JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 96, NO. B8, PAGES 13,561-13,575, JULY 30, 1991
CompositionalChangesin Trans-Pecos
TexasMagmatism
Coincidentwith CenozoicStressRealignment
EPac W. JAMESAND CHRISTOP•
D. HENRY
Bureauof EconornicGeology,Universityof Texasat Austin
The composition
and inferredsourcesof Cenozoicmagmasin Trans-Pecos
Texas changedabruptlybetween32 and
28 Ms, coincidentwith a fundamental
changein stressregime. Magmasemplacedbetween48 and 32 Ma comprise
extendeddifferentiationsuitesbeginningwith relatively evolvedbasalticmagmas. These rockshave low Nb and Ta
comparedto Zr, Hf, La, Ba, and K, characteristics
typicalof continental
volcanicarcs. However,the Texasrocksare
more alkalic, havehigherconcentrations
of incompatible
elements,andhavehigherratiosof Nb andTa to Zr, Hf, La,
Ba, and K than is typical of rocks from arcs near trenches. These differencesreflect relative positionwithin the
Cordilleran arc. Trace element models involving partial melting in the mantle wedge combinedwith fractional
crystallizationandassimilationof upperand lower crustcanaccountfor mostof the observedtrace elementconcentrations
and ratios. Trans-Pecos
magmatism
mayhaveinvolvedsmallerdegreesof partialmeltingthanis typicalof arcslocated
closerto the subduction
zone. Magmasemplacedafter 31 Ma were eitherbimodal,alkali basalt-rhyolite,
or exclusively
basaltic. They havehigh absoluteabundances
of incompatible
traceelementsand high ratiosof Nb and Ta to Zr, Hf,
La, Ba, andK. The maficrocksare typicalof continentalrift or oceanislandbasalts. Trace elementmodelsusingNb,
Ta, Y, Yb, Hf, andZr reproducethe rangeof concentrations
andratiosobservedin theserocksand requirevery small
degreesof partial melting,variableamountsof fractionalcrystallization,
but little or no assimilation.The changein
magmacompositions
and sourcesis best illustratedby a drop in Zr/Nb, from between18 and 6.25 in the earlier, arc
rocksto between6.25 and2 in thelater,rift-relatedrocks. Stressregimeappears
to playan important
rolein theprocess
of magmagenerationand evolution. The changein traceelementcompositions
indicativeof tectonicsettingsupports
interpretations
of palcostress
datathat activityup to 31 Ma occurredin a continental
volcanicarc, whereaslater activity
occurredin a settingof intraplateextension.The magmatic
changemayhavebeensimultaneous
with thechangein stress
regimeor may havelaggedby as muchas 3-4 m.y.
INTRODUCTION
ting was analogousto that of the Kenya rift. Further work
[McDowell and Clabaugh, 1979; Barker, 1979; Damon et al.,
Cenozoic volcanic and intrusive rocks in Trans-Pecos Texas
representthe easternmost
advanceof Cenozoicmagmatismin
theUnitedStatesandreflectconditions
relatedto theirposition
at the inboardfringe of the Cordilleranmagmaticarc. They
show the combinedeffects of location, age, and tectonicson
magmaticcomposition.The Trans-Pecosigneousprovince
extends(Figure 1) from the junction of the bordersof New
1981; Cameron and Cameron, 1986; Cameron et al., 1986,
1989] illustratedthe progressionin agesand compositions
in
rocks stretchingfrom the west coast of Mexico across the
Sierra Madre
Occidental
and into Trans-Pecos
Texas.
This
progressionsuggests
thatrocksin Trans-PecosTexaswith ages
from 48 Ma to at leastasyoungas31 Ma arepartof a single,
long-livedmagmaticarc.
In contrast,theyoungestigneousrocksin Trans-Pecos
Texas,
with Basin
andE1Paso,500 km southeast
throughthe Big Bendregionof 24-17 Ma alkali basalts, are contemporaneous
Texas andinto the stateof Coahuila,Mexico. Rock composi- andRangefaulting.Thesebasaltsclearlypostdatesubduction
tions become more alkalic from southwest to northeast across
at thislatitude[Atwater,1970]andarepartof widespread
Basin
the 150-km width of the province. This zonationparallels andRangemagmatismacrosswesternNorth America.Although
compositionaltrendsin the rest of the CenozoicCordilleran the originsof Basin and Rangemagmasare controversial,it is
sources
magmaticbelt. Emplacementagesrangefrom 48 Ma, closely generallyacceptedthatthebasaltshaveasthenospheric
postdatingthe end of Laramidefolding,to 17 Ma, concurrent and magmatichistoriessubstantiallydifferent from thoseof
with Basin and Rangefaulting. This time interval spansthe the earlier magmas.This contrastbetween arc and Basin and
transitionfrom a convergentto a transcurrentcontinentalmar- Range magmasposesthe questionof the exact time of the
gin and a coincidentchangein regionalstressregime at about transition. Many papersand argumentshave addressedthe
timing of Basin and Range faulting and magmatism in
31 Ma [Henryet al., thisissue].
Mexico, Texas,and Chihuahua,Mexico, near the citiesof Juarez
Thetectonic
setting
of theTrans-Pecos
igneous
province
areas
throughout
theCordillera.
Price
and
Henry
[1984,
1988],
hasbeen
amatter
ofinterest
since
Lipman
etal.[1972]
first Henry
andPrice[1986]
andHenry
etal.[this
issue]
present
proposed
thatCenozoic
igneous
rocks
asfareast
asColorado,
evidence
fora major
change
instress
regime
inTrans-Pecos
New Mexico, andTexas were the consequence
of subduction Texas at about 31 Ma. Dike, vein, fault, and slickenline
indicatethatthe maximumprinciplestress(c•)
along the westernmargin of the North Americanplate. The orientations
andtheminimumprinciplestress
(c•)was
Trans-Pecos
rocks,however,areunusuallyalkalinecompared waseast-northeast
from 48 Ma to 31 Ma. Henry et al. [this issue]
to most arc rocks,andBarker [ 1977] suggested
that their set- north-northwest
interpret
a change
to c• verticalandc•3east-northeast
at31Ma
to mark thebeginningof BasinandRangeextension.Price et al.
Copyright
1991bytheAmerican
Geot•ysicalUnion.
Papernumber91JB00203.
0148-0227/91/91JB-0(Y203505.00
[1987] demonstrated
differencesin major elementchemistry
betweenTrans-PecosTexas alkaline rocks emplacedduring
the earlier
arc-related
and those related
to later extensional
tectonicregimes.Cameron et al. [ 1986] and Gundersonet al.
13,561
13,562
SAMOS
ANDH•u•¾: COMPOSITION
CHANOF•
INTa•Ns-P•os T•XASMAOMATXSM
48-39 Ma
38-32 Ma
El, 106
1•04øNew
Mexico
I
, ø' .
A, 1060
I
•.,.ii.•l
Paso
1•04
øNew
Mexico
I
Texas•/r--•.--,
.-."
• ' ßDiablo
Plateau
Texas
x,-•,
• FinlayMtns.
"i•
x•47-48•
Chihuah•aX•
'
•,•
Early
•
•, .
+31o
•'%o"•
•Q E
W
C.
106
ø
oAIpine
Main
[ 32371
\ poAlpine
ø33
(
S34+29%
24-17
Ma
1•04øNew
Mexico
i
Texas
D.
Ma
106
ø
1?4øNew
Mexico
I
I
•.•1
Texas
Paso
•,_
•
•
Cox
Mountain
•/•e+/•'ø•
*
Davis
+31
o
'•.
RimRock
'•
Earl
oAlpine
extensional
/'••Mountains
Bofecillos
7-28
•28
:M.28.BM
+31o
""•••.•
o37\
•Ee
M
•a
•46
'! ,.,,Alamo
/ Creek
• ......
•
.IBasalt
',.? ?.... ._....•_/+29
ø
31-27
•.•.37
dikes
Mountains
""
i,.•,
24-18
Basin•X,
øAlpine
and
range •
Black
Gap,•22
•18
22
%22
oø23
•'24-••023
•:
'\,
+29 ø
Fig. 1. Distribution
of magmatism
through
timein Trans-Pecos
Texas.Numbers
indicatetimesof majoractivityof individual
centers
orareasofmagmatism.
Ruledareasarecalderas
andsolidareasaremajorintrusions.
(a) Earlymainphase,
48-39Ma. Cross,
Christmas
Mountainscalderacomplex.(b) Main phase,38-31Ma. AC andA indicatealkali-ealcic
andalkalicbelts.Q, Quitman
Mountainscaldera;E, EagleMountainscaldera;VH, Van Hom Mountainscaldera;W, Wylie Mountainscaldera;B, Buckhom
caldera;EM, El Muertocaldera;P, Paisano
volcano;I, Infiemitocaldera;C, ChinatiMountains
caldera;PC,PineCanyoncaldera;
S, SierraQuemadacaldera.(c) earlyextensional
phase,31-27Ma. SC, SanCarloscaldera;Sa,Santanacaldera;RM, Rattlesnake
Mountainsill;BM, BurroMesa.(d) BasinandRangephase,24-17Ma.
[ 1986]firstnotedthatchanges
in traceelementconcentrations31-27 Ma, and 24-17 Ma (Figure1). The first two phases,
of rocksin theBig BendareaandadjacentChihuahua,
Mexico, earlyandmainphasearc-related
magmatism,
areclearlyrelated
by a significantincreasein volumeof
werealsocoincident
withthechange
in tectonic
reg!meat but are distinguished
about31 Ma. Thispaperpresents
newmajorandtraceelement magmaticactivityat 38 Ma. Theearlyext.
ensional
phasebegan
analysesfor rocks in Trans-PecosTexas, which more than at about31 Ma, closelyfollowingmainphaseactivity.As we
doublethenumberof traceelementanalyses
for rocksemplaced showbelow,magmaseraplacedbetween31 and 27 Ma have
in theextensional
regime.Thesenewdata,alongwithpublished significantlydifferentsourcesandevolutionthanearlierrocks.
analyses
from theentiregeographic
andagerangeof theTrans- The youngest
phasecomprises
maficlavasanddikesclearly
Pecosprovince,indicatea significantchangein magmasources relatedto Basinand Rangefaults.The next sectiondescribes
andevolutioncoincidentwith or slightlylaggingthechangein eachof thefourphasesin moredetail.
stressregime.
Initialactivitycentered
in twoareas,onein thenorthern
part
Igneousactivityin Trans-Pecos
Texasis dividedinto four of the province, stretchingfrom El Paso and Juarez to the
ageperiods[HenryandMcDowell, 1986];48-39 Ma, 38-31 Ma, FinleyMountains,anda southern
centerin theBig Bendregion
JAMESANDH•-•¾: COMPOSmON
C•4ANOF_.S
m T•,ss-P•os
T•,•s •IAGMATISM
13,563
(Figure1a). Activityin thenorthernareacomprises
hypabyssal
trachyandesite
intrusions[Hooveret al., 1988]. The rocksare
quartz normatire and contain amphibolephenocrysts.Amphibolephenocrysts
are absentin almostall rocksof the same
age to the southeastand in all youngerrocks. K-At ageson
Mountains,Big Bend area, and near Black Gap. Dikes are
commonlyrelated to Basin and Range faulting [Henry and
Price, 1986; Henry eta/., this issue].These lavas have Mg
numbersin the sixties,Ni contentsgreaterthan 100 ppm, and
several of these intrusionsare 47 and 48 Ma, and field relations
themostprimitivein Trans-Pecos
Texas.
some dikes contain mantle
showthattheypostdateLaramidefoldsandfaults.
Early magmatismin the Big Bend area is recordedby the
4742 Ma Alamo Creek and Ash Springbasaltsof the Chisos
xenoliths.
These mafic
rocks are
MAJOR Am) TaAc•. ELEMENT DATA
Formation and related small intrusions [Maxwell et al., 1967;
Stewart, 1984; Schucker, 1986; Lonsdale, 1940; Gunderson et
New major and traceelementanalysesof low-silicarocks
from the Trans-Pecosmagmaticprovinceare presentedin
between 47 and 40 Ma extend northward into the Christmas
Table 1. Additionally,this paperusesrepresentative
analyses
Mount-•..• where a small calderacomplexhas _l•_.en
mapped compiledfromthesources
listedbelowto illustratethechanges
[Henry and Price, 1989]. Rock compositionsrange from in magmaticcompositions
with time.We haveusedanalyses
basalticto rhyolitic and include both nephelineand quartz for which we have goodgeochronologic
controlfrom Barker
al., 1986]. Numerous sills, laccoliths, and stocks with ages
nonnative
series.
[1977, 1987], Barker et al. [1977], Cameron and Cameron
Main phasevolcanismbeganat about38 Ma and continued
through32 Ma acrossthe entire province(Figure lb). This
periodis typifiedby large-volumeashflow tuffs eruptedfrom
small to moderatelylarge calderasand widespreadintrusions
and lava flows. These rocks representthe largestvolume of
volcanic material in Trans-PecosTexas. Magmatism during
this phaseshowsa clear zonationwith respectto alkalinity
[Barker,1977].An eastern,alkalicbelt is deftnedby Peacock
indices and the presenceof clinopyroxenewithout orthopyroxene.Contemporaneous
igneouscentersin the western
belt are alkali-calcicand have two pyroxenes.Cameronand
Cameron[ 1986] considerthe westernbelt to be broadlycalcalkalic based on mineralogyand geochemistry.Peralkaline
rhyolite occursin both belts, but nepheline-normativeand
nepheline-bearing
rocks are found almost exclusivelyin the
[ 1986],Cameronet al. [ 1986],Gunderson
et al. [ 1986],Henry
andPrice [ 1986],Hooveret al. [ 1988],Irving andFrey [ 1984],
Mathews and Adams [1986], Nelson and Nelson [1986],
Nelson et al. [1987], Price et al. [ 1986, 1987], and Setter and
Adams [1986]. Where we have new trace element data for
previouslyanalyzedsamples,the newer,more accurateinstrumentalneutronactivationanalysis(INAA) datasupplantolder
X ray fluorescence(XRF) numbers.
To decreasechemicalvariationsintroducedby protracted
fractionationand possibleattendantassimilationand mixing,
we have selectedonly the mostSiO,_-poor
and MgO-rich
samplesavailablefrom eacharea.Unfortunately,
the scarcity
of mafic compositions
in someareasrequiresthe inclusionof
afewsamples
withasmuchas57%SiOn_.
All thevolcanic
rocks
usedin this studycanbe confidentlyplacedin one of the age
eastern alkalic belt.
categories outlined in the introduction using volcanic
Throughoutmain phaseactivitybothalkalicandalkali-calcic stratigraphy,
field relations,andabundantK/Ar data[Henryet
magmasequences
compriseextendeddifferentiationsuitesthat al., 1986].
startwith basalticbut relativelyevolvedmagmas.Mafic endThe chemical data we have gatheredcome from several
membersare scarceand generallyhave low Mg numbersand laboratories
andweregenerated
at differenttimesusingdifferent
Ni contents(<50 ppm), indicatingthat they are not primary techniques.
Investigators
whousedINAA typicallydetermined
compositions.
Derivationof the evolvedmembersis primarily concentrations
of Ta, Hf, and rare earth elements(REE) but
by crystalfractionation,combinedwith variable amountsof not Nb, Zr, or Y. WorkersusingXRF havedeterminedNb, Zr,
assimilationof wall rock [Cameron et al., 1986; Nelson et al.,
andY but lack Ta, Hf, andheavyrare earthelement(HREE)
1987;Parker, 1983;Henry et al., 1988;Price et al., 1986].
data.To allowcomparison
betweensuitesanalyzedfor different
Between
31 and 27 Ma
volcanic
and intrusive
rocks are
found only in the southernpart of the province(Figure l c).
The San Carlos and the Santanacalderas,large but shallow,
nestedcalderasjust southof the Rio Grandeformedat 30 and
28 Ma. Igneousrocksof thesecalderasareexclusivelysilicic,
includingferroaugiterhyolitesand granite[Gundersonet al.,
1986]. Significantvolumesof mafic to intermediatelavaswere
eruptednearby, north of the Rio Grande in the Bofecillos
mountainsat 28-27 Ma [McKnight, 1970; Urbanczyk,1987].
Numerousalkalineandperalkalinelaccoliths,dikes,and sills,
typifiedby the RattlesnakeMountainsill [Carmanet al., 1975;
Cameronet al., 1986],wereemplacedin theBofecillosMountainsandBig Bendarea.Again,rocksare bothnephelineand
quartz normatire with perhapsa slight shift toward greater
volumesof undersaturated
compositions.
Taken together,rocks
of thisearlyextensionalphaseappearto be closerto a bimodal
rhyolite-alkalibasaltsuitethanmainphasevolcanicrocks.
The most recent phase of volcanic activity (24-17 Ma)
includesonly very smallvolumesof alkalic basaltto hawaiite
(Figure l d). Dikes and flows occur on Cox Mountain, in the
Davis Mountains,Sierra Vieja (Rim Rock dikes),Bofecillos
traceelements,Ta, Hf, and Yb have beenscaledto Nb, Zr, and
Y. Niobium,Zr, Ta, andHf are all highfield strengthelements
(I-[FSE)thatareusuallyhighlyincompatible
in maficmagmas.
The similarionicsizeandchargeof Nb compared
to Ta andof
Zr comparedto Hf mean that their geochemical
behavioris
similarregardless
of tectonicregime.This allowsscalingof Ta
andHf to Nb andZr usingtypicalbasaltabundances.
We have
usedconcentrations
of Ta times 16 andHf times39 [Gill, 1981]
to substitutefor missingNb andZr concentration
data. Rocks
for whichwe haveY andYb concentration
dataaverageY/Yb
of about12.5.Therefore,we substitute
Yb multipliedby 12.5
for rockswithoutY concentration
data.Thesescalingfactors
are approximations,
but we believethey are appropriate
for
reconnaissance studies. The similarities
between scaled and
unscaleddata are apparentin Figure 3. In any case,scaling
usingchrondriticor mid-oceanridgebasalt(MORB) abundances
for Ta, Hf, or Yb yields similarresults.In the rest of this
report we will refer to the combineddata setsof unscaledand
scaledconcentrationswith asterisks,i.e., Zr* for the combined
Zr and scaledHf data set; Nb* for Nb and Ta; and Y* for Y
and Yb.
13,564
JAM•S
ASDH•'•¾: CoM•n•smos
C•ASO•sm T•s-P•cos T•ns MAo•tn'nsM
o
•
•'
o
o
•
o
•
o
•
o
•
t,,,I
,,•
o
e,I
o
e,I
o
o
o
0q
o
13,566
J^MES^NDH•,m¾: COMPOSITION
CHANGES
IN TanNs-PEcosT•o,s M•OM,•'r•M
GEOCHF_•CAL EVOLUTION OF TaANs-PEcos MA•T•S•
1
Major Elements
A
17-24
0
28
Ma
Ma
-
The influence of tectonic regime on the major element
compositions
of Trans-Pecosprovincerockshas beenbriefly
outlinedin theintroduction
andmorefully treatedby Priceet al.
[ 1987]. Early andmain phase(48-32 Ma) volcanicrockshave
the evolvedcompositionsand compositionalrangestypically
seen in arcs, althoughmore alkaline than arcs closer to the
subductionzone [Hildreth and Moorbath, 1988; Bacon, 1990].
'•
100
c-
_
_c
ø 500
-
0
EC
-
The earlyextensional
phase(31-27 Ma) rocksappearbimodal,
and the basalticvolcanism (24-17 Ma) associatedwith Basin
10--
and Rangefaultingis exclusivelybasaltic.The percentage
of
mafic material increaseswith time, althoughthe total volume
of magma decreasesfrom the main phase to the early
Ba
Th
Rb
Nb
K
La
Ta
Sr
Ce
P
Nd
Zr
Sm
•
Hf
Yb
Tb
Tm
extensionalto the Basin and Rangebasalts.The questionof
are versuscontinentalextensionalsettingand the sourcesof
Element
the magmascanbe addressed
only partlyin termsof themajor
Fig. 2. Elementnormalization
diagramfor averagedcompositions
of mafic
elementcompositions
andmagmaticstyle.The early andmain igneousrocksfrom the main (48-31 Ma), early extensional(28-27 Ma),
phasesare clearly arc-relatedand the Basinand Rangedikes and Basinand Range(24-17 Ma) phasesof Trans-Pecos
Texas.Dashed
listedin text.Normalization
andlavaflows areextension-related
basaltswith asthenospheric linesindicatemissingvalues.Datafrom souroes
sources.The settingandsourcesof the earlyextensional
phase factorsfrom Thompsonet al. [ 1984].
rocks are less obvious. However, the trace element contentsof
rocksfrom all phasesprovidea clearerindicationof parentage.
Trace Elements
This studyusesthe consistentcorrelationbetweenrelative
concentrations
of certaintraceelementsand tectonicsettingto
documenta changein magmasourcesandevolutioncoincident
with a major stress reorientation. Many workers have
documented differences between the concentrationsof Nb, Ta,
largerion lithophileelements(L]LE), Y, andREE commonly
seenin arcversusintraplate
maficrocks[Gill, 1981;Pearceand
Norry, 1979; Wood et al., 1979; Pearce, 1983]. Commonly
cited indicationsof arc settinginclude low Nb, Ta, Zr, Ti, and
Hf versushigh Cs, Ba, Rb, K, Th, La, and Ce. High Nb, Ta,
Zr, Ti, and Nb/Zr versus low HREE and Y are considered
typical of intraplateor ocean island basalts.The petrologic
mechanisms
proposedfor thesecontrastsbetweenarc basalts
and intraplatebasaltsrangefrom generallywell acceptedto
highlycontroversial.
Currentexplanations
include(1) residual
garnetin the sourceregionof intraplatebasaltsto retainHREE
and Y, (2) a subductedslab componentgreatlyenrichedin
LREE, Ba, andK whichcan give the appearance
of a relative
depletion
of Ta andNb [Kay,1980;Pearce,1983;Davidson
and
Wolff, 1989],(3) reactionof ascending
arcmeltswith depleted
peridotiticmaterialsat the baseof the crust[MeenandAyres,
1989;Kelemen,1990], (4) the presenceof a Ta, Nb, and Ti
retainingphasein the sourceregionof arc magmas[Saunders
et al., 1980], and (5) contamination
of ascendingmagmain
continental settings [Thompsonet al., 1983; Hildreth and
Moorbath, 1988]. These mechanismsare not mutually
andNb, alsowith relativelyhighLILE, moderateto highLREE,
and low HREE and Y. Typical intraplatepatternsare smooth
andconvexupward.
Most Trans-Pecosrocks emplacedbetween48 and 31 Ma
have "arc-like" traceson normalizationdiagrams(Figure 2);
theyhaveanobvious,but shallow,dip at Nb or Ta. In contrast,
28 Ma rocksfrom Big Bend and the BofecillosMountainsand
24-17 Ma BasinandRangebasaltshavebroadhighsat Nb and
Ta (Figure2). The traceelementsfrom the threeagegroupsof
TransPecosrocksform a seriesrepresenting
changesin source
andmagmaticevolutionwith time. Noneof therocksanalyzed
has the very low relativeconcentrations
of Ta andNb typical
of thevolcanicfrontof arcsadjacentto subduction
zones.
Althoughelementnormalizationdiagramsallow comparisons
betweensamplesbasedon many traceelementsat one time, it
is difficult to compareelementsthat are not adjacenton the
diagram. The visual trend of the data, together with
normalization and log scaling, can obscure information.
Individual trace elementratios avoid theseshortcomings.
We
consider
ratioscommonlyusedto distinguish
arcfromintraplate
magmatism: Zr*/Nb*, Y*/Nb*, and Ba/Nb*. We first show
the changeof theseratioswith time andstressregimeandthen
presentgeneralizedmodelsof magmaticprocesses
that may
account for their variation.
Change in trace element ratios at 31 Ma. Substantial
changesin Zr*/Nb*, Y*/Nb*, and Ba/Nb* (Figures3 and4) at
about 31 Ma amplify the differencesevident in element
normalizationdiagramsand substantiate
the changein magma
typescorrelativewith changein stressregime[Priceet al. 1987;
exclusive, and the actual source of trace element enrichments Henry eta/., thisissue].Trans-Pecos
igneousrockshave several
anddepletionsis certainlycomplex.
trace element characteristicsthat are indicative of differing
The typical trace elementconcentrations
and ratios for arc magmaticprocesses.The youngestrocks, Basin and Range
and intraplate basalts can be illustrated on a element basalts,havehigh Nb* comparedwith the otherincompatible
normalizationdiagram ("spidergram"of Thompsonet al. traceelementsZr*, Y*, andBa (Figure3). This is particularly
[1984]) (Figure 2). As presentedon this diagramarc basalts clear in Nb* versusZr*. With one exception,Zr*/Nb* falls
have lows at Ta and Nb and, to a lesserdegree,at Ti. Arc below5 in post-24Ma rocks.Rockswith agesaround28 Ma,
basaltsalsohave high concentrations
of the LILE, Ba, Rb, K, which include rocks from the Bofecillos Mountains, the
andLREE. Basaltsthoughtto havesubstantial
asthenosphericRattlesnakeMountain sill, and a mugearite intercalatedwith
sources,ocean island or intraplatebasalts,have highs at Ta the BurroMesa Rhyolite(Figure 1C) haveintermediate
ratios,
JAMES
ANDHENRY:COMPOSITION
CHANGES
IN Tm•s-P•os
2500
e9
11
A
15oo
1000
5oo
ß
60
t
4O
ß
/"Earlyalkalic"
•1ß
1% ß
3O
slll•
2O
13,567
province[Hooveret al., 1988]. The originof Ba enrichmentin
thesesamplesis unknownbut may reflect their evolvedcompositionsandthepresenceof abundantamphibolephenocrysts.
All three sets of trace element ratios change between
31 and 28 Ma. This change is best illustrated by Zr*/Nb*
(Figure4). Thisratiodecreases
abruptlybetween31 and28 Ma
(Figure4). Pre-31 Ma rockshavehigh Zr*/Nb* typicalof arc
rocks.Early extensional(28 Ma) rockshavelowerratios;ratios
in 24-17 Ma rocks,eraplacedafter the initiationof basinand
range faulting, range to still lower values. The ratios fr 'ore
rocksyoungerthan 31 Ma are comparableto an averageof 97
Basin and Range basalts from the southernU.S. Cordillera
-
2000
T•XASMAOMATISM
[ Omerod et al., 1988].
A differentmeasureof arcversusintraplatecomponents
with
time is shownin Figure 5. This figure showsthe changewith
time of concentrations of Ta or Nb normalized to chondrites,
ß
dividedby the sumof normalizedK andLa dividedby two. Ta
'Aa•Aa' A
and Nb fall between
K and La on an element
normalization
diagram[Thompsonet al. 1983]. Sampleswith valuesgreater
than 1 on Figure 5 have peakson normalizationdiagramsat
Nb or Ta. Sampleswith values less than 1 have lows. The
10
0
700]
600
ß
,,,,
..q:).•
ß
increaseafter 31 Ma indicates a switch from magmas with
highLILE andLREE andlow HFSE typicalof arcsto magmas
with high HFSE associatedwith intraplate sources and
processes.
400ßR
_i I
ß
_
_
lOO
0
i - '
0
' 1 - - - i
20
40
I
i
i
i
60
80
100
120
Nb* ppm
The time of changein traceelementsignatureis constrained
to be between 32 and 28 Ma, while the stress change is
constrainedto have occurrednear 31 Ma. Analyzed rocks at
28 and32 Ma arewell dated[Henryet al., 1986]andbracketthe
changein stressregime and magma sources.Few mafic rocks
were eraplacedin Trans-Pecosbetweenthesetimes;the one
datapointat 30 Ma (Figures4 and5 andTable 1, analysis14)
is stratigraphically
constrainedonly between32 and 28 Ma.
Therefore, we currently cannot resolve the precise
contemporaniety
of changesin magmasourcesandstressregime
in Trans-Pecos
Texas.
Fig. 3. Variationof Zff, Y*, andBa versusNb* in Trans-Pecos
igneous
rocks.Squares,
48-31Ma samples;
diamonds,
28-27Ma samples;
triangles,
Processesaffectingtrace elementsignatures.Despitethe
slight
temporaluncertainty,clearly thereis a changein trace
24-17Ma samples.
Solidsymbols:
Nb,Zr, andY; opensymbols
represent
normalizeddata, Ta X 16, Hf X 39, and Yb X 12.5 (see text). Lines on element chemistry of Trans-Pecosmafic rocks broadly
Nb* versusZr* diagramshowratiosof Nb* to Zr*. Labeledfieldsand
numbered
datapointsaresamples
referredto in thetext.
coincidentwith the change in stressregime at 31 Ma. This
observationposesseveral interestingquestions.Is the contemporanietycoincidental?What mechanismsaccountfor the
between6.25 and4. Rocksemplacedprior to 31 Ma, with one evolution of different trace element signatures?How could
stressregime and theseevolutionarymechanisms
be linked?
exception,haveratiosgreaterthan6.25.
We
suggest
that
processes
of
partial
melting,
fractional
Similardistinctions
betweenpre- andpost-31Ma rocksare
crystallization
and
assimilation
generated
the
trace
element
evidentin Nb* versusY* (Figure3). Exceptions
includesome
47-42 Ma rocks from the Big Bend area and adjacent variationandarelinkedto stressregime.The contemporaniety
Chihuahua, which have Nb* and Y* concentrationssimilar to
thoseof the post-31Ma rocks.Theseexceptionshave been
described as an "early alkalic" phase of magmatism by
Gundersoneta/. [ 1986].The two 28 Ma rockswith relatively
high Y*/Nb* (Table 1, analyses9 and 11; Figure 3) arefrom
one stratigraphic
intervil in the BofecillosMountains.These
two aphyricbasalt sampleshave unusuallyhigh Y and Ba
contents,whichmay reflectcrustalcontamination.
The agegroupsshowconsiderablylesscoherenceandbroad
overlap in Ba versus Nb* (Figure 3). Nevertheless,most
of changeis not coincidental.
The threesetsof traceelementratiospresented
in Figure3
suggestsomepossiblesourcesandevolutionaryinfluenceson
Trans-Pecos
rocks.In themostprimitiverockstheseratioscan
help suggestpossiblemantlesourcecompositions.
Zr*/Nb* is
sensitiveto the degreeof partialmeltingand proportionof
clinopyroxene
andgarnet[Kemptonet al., 1987],while Y*/Nb*
is more sensitiveto garnet in the source.Ba/Nb* indicates
LILE-enrichedcomponents
attributedto dehydrationof the
subducting
slab[Kay, 1980]andalsomay indicateassimilation
rockslessthan 31 Ma have lower Ba at a given Nb* content. of crest.However,thebehaviorof Ba in continental
arcsettings
The early alkalic group defined by relatively low Y*/Nb*
is complicated,
andhighBa/Nbmay not be specificto either
(Figure 3) has low Ba/Nb* as well. Samples9 and 11 (Table slabor crustalsources[Hildreth and Moorbath, 1988]. All three
1), with high Y*/Nb*, alsohave exceptionallyhigh Ba. The setsof ratiosin Figure3 aresensitive
to fractionalcrystallization
circled samples labeled NW on Figure 3 are 48-47 Ma and assimilationof crust. The basis for these generalized
trachyandesites
from the northwesternend of the Trans-Pecos correlations
of traceelementratioswith specificsourcesand
13,568
5AM• ANDHF•'ItY:COMPOSITION
CHANGF•IN TI•,NS-PF•osT•XASMAOMAT•M
20 0
-
180
-
160
-
140
-
120
-
L
100-
80-
6.0
-
4.0
-
2.0
--
Basin
and
Range
Average.
Z•AA O•
0.0
'
'
'
15
'
I
'
'
'
20
'
I
25
'
"
'
'••
'
I
30
'
'
35
40
45
50
Age (Ma)
Fig. 4. Variationof Zr*/Nb* with agein Trans-Pecos
marierocks.TrianglesareBasinandRangealkalibasalts;diamonds,
early
extensional
basaltsto mugearites;
squares,
mainphasebasaltsto traehyandesites.
LineindicatingBasinandRangeaverageis from
dataof Omerodetal. [1988].Barsonsamples
near28 Ma areuncertainties
fromHenryetal. [1986]andaretypicalformostpoints.
Barson30 Ma samplerepresent
uncertainties
in agebasedonstratigraphic
constraints.
Ageof stress
regimetransition
fromPriceand
Henry [ 1984] andHenry et al. [thisissue].
processes
canbe illustratedby modelingtracedementbehavior.
Thesemodelsalsosuggestseverallinksbetweenstressregime
andtheproductsof magmaticevolution.
Three typesof processes
are importantin the evolutionof
pathsreflectingtheseprocesses
andcomparethesepathsto the
analyses
we havecompiled.Thesemodelsareintendedonlyto
indicatepossibletrendsor influenceson magmaticevolution
in Trans-Pecos
rocks,notasexamplesof whatdid happen.
Trans-Pecos
igneous
rocks:partialmeltingin thesource
region,
Partial melting: The partial meltingmodel usedis modal
fractionalcrystallization,
andassimilation
or mixingof crustal batchmelting:
materials.
Ouraimis tomodelsomeof thepossible
evolutionary
q=Col[O+F(1-P)1
2.0
18
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Fig.5. Nb or Ta, normalized
to chondrites,
dividedby normalized
K plusLa, dividedby 2 versus
age.Valuesgreater
than1 have
Nb or Ta "highs"ona elementnormalization
diagram;
valueslessthan1 havelows.Symbols
asin Figure3.
JAMESANDHF.NRY:COMPOSITION
CHANOF_..S
IN T•',Ns-P•os TEXASMAOMATISM
13,569
TABLE 2. Data for PartialMelting andAFC Models
Distribution
Nb
Coefficents*
Zr
Lherzolite
Mode
Melting
Proportions
Fractionating
Assemblage
Y
Ba
.001
0.0001
0.5 8
0.09
0.4
0.001
0.15
0.2
0.3
0.7
Olivine
0.005
0.01
Clinopyroxene
0.01
0.27
0.36
Orthopyroxene
0.015
0.03
0.01
0.001
0.22
Spinel
0.4
0.1
0.2
0.001
0.05
Garnet
0.1
0.3
2.0
0.08
Plagioclase
0.01
0.01
0.03
0.15
0.25
Apatite
0.001
0.001
0.001
0.02
Spinel lherzolite bulk D
0.028
0.058
0.067
0.0005
Garnet lherzolite bulk D
0.013
.068
0.157
0.004
Fractionatingbulk D
0.0195
.091
0.322
0.038
Lherzolite source t
1.1
10.0
or 0.05
0.01
0.03
0.01
Compositions,
ppm
8
3.1
12
Lower crust assimilant õ
35
431
59
968
Upper crustassimalantõ
12
175
34
652
* Partitioncoefficients
arefromtabulationof Kemptonet al. [1987],Kuehneret al. [1989],andPearceandNorry [1979].
•' Lherzolitestartingmaterialis an averageof enriched
lherzolite[O'ReillyandGriffin,1988;HartmannandWedepohl,1990]anddepletedlherzolite[O'Reilly
and Griffin, 1988; $alters and $himizu, 1988].
õ Lowercrustcomposition
is "ADC" fromCameronet al. [1989].Uppercrustis an averageof Precambrian
rhyolitesfromtheCarrizoMountains
Group
[Rudnick.1983].
whereC•is theconcentration
of thetraceelement
in themelt, the melting lherzolite, marked by a circle near the origin
Cotheconcentration
in themeltingrook,D thebulkdistribution(Figure6). The spinellherzolitemelting curveslie abovethe
coefficientwith the mineralcomponents
weightedby modein
the melting rook, F the fraction melted, and P the bulk distributioncoefficientwith themineralcomponents
weightedby
the proportionof the mineral enteringthe melt. Two source
mineralogies,a garnetlherzoliteand a spinellherzolite,with
the samechemicalcompositionareusedto representdifferent
garnet-bearing
compositions
with higherconcentrations
of Zr*,
Y*, andBa andhigherratioswithrespectto Nb. Partialmelts
of morethan3% of eitherstartinglithologyhavesimilarNb*,
Zr*, and Ba concentrations,
but Y* remainsdistinctlyhigher
at over 10% meltingof spinellherzolite.
This model suggeststhat small degreesof partialmeltingof
depths
ofmelting.
Distribution
coefficienis,
modal
andmelting garnetlherzolitecanyield meltswith traceelementratiosand
approaching
thoseof thepost-31Ma maticrooks
proportions,and traceelementcontentsusedin the model are concentrations
Texas.Model partialmeltingof spinellherzolite
listedin Table2. Thesevariablesarenecessarily
compromises; of Trans-Pecos
they ignore the temperature,pressure,and compositional producesY*/Nb* ratiosthat are too high to matchmostof the
dependence
of partitioncoefficientsand assumesimpleinitial post-31 Ma rooks, but similar to those measuredin mafic
pre-31 Ma rooks.Higher degreesof partial melting of either
composition,
fractionatingassemblages,
andcontaminants.
Figure 6 comparestraceelementconcentrations
and ratios garnetor spinellherzoliteyield Zr*/Nb* and Ba/Nb* ratios
calculated
for partialmelting,fractional
crystallization
and closerto, but not equalling,mostof thoseof the older,pre-31
assimilation to the observed concentrations. A ratio can be
Ma suite. The concentrations
calculatedfor 15-20% partial
visualizedas the slope of a line from the origin througha melting are much lower than thosemeasuredin any of the
point.In thesemodels,ratiosof concentrations
are probably pre-31 Ma rocks. Given that most of the pre-31 Ma Transmore accurate than the calculated
concentrations
due to
Pecosrooksare not primitive, it is not surprisingthat partial
uncertainties and variations in distribution coefficients. These
melting aloneis insufficientto modeltheir traceelementratios
variationswouldbe expectedto cancel,in part,whenexpressed and concentrations.
as ratios.
Fractional crystallization: Rayleigh crystallization of
Curveslabeledgt andspon Figure6 showthe traceelement magmacompositions
derivedby 3% partialmeltingof garnet
.
concentrations
calculatedfor between0.1 and 20% partial lherzolitearemodeledfor comparison
with thecompositions
meltingof garnetandspinellherzolite.
Theproportion
of garnet of post-31Ma rooks(Figure6). The heavy curveson these
or spinelis 5%. This is high comparedto most lherzolite figuresshow0-50%fracfionation
of a low-pressure
assemblage
xenoliths and tends to accentuate calculated differences in trace
consistingof olivine, clinopyroxene,
spinel,plagioclase,and
elementratios.This is particularlytrue of Y* concentrations, apatite(40:30:3:25:2).Fractionationof a moderatepressure
whichareheavilydependenton thepercentageof garnet. Low assemblage,typical of type II lherzolite xenoliths, of
degreesof partial meltingof garnetlherzoliteproducemelts clinopyroxene,spinel, and olivine (85:10:5) yields nearly
with low Zx*/Nb* and Y*/Nb* (Figure 6) similar to thoseof identicalcurves.Althoughthemodelcrystallization
trajectories
and maintain the trace'element
the post-31Ma rooks.With progressivelylarger degreesof follow the trend of •trations
partial melting the curve trendstoward the compositionof ratiosmeasured
in post-31Ma rooks,theydo not generatethe
25OO
2000
0.1%
1500
f =0.5
1000
A
5OO
0
70--
f=0.5
60-
•.,%½,..,•"'•,•
'x/
ß
50-
ß ,,,,,.
O.1%apartial
melt
p
% ql
40-
30-
ia
ß
ßß
20-
lO%
ßß
50%'*'
A
A
1%
3%
A
- O.1% partial melt
gt
lO%
102O%
0
,
ß
700
-
600
-
ß
ß
f=0.5
5oo-.
x/
4OO
300
200-
'
A
A
•u.-o
• •A AA
1"/o
_•'•
.•
0.1%
partial
lOO.
o
melt
.
0
I
20
'
'
'
I
'
'
'
40
I
60
8O
100
120
Nb* (ppm)
Fig. 6. Traceelementconcentrations
calculated
frompartialmelting,fractional
crystallization,
andassimilation-fractional
crystallization
models
compared
to observed
concentrations
(symbols
asin Figure
5). Constants
andcompositions
of starting
materials
andassimilants
arepresented
in Table2. Thinlineslabeled
gtandspshow
calculated
concentrations
forbatch
partial
melting
ofgarnet
andspinel
lherzolite
compositions
shown
bycircles
neartheorigin.
Ticksshow
percentage
melting.
Boldlines
labeled
fxl show
effectof 0-50%Rayleigh
fractionation
on3%partial
meltof gamet
lherzolite
andontheproducts
oflower
crustal
AFC.Dashed
lineslabeled
LC AFCshow
calculated
effectof lowercrustal
AFCusing
f (fraction
crystallized)
= 0.05,r
(ratioof rateof assimilation
to rateof crystallization)
= 0.9andlowercrustassimilant
marked
byasterisk.
Dash-dotted
curves
labeled
UCAFCshow
effect
ofmodeled
upper
crustal
AFContheproduct
oflowercrustal
AFC.Theupper
crust
assimilant
is
shown
bycross,
f = 0 to0.5,r = 0.4.Ticksindicate
0.1changes
inf.
JAMES
ANDH•'•r•¾: COMPO$mON
CI-IANOF_•
IN T•a•s-Pr•os T•x•s MAOMATISM
13,571
high traceelementconcentrations
observed.The high concen- of relatively large partial melts into the range of the least
trationswould requireeitherlarge amountsof fractionationor enrichedpre-31Ma maficrocks.
The thick solid lines, continuingupwardfrom the dashed
startingmeltsgenerated
by verysmalldegreesof partialmelting.
Our modelsshowup to 50% crystallization,which seemsa lines modelinglower crustalAFC, show the calculatedeffects
reasonableupper limit consideringthat all the measuredcom- of 0-50% fractionalcrystallizationaloneon the composition
positions
havelessthan57%SiO2andthegreatmajorityhave derivedby lower crustalAFC. The traceelementratiosremain
lessthan52% SiOz However,experiments
on naturalalkali nearthe analysesof pre-31Ma rocksbut modelconcentrations
basalts [Mahood and Baker, 1986] show that moderately are lower than most of the observed concentrations.
The additionof an uppercrustalcontaminant
in concertwith
primitive
basalts
with46%SiO2may
fractionate
asmuchas70%
andstillyieldbasaltic
compositions
with50%SiO2.Thislarger furtherfractionalcrystallizationcan providea bettermatchto
data.The dash-dottedlinesin Figure 6 show
percentage
of crystallization
wouldimprovethefit of ourmodels the concentration
to the observed concentrations.
the calculatedeffect of AFC in the uppercruston the magma
Model meltingcalculationscommonlyyield concentrations modified by AFC in the lower crust. The secondstage of
that are lower than thoseobservedin rocks [Frey et al., 1978; assimilation
andfractionalcrystallization
assumes
an r of 0.4
Fitton and Dunlop, 1985;Kemptonet al., 1987]. Our simple anda rangeof crystallizationproportions(f) from 0 to 0.5. The
by
modelsignoreor only approximate
severalprocesses
thatmight compositionof the uppercrustalassimilantis represented
yieldhigherconcentrations.
A moreprimitiveor metasomatized an averageof rhyolitesfrom the PrecambrianCarrizoMountain
mantlesourcethanthe averageof enrichedanddepletedlher- Group [Rudnick, 1983]. These rocks crop out in the northzolitecomprisingthemodelsourcewouldcertainlyyieldhigher westernpart of the Trans-Pecosvolcanicprovincenear Van
concentrations.
Model concentrations
could be increasedby Horn, Texas. Lead isotopicdata [Jamesand Henry, 1990]
calling on zone ref'ming,magmamixing, or fractionationof an suggestsimilar basementunderliesmuch of Trans-PecosTexas,
assemblage with lower bulk distribution coefficients. at least as far southeast as the Davis and Bofecillos mountains.
Additionally,reactionof smallpartialmeltswith largevolumes
Many of the analysesof pre-31 Ma rockshave Nb*, Zr*,
of mantle may increaseconcentrations
of incompatibleele- and Y* concentrations
that might be explainedby a single
ments[Fitton and Dunlop, 1985]. Nonetheless,the modelsfit stageof AFC in the lower crustand fractionalcrystallization
the data reasonablygiven the chemical and mineralogical with little or no upper crustal contamination.Nevertheless,
some rocks have concentrationsabove the highest modeled
assumptions.
generated
from combinedupperandlowercrustal
The partial melting model suggestslarger degreesof melt- concentrations
ing to duplicatethe evolutionarytrendsof the pre 31-Ma arc assimilationandfractionalcrystallization,indicatingprocesses
rocks.Calculatedtraceelementratiosfor 20% partialmelting outside our model. Furthermore, the fit between model Nb*
of either spinel or garnet lherzolite approachthe measured versus Ba and the observed concentrations is not as clear as
ratios in the pre-31 Ma rocks;however,the calculatedcon- Nb*/Zr* or Nb*/Y*.
centrations are much lower than those observed. Fractional
The modelspresentedso far havenot consideredthe effects
crystallizationalone cannotincreasethe concentrations
in the of the different sourcesrepresentedby the subductingslab
partialmeltssufficiently
withoutassuming
anunrealistic
amount versusthe mantle.Obviously,the slabcontributessubstantially
of fractionation.Also, becausethe elementsusedin modeling to the trace element budget of arc magmas [Kay, 1980; Gill,
1981]. The trace element signatureof arcs, a high ratio of
are nearly equally incompatible,fractional crystallizationof
reasonablemineral assemblages
doesnot appreciablychange LILE to I-IFSE, can be ascribedto both the compositionand
the startingratios to bring them closerto the measuredratios. effectsof fluids evolvedfrom the slab. Thesefluids promote
We suggestthat models that involve assimilationof crust partial melting in the overlying mantle wedge. As modeled
may accountfor higher Y*/Nb*,'Zr*•b*,
and Ba/Nb* in the above, with increasingpartial melting the concentrations
of
pre-31Ma rocks.
HFSE, particularlyNb, decrease.The slabfluids alsocontribute
Assimilation-fractional
crystallization:Fractionalcrystalli- high concentrationsof LILE elements,K, Cs, Rb, and Ba. The
zation with assimilationin two different settingsare modeled varioustrendsmodelingNb* versusBa (Figure 6) follow the
belowusingtheequations
of DePaolo[ 1981].We assumea high data rather poorly and the lack of a LILE-enriched slab
ratioof assimilation
to fractionalcrystallization
(r = 0.9) anda componentin our model partially accountsfor the poor fit.
smallamountof crystallization,f=0.05, in thelowercrust.The However, these data also show less correlationbetween age
relativelyhigh temperature
of the deepwall rock is assumed
to groupand Ba/Nb*. This suggestsdecouplingof the behavior
slowcrystallization
butfacilitatesassimilation.
Bulk distribution of LILE and HFSE or multiple crustaland mantle sources.
coefficients are derived from the data in Table 2. The Nb, Zr,
Notably,Hildreth and Moorbath [1988] alsofound complex
Y, and Ba concentrationsassumedfor the lower crustal rock
assimilatedare thoseof averagedeepcrustof Cameronet al.
[ 1989]. Thesedatawere derivedfrom studiesof crustalxenoliths
from near La Olivina, Chihuahua, Mexico, which lies about
behavior of Ba in Andean arc rocks. We find it difficult to
attribute differences in Ba concentration in the Trans-Pecos
provinceto a simplemechanism.
The effectsof stressregime: All the componentsin the
200 km south of the Trans-Pecosprovince. The initial modelspresentedabovewould be influencext
by stressregime.
compositionof the magmais modeledas a 20% melt of model Intuitively,the proportionof partial meltingwould be lessin
spinellherzolite,althoughgarnetlherzolitewouldyield similar an extensionalenvironmentthanin a compressional
or neutral
results.
setting. Extensionalenvironmentsshould allow melting and
The dashedlines extendingup from the 20% tick on the melt migration at deeper,less depletedlevels in the mantle.
partialmeltingcurveson Figure6 showthe calculatedeffects Melts would coalesce and migrate upward easily in an
of assimilation-fractional
crystallization
in thelowercrust.Small extensional
regh'ne,reactingrelativelylittle with theuppermost,
amountsof fractionalcrystallizationand assimilationof deep depletedmantleor the crust. In a compressional
environment,
crust could bring the trace element ratios and concentrations ascendingmelts would rise more slowly, allowing more
13,572
JAMESANDHENRY:COMPOSmON
CHANGES
IN TRANs-PEcos
TEXASMAGMATISM
"Transitional"Patagonianplateaubasalts(field T, Figure7)
600
in and east of the southern
500
•
'•
Andean
arc have
Zr
and Nb
compositionsintermediatebetween the Andean arc lavas to
the west and intraplate"cratonic"basalts(field C) to the east
[Sternet al., 1990]. They are more Nb-rich than the arcrocks
400
and have Nb and Zr contents at the lower end of the trends
formed by 28 Ma and post-24Ma Trans-Pecosbasaltsand
hawaiites. Rocks farther east, cratonic Patagonianplateau
basalts,show similar concentrations
to thoseof post-31Ma
rocks in Trans-PecosTexas. These plateau basaltscontain
mantle xenoliths and have major and trace elements
compositions
thatclearlyreflecttheir intraplatesources.
300
200
1 O0
There are further contrasts between Trans-Pecos
0
20
40
60
80
100
120
Nb* (ppm)
Fig. 7. Comparisonof Nb* and Zr* in Trans-Pecos
igneousrocks(symbols as in Figure 3) to Nb and Zr in rocksfrom the southemAndesand
Patagonia.
Shadedfieldsindicatewesternpartof the southern
Andes[Hickey
et al., 1986], Wa; eastemvolcaniccentersin the southemAndes[Hickeyet
al., 1989], Ea; "transitional,"T, and "eratonic,"C, Patagonianplateau
lavas [Stern et al., 1990].
extensivefractionalcrystallizationandthereforeadditionaltime
and heat for assimilation.These processeswould influence
both the initial melt compositionsand their subsequent
evolution.Our simplemodelsassumea singlestartingchemical
composition,yet we are able to generatedistincttraceelement
suitessimilar to thoseobservedfor rocks that evolvedduring
differentstressregimesin Trans-Pecos
Texas.Separate
depleted
and enrichedstartingcompositions,rather than an average,
and additionof slab sourcesshouldprovidean evenbetterfit
between model and observed concentrations
and ratios.
Texas and
the Andean/Patagoniansuites. The variations in Andean/
Patagoniantrace elementabundances
reflect geographicand
probablytectonicvariations.The Andeanrocksformed in an
arc environmentclose to a continentalmargin. The back arc
setting and continental sedimentation[Stern et al., 1990]
associatedwith the transitionaland cratonicPatagonianlavas
suggestsan extensionalenvironmenteast of the arc. TransPecosrocksshowa parallelvariation,but thechangein tectonic
settingis a function of age rather than location.Older TransPecosrocks(pre-31Ma) were emplacedin an arc fartherfrom
the continentalmarginthanthe Andeanrocks.The post-31Ma
rocks were emplacedin an extensional,perhapsback arc,
setting.
The geographicdistributionof magmatypesand sourcesin
Andean/Patagoniansuites has been attributed to decreasing
degreesof partial meltingin the mantle wedgefrom west to
east,a concurrentdecreasein LILE-enrichedslabcomponent,
andan increasingcontributionfrom convectingasthenospheric
sources[Stern et al., 1990]. Furthervariability developedby
assimilationasmeltsascended
throughthickcrust[Hildrethand
Moorbath, 1988]. We believe similar sourcesand processes
also affected the Trans-Pecos rocks.
COMPARISONOF ]•[AGMATISM IN TRA•S-PECOS TEXAS
•
t•.
Sotrm•.R•
Atones
DISCUSSION
Changesin the major and trace elementcompositions
of
Resultsof trace elementmodelingsuggestthat differences
in Trans-Pecosrocks associatedwith a changein stressregime theTertiary igneousrocks of Trans-PecosTexas with time
in turnlinked
arerelatedto differencesin degreeof partialmelting,fractional indicatemajor changesin sourcesandprocesses,
crystallization,
andassimilation.
Similarprocesses
andtectonic to a changein tectonicsetting.These findingssupportand
of paleostress
data [Henry et al., this
environmentshave •
proposedfor rocks of the southern amplify interpretations
Andeanarc andPatagonian
plateau[Hickeyeta/., 1986,1989; issue]that showo• was east-northeast
and o3 was northSternet al., 1990;Hildreth and Moorbath, 1988]. Comparison northwest
from48-31Ma andchanged
abruptlyto o• vertical
of Zr and Nb concentrations
in the Andean suites with those of
ando3 waseast-northeast
at 31 Ma. Henryet al. infera con-
the Trans-Pecosigneousprovince showssome interesting tinentalarc settingup to 31 Ma andoneof intraplateextension
parallelsanddifferences.The westernAndeanarcrocks,those afterwards.Rocksemplacedduringthe 48-31 Ma period are
closestto the trench, have low Zr, very low Nb, and high differentiated alkali-calcic and alkalic rocks with lower Nb
Zr/Nb. EasternAndean arc rocks, slightly farther from the andTa comparedto Zr, Hf, La, Ba, and K than later rocks.
aretypicalof theinterioredgeof continental
trench,have slightlyhigherZr and Nb concentrations.
These Thesecompositions
concentrations
overlapwith the lower rangeof concentrations magmaticarcs.Rocksemplacedafter the changeto an extenin 48-31 Ma Texas samples.However, Zr and Nb concentra- sionalregime at 31 Ma are initially bimodal and then, after
tions in Trans-Pecosrocks extend to much highervalues.We 24 Ma, exclusivelybasaltic.These rocks have higherNb and
suggesttwo possiblereasons.The more alkalinecompositions Ta comparedto Zr, Hf, La, and K. Compositionsare charof the Trans-Pecosrocks imply that they representsmaller acteristicof intraplateandoceanislandbasalts.
Although element normalizationdiagramsand the various
degreesof partialmeltingandconsequently
wouldhavehigher
incompatibleelementabundances.
Additionally,many of the traceelementratiosusedin this studyclearly showa contrast
Texas samples are more differentiated and perhapsmore betweenthe pre- andpost-31Ma rocks,they alsoindicatethat
contaminated
thantheAndeansamples,whichhavea maximum the pre-31 Ma rocks do not have the samemix of sourcesas
in arc
SiO2 contentof 50.2%.Differentiation
of the enriched
west arcsadjacentto a trench.The traceelementabundances
Texas magmaswould further increaseincompatibleelement rocks from Trans-PecosTexas are higher, and relative conconcentrations.
centrationslie between thoseof arcs adjacentto subduction
JAMES
ANDH•'t•¾: COMPOSITION
CHANGES
IN T•s-P•cos
zonesand intraplatebasalts.Furthermore,a few analysesof
48-31Ma rockshavediagnostic
traceelementratiosthatoverlap
with post-31Ma rocks. On the basisof trace elementabundances,Gundersonet al. [1986] suggestedthat theseearly
alkalicrockswere only indirectlyrelatedto subduction.
These
rocksare, however,intimatelyassociated
with rocksthat have
anunequivocal
subduction
traceelementcomponent.
We suggestthat all the pre-31 Ma rocks are indeed
subduction-related
but thattheyhavecompositions
controlled
by a distinctenvironment.The positionof the Trans-Pecos
magmaticprovinceat the easternedgeof the Cordilleranarc
impliesgeneration
of meltsabovethe deeperpartsof the subducting slab comparedto more western magmatism.We
speculatethat this deeper,distal processof melt generation
wouldinvolve(1) a slabpartiallydepletedof waterandLILErich components,
(i.e., the slabcomponen0,(2) relativelysmall
degreesof partial meltingfor an arc, (3) possiblya greater
fractionof F in anyremainingfluid liberatedfrom theslabdue
to breakdownof F carryingphases,(4) a thick mantlewedge,
and (5) a substantialthicknessof continentalcrust. The first
two factors,a fluid-depleted
slabandsmalldegreesof partial
melting,would reducethe slab-derivedfluid flux. In turn, this
TEXASMAGMATISM
13,573
In contrastto the pre-31 Ma rocks,the 28-17 Ma suitesare
dominatedby intraplatemantle sourceswith no obviousarcrelated components.DecreasingZr*/Nb* in the post-31 Ma
suites(Figure 3) suggeststhat deeperor more fertile sources
were tappedwith time. This changeis evidentin the 24-17 Ma
suite[Daschet cd.,1987].The temporalchangeof traceelement
compositions
from arcto intraplatemagmatismin Trans-Pecos
Texasis paralleledby the geographicchangeof compositions
in southernAndeanarc andPatagonianplateaulavas.
Although we have emphasizedthe temporalevolutionof
magmatic compositionswithin the Trans-Pecosmagmatic
province, there are larger regional variationswe have not
discussed.After 31 Ma, substantialmagmaticactivity to the
west is arc related and has a clear arc trace elementsignature.
This includes the southern cordilleran
basaltic andesite suite of
Cameronet al. [1989], which comprisesextensivelavaswith
arc geochemistryeruptedin a probableextensionaltectonic
environment.
Consideration
of these rocks
shows
that the
variationin compositions
in Trans-Pecos
Texasis not simply
temporal but is also part of a geographicarc-to-intraplate
gradient.This perspectivevalidatesthe analogybetweenthe
southernAndes/Patagonia
andTrans-PecosTexas-Chihuahua.
effect would reduce the contrast in concentrations between
Acknowledgments.
We thankPamelaKemptonandJuliusDaschof
HFSE and LILE plus LREE typicallyseenin arc magrnas NASA, Houston,for INAA analyses.We are gratefulfor reviewsof
while concurrently
keepingalkalieshigh. Derivationof this an early versionof this manuscriptby A. Laughlin,C. Chapin,D.
melt from the mantle wedge and further interactionwith it
duringascentcouldalsomute an arc traceelementsignature.
High-fluorineactivitiescouldaccentuate
the ability of these
meltsto extractandcarryHFSE from thewedgeandcontinental
Barker,K. Cameron,andA. Glazaer.Researchsupportwasprovided
by the U.S. Bureauof Mines grantG1194148to the TexasMining
and MineralResources
ResearchInstituteand by the COGEOMAP
programof the U.S. Geological Survey (CooperativeAgreement
14-08-0001-A0408 and 14-08-0001-A0662).
crust.
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