64
0.78
0.02
0.26
97.43
39.8
14.8
12.7
1 1. 7
2.41
0.18
0.07
10.7
2.67
1 . 11
0.05
0.25
96.46
40.9
14.1
10.7
13.0
2.48
0 . 11
0.11
11.1
2.71
1.09
0.03
0.30
96.64
48.5
5.59
11.2
15.3
1.71
0.40
0.03
11.5
1.59
0.52
0.12
0.36
96.77
6.414
1.987
1.046
3.531
0.239
0.014
0.004
1.665
0.744
0.161
0.00s
0.000
15.809
77.1
6.145
2.280
1.516
2.914
0.281
0.021
0.002
1.758
0.759
0.148
0.005
0.122
15.951
65.8
5.988
2.632
1.598
2.621
0.273
0.023
0.004
1.723
0.779
0.213
0.013
0.119
15.985
62j
6.082
2.473
1.335
2.879
0.277
0.014
0.006
1.769
0.781
0.207
0.008
0.141
15.973
68.3
Anorthosite
Gb. anorth.
cR2b
mantle
cR81
core
cR81
mantle
cRTt
mantle
cRTf
nm
CRTf
@re
THR17
core
39.5
14.3
17.6
8.50
1.63
0.33
0.07
11.2
2.16
1.27
0.09
0.18
96.82
44.1
9.40
16.0
11.1
1.11
0.54
0.00
11.3
1.91
0.87
0.07
0.03
96.34
43.3
10.1
17.2
10.2
1.43
0.43
0.00
10.2
2.08
0.96
0.08
0.34
96.21
42.5
10.6
16.0
10.8
1.59
0.45
0.07
10.3
2.O7
1.08
0.09
0.16
95.74
41.6
11.9
14.8
1 1. 1
2.77
0.16
0.08
11.6
2.23
1.45
0.29
0.00
97.54
6.070
2.s93
2.255
1.946
0.188
0.043
0.004
1.845
0.643
0.249
0.023
0.087
15.947
46.3
6.705
1.683
2.026
2.501
0.127
0.069
0.000
1.833
0.563
0.169
0.018
0.014
15.708
55.2
6.608
1.820
2.193
2.320
0.164
0.056
0.000
1.663
0.616
0.187
0.021
0.164
15.812
51.4
6.512
1.920
2.055
2.457
0.183
0.058
0.004
1.691
0.615
0.211
0.023
0.078
15.808
54.5
6.239
2.113
1.861
2.479
0.313
0.020
0.005
1.861
0.649
0.278
0.074
0.000
15.892
57.1
Oxidenrtolo
43.8
39.6
14.8
12.5
12.1
9.80
11.5
14.7
2.43
1.72
0j2
0.15
0.07
0.07
1 1. 1
11 . 9
2.63
2.s3
0.76
0.79
o.o2
0.04
0.29
0.00
95.42
97.95
per
23 O atoms
Cations
s.999
7.114
6.361
2.638
2.144
0.967
'l.528
1.370
1.191
2.604
3.349
3.183
0.277
0.189
0.188
0.015
0.018
0.050
0.004
0.004
0.002
1.805
1.801
1.858
0.773
0.452
0.713
0.147
0j47
0.097
0.010
0.005
0.030
0.139
0.000
0.167
15.939
15.588 15.812
63.0
71.O
72.8
maiorelementcompositions
Trau 6, Representative
Oxide wtc6, normalized to 100
Sample
sio,
Tio,
CR6F
CRTF
CRSTRT
CR74A
CR79A
CR79B
UNI86
CA
SDJ
SDJl
61.9
63.7
63.5
61.7
62.2
62.3
59.3
59.3
62.7
63.5
64.8
64.2
63.4
0.72
0.61
0.57
0.70
0.63
0.66
0.90
0.89
0.66
0.50
0.48
0.53
0.52
17.2
17.2
18.4
18.0
17.8
17.7
16.6
16.6
17.9
18.4
17.8
17.3
18.7
2.54
1.62
2.29
3.34
3.00
3.01
6.03
5.77
4.02
3.60
3.38
4.07
3.57
FE1
FE2
74.3
74.3
0.04
0.0s
't5.2
13.6
0.13
1.36
CR5E
60.0
0.83
18.5
CR4OA
69ENY
CR69EN
CRFLEN
61.1
59.8
60.4
61.8
0.81
0.90
0.92
0.89
17.O
16.6
17.4
17.4
3.47
5.70
3.34
2.88
CR159
CR82A
THRl 89
THR101
54.4
57.1
61.9
54.6
51.2
0.81
0.67
0.59
0.96
1.87
18.3
20.3
19.3
23.4
23.4
5.08
2.91
3.26
3.94
5.34
48.0
44.0
52.8
1.39
1.28
1.09
'17.4
4.28
14.15
7.OB
coR2
coR86
THR86
c5
CR2B
CRMA
co25
Al2o3
17.6
15.2
FerO"
FeO
MnO
Trachyandesite
2.22
0.09
0.07
2.23
0.07
1.32
1.15
0.08
1.12
0.09
1.14
0.09
0.09
n.d.
n.d.
0.09
n.d.
0.06
n.d.
0.06
n.d.
0.06
0.06
n.d.
n.d.
0.07
Rhyolite
Naro
GO
P.o"
LOI
4.28
3.48
3.45
3.94
4.08
4.13
4.54
4.45
3.91
3.57
3.32
4.40
4.30
5.29
5.88
5.75
5.89
5.81
5.72
5.22
6.16
6.04
3.94
6.15
2.95
3.08
2.85
2.98
2.66
2.99
2.75
3.03
3.30
2.49
2.54
3.38
1.52
0.48
0.48
0.45
0.44
0.41
0.46
0.38
0.40
0.29
0.30
0.28
0.28
0.36
1.83
0.90
1.37
1.15
1j2
1.26
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.67
0.63
5.23
4.93
4.36
4.30
0.03
0.14
n.d.
1.89
4.47
5.34
3.04
0.71
2.22
2.16
3.72
2.74
1.83
4.81
6.04
4.45
3.67
5.31
4.98
5.37
5.53
2.88
1.75
2.41
3.07
0.53
0.41
0.56
0.58
2.03
2.44
2.35
1.86
3.72
2.12
0.s0
1.47
2.29
7.83
6.47
2.39
9.23
10.30
5.54
6.17
5.28
5.72
4.67
0.86
1.03
6.52
0.49
0.70
0.48
0.48
0.24
0.13
0.05
1.61
0.45
n.d.
n.d.
n.o.
8.97
7.27
s.82
8.07
11.53
10.70
3.36
2.80
5.12
1.82
0.68
1.58
0.48
0.41
0.39
n.d.
n.o,
n.d.
Mgo
2.29
1.62
1.43
1.80
2.13
I
aF
4.17
3.95
2.01
1.38
1.30
1.81
1.36
0.10
n.d.
0.02
0.55
0.04
n.d.
Porphyritic trachyandesite
1.79
0.16
3.16
2.05
Monzodiotite
1.88
0.11
n.d.
0.11
0.10
2.30
0.10
2.22
Anorthosite
2.53
0.18
2.56
0.13
n.d.
0.07
n.d.
0.06
n.d.
0.09
Diotite
6.07
0.21
n.d.
0.24
n.d.
0.20
Cao
s.20
5.CC
t3t2
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
no
o
L
a
E
l^
I-
nz
Oi)
o6
^v
u{o
o*
^ *
.t&_
ti
A
Fco
tr
si ''"
A SOUTHERN
)
o cENTFTAL
HOST
I
V N O R T H E B N'
I P O R P H Y R T T T C) - . . ^ , _
ENUL'
a MoNzoDroRrrrct
1.
v
^
coR
N
Y
+
oo
z
$R
o.7
o
cR -
v ua./
ra^cHvaNDEstrE
I
PoRtr ENCLAVE
.
MoNzoDIoRITIcENCLAVES
O
oroRrrc ENCLAVES
o DroRrTrc
\
ft rHonrros,rrc f
o
oL-
0-8
\
F
o
1.0
\
o
o
o
rrE
v-v I
la
-rH
ffi
au
ANoRTHoslrE
o
B
1.0
2.O
si02
t4lAl
Fig. 3. Amphibole compositions. All cation proportions per
23 O atoms. Fields shown for amphiboles from Coronado sill
(COR) and anorthositic enclaves.(A) Variation of Mg/(Mg +
Fe) with Si; (B) variation of Na/(Na + K) with Al in tetrahedral
sites.
c
The trachyandesitic intrusions can be subdivided on
the basis of trace element compositions (Hoover et al.,
1988). The central intrusions contain relatively larger
abundances of Sr at a given Ba concentration (Fig. 64,)
aoo
xENo
N
^-
o
o
r
\e
f
'9Y-o "
-iEo
w
V*
B oo*
^a
oa"
D
:/
o
A
-N4
SOUTHERNINTRT'SIONS
tr CENTRALINTFUSIONS
2
V
NoRTHERNtNTRUstoNs
I
PORPH ENCLAVE
r,
MoNzoDroRtTtcENcLAvE
^5
N
d
z
O oroRtTrc ENCLAVE
ft rNonrxosmcencuve
Kzo
Fig. 4. Alkali contentsof trachyandesiteand enclaves.Fields
shown are from Gill and Whelan (1989): high-K calc-alkaline
(HKCA), low-K shoshonitic (LKSH), and medium-K shoshonitic (MKSH).
70
sio2
Fig. 5. Major element variation as a function of SiO, content. (A) MgO; (B) TiO,; (C) ALO,; (D) Na,O. Porphyritic and
monzodioritic enclavesare cognate;dioritic and anorthositic enclavesare xenolithic.
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
1313
a
Fig. 6. Trace element variation diagrams.Short-dashedline
enclosescompositional field ofanorthositic enclavesand dioritic
enclaves with Fe-rich amphibole. Long-dashed line separates
central (Sr-rich) from northern (Sr-poor) trachyandesites.(A) Sr
vs. Ba; (B) Ba vs. Rb; (C) Rb/Sr x 100 vs. Rb; (D) Sc vs. Ba;
(E) Y vs. Ba.
&
60
80
1oo
Rb
and generally have Ce*/Yb* between 14 and 22. The
northern intrusions contain less Sr but higher concentrations of Sc, Cr, V, and Y(?) at a given Ba concentration
and higher Rb/Sr ratios than the central intrusions (Fig.
6). Ce"/Yb* in the northern group ranges from 10.5 to
15. Samplesfrom the southern part of the area plot with
both groups.
The moderately steep REE patterns of the trachyandesitic host rocks show little or no inflection and no Eu
anomaly (Fig. 7A). The diference in Ce*/yb* between
northern and central sills cited above is largely due to
variations in Yb content. In contrast, the rhyolitic intrusion at Cristo Rey contains lower REE abundancesthan
the trachyandesiticintrusions. It also displays an unusual
REE pattern with relatively flat LREE, steeper HREE,
and no Eu anomaly (Fig. 7A). This pattern probably reflects loss of LREE during alteration and weathering.
The REE pattern ofthe porphyritic trachyandesiteenclave from Cristo Rey is identical to that of the host rocks
(Fig. 7B). Monzodioritic enclave REE patlerns have slopes
similar to the host trachyandesitesbut contain somewhat
lower REE abundances.
Anorthosite xenoliths contain lower abundances of
REE, have Ce*/Yb* lessthan 12,and display pronounced
positive Eu anomalies (Fig. 8C).
Dioritic enclaveshave Ce*/Yb" less than 12 and generally less than 6. The two enclavesthat are chemically
and mineralogically similar to the trachyandesite(CR2B
and CRMA) contain lower LREE abundancesthan do the
other dioritic samples (Fig. 7C). Although plagioclase
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
V
NORTHERN SILLS
o
CENTFAL INTRUSIONS
.
MONZODIORITE
.
POBPH TRACHYANDESITE
t>
Nd
SmEu
ANORfrOSITE
DtoRlrE
cognate enclaves
trachyandesite
LaCe
O
Yb Lu
LaCe
Tb
Nd
SmEu
xenoliths
LaCe
YbLu
Tb
Nd
SmEu
Tb
and rhyolitic intrusions;(B)
(Haskinet al., 1968)rare earthelementplots.(A) Trachyandesitic
Fig.1. Chondrite-normalized
enclave;(C) dioritic and anorthositicenclaves.
and porphyritictrachyandesitic
monzodioriticenclaves
Dioritic and anorthositic xenoliths have distinctly difphenocrystsare sparseor absent in the dioritic enclaves,
patternsfrom the trachyandesite(Figs. 8C and 8D)'
ferent
ihree diiplay slight positive Eu anomalies.
in keeping with their xenolithic character.
Incompatible element patterns
The trachyandesitic rocks plot in a narrow band on
DrscussroN
"spider diagrams" (Fig. 8A; normallization factors of r 1,-,!--- --variables
Intensive
Thompson et al., 1984). Porphyritic trachyandesitic and
Qualitative estimatesof the ?"and /o, of the trachyanmonzodioritic enclaves have similar patterns (Figs. 88
gD).
desitic magmas can be made on the basis of comparison
and
traceelementconcentrations
TABLE
7. Representative
La
Ce
Nd
Sm
21.6
40.7
23.3
42
26.0
30.4
54.9
61.7
20.6
24.4
4.0
4.5
FE1
s.2
21.5
cRsE
18.9
36.7
1 7. 8
cR10A
cR13A
CR4OA
CR69ENY
CR69ENZ
CRT2AEN
26.1
25.9
45.0
46.2
17.5
223
co25
cR2B
CRMA
THR29
30.2
17.6
13.5
25.3
70.0
33.6
36.9
67.0
cR6f
CR74A
CR79A
CR79B
CRIST3
CRIST3
Eu
Tb
Trachyandesite
0.40
1.03
1.21
1.42
Rhyolite
0.78
3.4
0.31
0.51
Cr
Co
0.14
0.14
6.1
6.7
6.2
6.3
3.8
4.2
40
32
32
40
17
12
72
29
0.04
1.1
1
0
0.77
0.12
4.5
4
8
0.72
0.91
0'11
0.13
1.2
7.0
8.1
11.0
9.8
12.1
3
39
2
48
13
122
2
12
3.90
1.52
1.63
1.34
0.51
0.21
0.22
0.18
16.0
31'3
39.0
15.5
175
316
I
149
21
34
42
27
Lu
0.80
0.12
o.77
o.72
0.22
Porphyritictrachyandesite
0.38
1.13
4.1
Monzodiorite
0.28
0.71
3.1
0.44
1.19
48
8.9
5.6
5.2
6.5
Sc
Yb
Diorite
1.13
2.OO
0.72
2.05
1.88
0.57
o.8o
2.45
Anorthositicgabbro
CR82A
THR10
c5
10.8
3.9
22.5
9.3
Note.'Blank entry indicateselement not determined,
2.O
0.9
Anorthosite
1.77
1.00
0.17
013
0.07
0.03
A.4
0
6.0
2.4
2
10
I
19
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
with experimental studies.The presenceof early-formed
amphibole phenocrystsis suggestiveof HrO contents of
3.0 to 3.5 wto/o(Eggler, 1972). euartz and biotite phe_
nocrysts in magmas with comparable HrO contents are
stableat Zless than 800 "C and probably at lessthan 750
l3l5
correspondsto a pressureof5.5 kbar (Johnsonand Rutherford calibration, 1989) and an estimated depth of crystallization of the monzodioritic enclavesof 16 km, with
minimum uncertainty of +4 km. By contrast, the probable depth of emplacement (estimated on the basis of
stratigraphic reconstruction)was less than 2 km.
Mass balancecalculations
Fractional crystallization relationships among rock types
in the Cristo Rey intrusion were testedfor major element
positive correlation between Mg/Mg + Fe) and Si in compositions with least-squares
mass-balancecalculaamphibole from the Cristo Rey intrusion (e.g., Cza- tions (Bryan et al., 1969). Parent-daughterrelationships
manske et al., 198l). In support of this inference, it can that were tested include diorite (sample CR2B) to trachybe noted that the assemblagecalcic amphibolo * mag- andesite,monzodiorite to trachyandesite,trachyandesite
netite + titanite * quartz + ilmenite is presentin several to rhyolite, and low-Si trachyandesiteto high-Si
trachymonzodioritic (cognate) enclaves. It corresponds to f,
andesite (Table 8). None of the attempts to produce
conditions between I and 2 log units higher than the NNCi trachyandesitefrom dioritic enclavemagma was successbuffer (Noyes et al., 1983). Similar conditions of crystal- ful. Evolved trachyandesite can be produced from maglization are reasonablein view of the sirnilarity of mineral ma of monzodioritic composition by removal of plagioand elemental compositions. Although coexisting Fe-Ti clase * amphibole + magnetite + ilmenite and from less
oxide phenocrystsare present, subsolidusexsolution and evolved trachyandesiteby removal of amphibole + mialteration have rendered these phasesuselessfor deter- nor plagioclase + apatite. The rhyolite can be derived
mination of T and fo,.
from evolved trachyandesiteby removal of plagioclase,
K-rich monzodioritic enclavescontain the assemblage amphibole, biotite, magnetite, and apatite. This calculacalcic amphibole + biotite + plagioclase + alkali feld- tion requires removal of approximately 510/ocrystals
Oy
spar + qtuartz+ magnetite + titanite + ilmenite. If equi_ weight). Unfortunately, when acceptable (i.e., sum of
librium was attained with near-minimum interstitial melt, squares of residuals < 1.0) mass balance solutions
are
then the Al content of calcic amphiboles is pressurede- examined in detail, they either fail to fit important elependent (Hammarstrom and Z,en, 1986 Hollister et al., ments adequately (especiallyTiOr,
PrOr, and NarO), or
1987). The averageAl content (cations per formula unit) the daughter product is not typical of the bulk
of the
of the rims of magmatic amphibole from appropriate trachyandesites. In addition, fractional crystallization
monzodioritic enclavesis 2.12 + 0.15 (lo). This value cannot explain the positive correlation ofSc and Ba
and
Taete7-Continued
NiRbSrZrNbBaCsHfUThTay
30
16
25
33
12
13
68
64
60
61
52
59
1328
1403
1201
1163
1126
1189
0
103
242
62
3
12
55
1270
132
6
10
28
0
32
o
55
31
35
64
33
4't
20
1065
1190
1062
1707
941
627
114
161
143
157
152
111
82
33
13
22
40
24
7
48
730
700
1037
1510
63
84
65
49
7
15
18
6
0
12
1056
112
5
n
58
4
13
1013
772
158
149
138
143
142
143
56
263
3
3
3
J
o
o
5.5
5.4
5
5
5
5
1
Trachyandesite
1736
1.62
1777
1633
1678
1390
1684
2.84
2.04
Rhyolite
808
1.26
Porphyritic trachyandesite
1580
1.06
Monzodiorite
521
8.62
522
6.76
2013
1595
1657
312
Diorite
1036
1.58
't.70
940
449
2.'t0
't
864
.AS
Anorthositic gabbro
554
Anorthosite
461
468
U.CJ
'1.73
4.49
0.51
12
12
12
11
3.87
4.41
1.38
1.62
3.97
4.04
1.46
0.86
10
2.44
2.04
1.25
1.39
I
2.70
1.34
2.86
0.26
3.08
3.15
1.09
1.46
4.60
4.21
0.48
0.47
15
16
17
13
1.70
3.00
1.33
1.39
0.64
0.69
o.27
0.31
1.78
1.47
0.56
0.80
0.37
45
22
22
24
18
1.03
0.32
0.10
0.18
0.36
0.23
0.12
6
3
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
l 3l 6
10 0 0
porph
rhyolite
t r a c h y a n d e si t e
monzodiorite
gabbro
anorthosite
BaRbThK NbTaLacesrM
P S m Z I I + T T ]T b Y T m Y b
B a R b T h K N b T A L A C C S Tl i d P S M Z T I I T T IT b Y T M Y b
Fig. 8. Incompatibleelementvariationdiagrams;normalizationfactorsfrom Thompsonet d. (1984).(A) Typical field for
enclave(porph) and rhyolitic intrusion southof Cristo
trachyandesiticintrusions.(B) Compositionsof porphyritic trachyandesite
dioritic
with Fe-richamphibole;opensymbolsrepresent
filled symbols,.pr.r"nt dioritic enclaves
Rey.(C) Dioritic compositions;
xenolithic
and
enclaves
monzodioritic
of
cognate
(D)
Compositions
host
trachyandesite.
similar
to
enclaveswith Mg-rich amphibole
anorthositeand anorthositicgabbro(gabbro).
from the Cristo Rey intruof Y and Ba among samples
sion (Figs. 6D and 6E).
The scatter observed in many variation diagrams suggests that problems in obtaining acceptable solutions to
massbalancecalculationsmay bi becauseof variable degreesof crystal accumulation (see Hoover et al., 1988).
ih" ,.utt"i is particularly apparent in plots of AlrOr, Sr,
and compatibli trace elements(Figs. 5 and 6). If crystal
sorting and accumulation occurred during flow and em-
placement of the trachyandesiticmagma' then the effects
of both fractional crystallization and crystal accumulation would be present in the suite. Part of the scatter
observed in the northern sills, particularly the higher concentrationsof Sc, Cr, Ni, and MgO, can also be explained
as contamination by dioritic and anorthositic xenoliths.
Such contamination would be the result of fragmentation
and resorption of the xenoliths and would explain the
increasein compatible elementconcentrationsin samples
TA
Phases removed
Daughter Amph.
CR6F
trach.
CR6F
trach.
69ENZ
monzo.
CRTF
trach.
CRTF
trach.
CRTF
trach.
CRTF
trach.
CRTF
trach.
FE2
rnyo.
FE2
rhyo.
7.5
7.2
12.8
4.1
6.8
6.2
6.2
4.3
(AN"J
3.7
(An*)
7.9
(An*)
3 7. 2
(An.u)
38.0
(An*)
ft
MT
Plag.
0.3
87.9
0.33
poor fit for Na
0.1
89.0
0.20
78.9
0.37
poor fit for Na, plag
too calcic
poor fit for P, with or
without apatite
poor fit for Na
0.4
1. 1
1.5
49.4
0.83
1. 3
1.2
47.1
0.40
Note.'Abbreviationsare trachyandesite(trach.),monzodiorite(monzo.),rhyolite (rhyo.).
poor fit for Na and
Mn
BARNES ET AL.: INTRUSIVE ROCKS AND THEIR ENCLAVES
t3r7
ofthe host trachyandesite.The calcic plagioclaseand the continental arc magmas (Fig. 8; Gill, l98l; Thompson
et
resorbed, Fe-rich amphibole in the northern intrusions al., 1984). As such, these rocks representearly manifesare probably xenocrystsbroken from dioritic and anor_ tations of Tertiary arc activity in the Trans-Pecosof westthositic xenoliths. Finally, the low abundanceof incom- ern Texas. Further constraints on magma sources
will
patible elements (especially LREE) in the rhyolite sug_ require detailed isotopic investigations.
gests equilibrium with LREE-rich accessory minerals.
either in the sourceor during fractionalcryst;llization.
AcrNowr,nncMENTs
Origin of the enclaves
The presenceof deformed crustal xenoliths within some
cognatemonzodioritic enclavesindicates that they began
to crystallize from the trachyandesitic magma along the
margins of a magma chamber. If p estimatesare correct,
this chamber was in the middle crust. In any case,slow
cooling and crystallization of the monzodioritic rocks resulted in hypidiomorphic granular texture and in prismatic habits of accessoryminerals. parts of this monzodioritic rind were engulfed in and disrupted by the
trachyandesiticmagma as it rose to the site of final em_
placement in the shallow crust.
The anorthositic xenoliths (and most dioritic xenoliths)
are distinct from the host trachyandesitein terms of plagioclaseand amphibole compositions and in Ba, Sr, REE,
and HFSE concentrations.The fact that these xenoliths
occur not only in the host trachyandesitebut also in cog_
nate monzodioritic enclavessuggeststhat the wall rocks
ofthe midcrustal trachyandesitemagma reservoir(s)were
predominantly anorthositic and dioritic.
CoNcr,usroNs
The trachyandesiticintrusions in the El paso areawere
emplaced at high levels in the crust after possible resi_
dence in the middle crust. The trachyandesitic magma
was emplaced at temperaturesof 850 .C or lower, con_
tained approximately 3.0 wto/oHrO, and had,fo. I to 2
log units higher than NNO.
The wall rocks of the magma chamber consisted, at
least in part, of anorthosite and related diorite and anorthositic gabbro. The presenceof weakly deformed anorthosite in the crust beneath the El paso area is consis_
tent with widespread occurrence of proterozoic
anorthosite in the southwesternUnited States(e.g.,Ems_
lie, 1978;Anderson,1983;Mckmore and McKee. lggg:
Padovani and Carter, 1977). It seems likely that anorthositic xenoliths in the trachyandesitesrepresentan asyet unrecognized segment of Precambrian crust in the
area.
As the trachyandesitebroke out of its crustal chamber,
it incorporated part of its mostly crystalline rind to form
monzodioritic enclavesand also engulfed anorthositic and
dioritic wall rocks. Disruption of theserocks and contamination of the host trachyandesite resulted in bimodal
compositions among amphibole populations and in elevated abundancesof compatible elementsin the northern
sills.
The elemental abundances(i.e., enriched LILE, negative slope, and relatively depleted HFSE) of the trachyandesitic intrusions display patterns that are typical of
We are grateful to T. Frost, R. Wiebe, C. Bacon, T. Prestvik, and R.
Cullers for helpful reviews, to B May and W.M. Shannon for assistance
in the fleld, and to M.A. Barnes for help in the laboratory. S.E.E.'sfield
studies were funded by the New Mexico Bureau of Mines and Mineral
Resources,the Penrosebequestfrom the Geological Society of America,
and Texas Tech University. Analyses were funded by NSF grants EAR840319and EAR-8720141(C.c.B.).
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