Mineralogy of the La Posta pluton: Implications for the origin of zoned plutons in the eastern

American Mineralogist, Volume 74, pages 1258-1269, 1989

Mineralogy of the La Posta pluton: Implications for the origin of zoned plutonsin the eastern Peninsular Ranges batholith, southern and Baja California

J. P. Cr,rNxnNBEARD.* M. J. War,awnNonnDepartment of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.

Ansrucr

The 94 Ma La Posta pluton is located in the eastern zone of the Peninsular Rangesbatholith and is exposed over an area of approximately 1400 km2. It is zoned from asphene-hornblende-biotite tonalite rim inward to a core of muscovite-biotite granodioritewith internal contacts gradational over distances of several tens to hundreds of meters.This pluton is texturally distinct from the older synkinematic plutons of the western zoneof the batholith in that it is undeformed and contains sharply euhedral mafic minerals.

Microprobe analyses indicate that hornblendes from the eastern side of the pluton havehigher Al, and lower Si than those from the western side. This variation in compositionyields geologically unreasonable differences in hornblende geobarometry and reflects horn-blende equilibration under different thermochemical conditions on either side of the plu-ton. Compositional variations within individual mineral suites are consistent with a gen-eral model involving inward crystallization of a static magma chamber. However, biotitesin the interior facies are more oxidized and have abruptly higher Fel(Fe + Mg) than thosein the marginal facies. These data suggest that the muscovite-bearing core facies representsthe tail of a diapir that intruded a ballooned magma chamber prior to its complete solidifi-cation. A longer residence time for the tail adjacent to a cratonal source resulted in itsenrichment in, at least, Fe3* and Fe/(Fe + Mg).

INrnooucrroN

The eastern side of the Peninsular Ranges batholith isdominated by a series of large, concentrically zoned plu-tons of intermediate composition that exhibit distinctsimilarities in age, rock type, and geochemistry (Wala-wender et al., 1989). The largest of these, the 94 Ma LaPosta pluton, is located approximately 65 km east of SanDiego, California, and straddles the international border(Fig. l). Although the northern portion of this pluton andthe section south of the international border have beenmapped only in reconnaissance fashion, the approxi-mately 750 km'zthat have been studied in detail (Kimzey,1982; Sinfield, 1983; Clinkenbeard, 1987) reveal a moreor less single magmatic pulse that crystallized inward toform a lithologic succession ranging from hornblende-biotite tonalite to muscovite-biotite granodiorite. Tex-tural evidence indicates that this pluton experienced littlesubsolidus deformation or alteration and thus providesan excellent opportunity to examine compositional rela-tionships between coexisting silicate minerals that haveretained their primary magmatic character. This paperdescribes these compositional relationships and inter-prets them with respect to various parameters that affectcooling magmatic systems.

+ Present address: Division of Mines and Geology, 650-B Ber-cut Drive, Sacramento. California. U.S.A.

Gnor,ocrc sETTTNG

The Peninsular Ranges batholith is part of the circum-Pacific chain of Mesozoic batholiths and extends fromRiverside, California, southward more than 1000 km tothe lat 28"N in Baja California. It consists of hundreds ofindividual plutons that range in maximum dimension upto 45 km and in composition from gabbro to granite. Thebatholith, as a whole, is calcic in composition (Silver etal.. 1979\ and can be divided into western and easternzones based on geochemical, lithologic, structural, andgeophysical criteria.

The western zone is characterized by smaller plutonsthat range in composition from gabbro to monzogranite.They were emplaced into greenschist- to lower amphi-bolite-grade host rocks dominated by volcanic and vol-canoclastic assemblages to the west and submarine fandeposits to the east (Todd et al., 1988). The eastern zoneof the batholith is essentially devoid of gabbroic plutonsand contains mainly large (hundreds of square kilome-ters) plutons of intermediate composition that were em-placed into locally migmatitic, sillimanite-bearing meta-sedimentary rocks. In addition, plutons in the easternzone are undeformed and contrast with the synkinemat-ically deformed plutons to the west (Todd and Shaw,r979).

The boundary between these plutonic zones crudelycoincides with a sharp gravity contrast that Oliver (1980)interpreted as the juxtaposition ofa western oceanic crus-

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23(3el

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POSTA PLUTON

HB-BTE FACIES

L G B T E F A C I E S

S M B T E F A C I E S

M U - B T E F A C I E S

W E S T E R NZ O N EPLUTONICR O C K S

M E T A M O R P H I CR O C K S

S A M P L EL O C A T I O N

tal basement and an eastem continental basement. Silveret al. (1979) showed that this boundary also approxi-mates a "step" in both the emplacement ages of the plu-tons and their oxygen-isotope character. Plutons in thewestern zone range in age from 120 Ma to 104 Ma withdtEO less than 8.5Vu, whereas those in the eastern zoneare generally younger than 100 Ma and have D'EO rangingfrom 9 to l2%oo. Walawender et al. (1989) argued thatalthough these regional patterns are generally valid, tran-sitional plutons with characteristics of both zones arepresent in the vicinity ofthe age and isotopic "steps" andthat this boundary represents a transition from mantle-derived melts to oceanic crust-derived melts.

Diamond et al. (1985) and Gastil et al. (1986) haveshown that the plutons of the western zone contain mag-netite and ilmenite as their primary opaque phaseswhereas those to the east are devoid of magnetite. Thismagnetite-ilmenite "line" approximates the lithologic,geochemical, and age "steps" described above. Gastil etal. (1989) have also considered it to reflect a transitionfrom mantle-derived to oceanic crust{erived melts withthe nature ofthe volatile phase released during anatexiscontrolling the presence or absence of magnetite.

The La Posta pluton is divided into two parts by a largemetasedimentary screen (Fig. l) that intemrpts the con-tinuity of the plutonic units. Sillimanite-bearing peliticschists and migmatitic gneisses along with lesser amountsof marble, quartzite, and amphibolite make up the bulkof the metamorphic complex. The metamorphic complex

1259

and adjacent parts of the La Posta pluton are intruded by89-93 Ma garnetiferous two-mica monzogranites. Thewestern hornblende-biotite and small-biotite facies, andthe eastern hornblende-biotite facies ofthe La Posta plu-ton have zircon U-Pb ages of 93, 94, and 95 Ma (+2Ma), respectively, and indicate an emplacement age ofapproximately 94 Ma (Walawender et al., 1989). ThisU-Pb age is consistent with a four-point Rb-Sr isochronage of 92 + 2.8 Ma for the western small-biotite facies.The continuity of ages across the pluton along with itsmap-unit symmetry and petrographic similarities suggestthat the La Posta pluton was emplaced during a singleintrusive event. In addition, the large-biotite facies occursin identical pseudostratigraphic position on either side ofthe pluton. That particular facies has not been recognizedas a map unit in any other La Posta-type pluton studiedto date.

Prrnocupnv

The La Posta pluton is crudely circular and estimatedto be approximately 1400 km'zin total exposed area. Fivelithologic facies (Fig. 1) have been recognized within thepluton (Kimzey, 1982). These, from the western contactinward, are the border, hornblende-biotite, large-biotite,small-biotite, and muscovite-biotite facies. The sequenceis repeated in reverse order from the muscovite-bearingcore eastward to the exposed edge of the pluton. Here,the pluton disappears beneath a cover of younger allu-

CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON

Fig. 1. Geologic map of the La Posta pluton (after Kimzey, 1982; Sinfield, 1983).

r260

Fig.2. Photomicrograph of sample 8 from the westem horn-blende-biotite facies. Q : quaft4H : hornblende, S : sphene.Idiomorphic homblende boundaries against quartz indicate thathornblende growth had ceased by the time quartz began to nu-cleate.

vium such that neither the host rocks nor the border fa-cies is exposed.

Border facies

The border facies, not shown in Figure l, crops out ina narrow discontinuous band up to 100 m wide only wherethe La Posta pluton is in contact with the older igneousrocks of the western zone of the batholith. Where thepluton intrudes metasedimentary or metaigneous rocks,the border facies is absent, and the hornblende-biotite orlarge-biotite facies forms the outermost unit. The borderfacies changes inward from a banded marginal zone,through a gneissic, melanocratic biotite-hornblende dio-rite into weakly foliated and then nonfoliated horn-blende-biotite facies rocks. All contacts are steep to ver-tical and subparallel to the margin of the pluton. Thissequence is cut by narrow dikes ofaplite and alkali feld-spar megacrystic granite that form less than l0o/o of theexposures.

The banded marginal zone ranges up to 50 m in thick-ness and consists of alternating bands of plagioclase +quartz and hornblende + biotite, which range from a fewcentimeters to more than a meter in thickness. The maficminerals are commonly aligned parallel to the bandingbut are typically subhedral to euhedral in form. The maficdiorites are in sharp contact with the banded zone butchange laterally via an increase in plagioclase and a lossof gneissosity into unfoliated hornblende-biotite faciesassemblages.

Hornblende-biotite facies

The hornblende-biotite facies is the main outer unit ofboth the eastern and western sides of the pluton. It istypically massive, but can exhibit a weak foliation nearand parallel to the margin of the pluton. The dominantrock type is a sphene-hornblende-biotite tonalite that be-comes progressively more enriched in alkali feldspar anddepleted in hornblende inward.


The most obvious feature of this unit is the euhedralcharacter ofthe hornblende, biotite, and sphene (Fig. 2),which occur as large discrete crystals up to 7 mm in length.Inclusions in the mafic minerals consist of ilmenite, apa-tite, zircon, and allanite (Clinkenbeard, 1987). This tex-tural character is distinct from that ofthe hornblende andbiotite in the older plutonic rocks to the west, which arecommonly anhedral, charged with euhedral plagioclaseinclusions, and intergrown with one another. Sphene inthe older plutonic rocks is typically anhedral and inter-grown with the biotite.

Plagioclase occurs as subhedral to euhedral crystals upto l0 mm in length that exhibit weak oscillatory or patchyzoning. They commonly have cores composed of two ormore crystals joined in a synneusis relationship; the coresare mantled by less calcic rims. Alkali feldspar, wherepresent, is interstitial and exhibits "gridiron" twinning.Locally, myrmekitic intergrowths occur at the contact be-tween the two feldspars. Quartz exists as discrete or com-posite grains with weak undulatory extinction and mod-erately sutured boundaries.

Large-biotite facies

The sphene-hornblende-biotite tonalite of the horn-blende-biotite facies grades inward into sphene-horn-blende-biotite and sphene-biotite granodiorite of the large-biotite facies by loss ofthe large euhedral hornblende andan increase in both the size and abundance of the inter-stitial alkali feldspar. The presence of large (5-10 mm)pseudohexagonal books of biotite gives this rock a dis-tinctive salt and pepper appearance. Small (l-3 mm) sub-hedral hornblende crystals are present in the outer por-tion ofthis facies but decrease in abundance toward thecore of the pluton. Both biotite and hornblende are oth-erwise similar to those found in the hornblende-biotitefacies.

Plagioclase occurs mainly as discrete subhedral crystalsthat average 3-4 mm in size but that may reach l0 mm.Oscillatory zoning is common. Quartz appears to beslightly coarser in this facies but is otherwise as describedfor the hornblende-biotite facies. Alkali feldspar is ingreater abundance and tends to form oikocrysts up to 3cm in size that enclose all of the above minerals. As inthe hornblende-biotite facies, euhedral sphene is the mainaccessory mineral.

Srnall-biotite facies

Proceeding inward, the next zone of the La Posta plu-ton consists of biotite granodiorites that are texturallydistinct from those in the large-biotite facies. The twofacies have a gradational contact on the western side ofthe pluton, but this contact on the eastern side is at leastin part intrusive (Sinfield, 1983).

The small-biotite facies is characterized by the pres-ence of small (l-4 mm) euhedral to subhedral biotitecrystals. Hornblende is scarce and, where present, occursas minute inclusions in biotite or plagioclase. Plagioclaseis present as l- to 3-mm subhedral crystals, some of which


Trele 1, Average modes of rocks in the study area

t 2 6 l

Mineral W. H W. LB W. SB E. SB E. LB E . H

nQuartzPlagioclaseK-feldsparBiotiteHornblendeMuscoviteSpheneApatiteZtconAllaniteOpaquesMyrmekiteSericiteChloriteEpidote

325.155 12.7

12.72.7

o.4Ir.Ir.Ir.Ir.0.4Ir.u . ctr.

229.454.44.28.41 . 9

0.6Ir.Ir.Ir.Ir.0.4Ir.0.6tr.

3 1 . 548.6

I - 6O A

Ir.

0 .1Ir.Ir.tr.tr.o.4tr.1 . 1tr.

36.146.98.08.1

335.849.43.2

12.20.9

0.8tr.tr.tr.Ir.0.1

329.845.20.8

1 A A

4.0

0.9tr.tr,tr.Ir.

326.349.70.2

14.88.7

tr.

tr.

0.2

Ir.f f

Ir.tr.

tr.tr.

34.550.34.6

10 .1

Ir.tr.

0.4

Ir.

tr.0.3tr.

tr.Ir.tr.Ir.

Note. 'Faciesdesignat ionasfol lows:H=hornblende-biot i te,LB: large-biot i te,SB:smal l -b iot i te,MB:muscovi te-biot i te,andT:western-zonetonalite. E. and W. represent the eastern and western sides of the pluton, respectively; n: number of analyses; tr. = trace.

are joined in a synneusis relationship. Weak oscillatoryzoning is present, but the patchy zoning observed in theplagioclase of the hornblende-biotite and large-biotite fa-cies is missing. Quartz is texturally similar to that foundin the other facies but exhibits stronger undulatory ex-tinction. Alkali feldspar typically forms oikocrysts up toseveral centimeters in size that have well developed"gridiron" twinning.

Muscovite-biotite facies

This facies is mainly found in the eastern portion ofthe La Posta pluton where it grades into the eastern small-biotite facies. Locally, however, muscovite-bearinggranodiorite also occurs just west of the large metasedi-mentary screen (Fig. l) in the central part of the studyarea. The main rock type is a muscovite-biotite grano-diorite that is similar to the small-biotite facies except forthe presence of 1- to 3-mm subhedral muscovite crystalsand the absence ofsphene. Based on the textural criteriaof Miller et al. (1981), the muscovite is considered to beprimary, i.e., magmatic in origin. Secondary muscovite(sericite) is present in trace amounts in the plagioclase.

ANnr,vrrclr, METHoDS

Chemical analyses of minerals in polished thin sectionswere made using the three-channel ceueca Msx electronmicroprobe at the Scripps Institute of Oceanography. Op-erating conditions were as follows: accelerating potential: 15 kV, sample current : 15 nA on brass, spot size :

2 to l0 pm. Standards used for calibration were well-characterized silicates and oxides. Data reduction wasperformed on-line using the program coRREx, which ispart of the Msxcon software package. Mineral formulasfor biotite, muscovite, plagioclase, alkali feldspar, and il-menite were calculated using the program ceo, also partof the Nasxcon package. Amphibole formulas were cal-culated by using an electronic spreadsheet program in themanner suggested by Leake (1978). Their ferrous/ferricratios were varied until the sum ofthe cations exclusive

of Ca. Na. and K totaled 13. The ferrous/ferric ratios ofselected biotite separates were determined with the tech-nique ofFritz and Popp (1985).

MrNtur,ocv

Amphibole

Amphibole is found only in the outer zones of the LaPosta pluton. It occurs in significant amounts in the horn-blende-biotite and large-biotite facies but decreases in-ward to trace amounts in the innermost portion of thelarge-biotite and throughout the small-biotite facies (Ta-ble l). Hornblende is absent in all muscovite-bearingrocks.

Analyses of amphiboles from seven samples within theLa Posta pluton and four samples in tonalitic rocks im-mediately west of the pluton (Fig. l) are reported in Table2. All samples plot close to the boundary between themagnesio- and ferro-hornblende fields (Fig. 3), but dif-ferences between the western and eastern sides ofthe plu-ton are apparent. Amphibole from the western horn-blende-biotite and large-biotite facies have Si > 7.0,whereas those from the eastern equivalents have Si < 6.8.This difference is also observed in the lower Alt of theamphiboles in the western half of the pluton (Table 2)and indicates that at the time of amphibole nucleation inthe melt, conditions of lower a",orartd higher c^ro, existedin what is now the eastern side of the pluton.

Hammarstrom and Znn (1986) and Hollister et al.( I 987) have defined a geobarometer based on the Al con-tent of hornblende. Constraints on the use of this geo-barometer were summarized by Hollister et al. (1987)and include the presence of magnetite among the opaquephases; a magmatic assemblage of hornblende, biotite,plagioclase, alkali feldspar, and quartz; and plagioclaserim compositions between An* and Anrr. The horn-blende-bearing rocks ofthe La Posta pluton have plagio-clase rim compositions between An, and Anrt and therequired mineral paragenesis with the exception of mag-

t262 CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON

Tlele 2. Average hornblende compositions of rocks in the study area

E . H T T T T2 0 2 3 1 2 3 94 7 7 8 4

W . H W . L B E . L B E . H2 2 4 3 5 1 24 7 4 5

E . H1 95

Unit :Sample:n:

W H8o

sio,Tio,AlrosCrrO"FerO.FeOMnOMgoCaONaroKrONioP"ou

Total

AIFee*

N iTiMgFe2+Mn

Sum

r-e.-MnCaNa

Sum

NaK

Sum

47.621.016.74

18.340.60

10.521 1 . 9 10.980.510.020 . 1 0

98.35

0.64

47.301.067 . 1 30.05

18.490.64

10.221 1 . 7 31.O20.600.050.10

98.39

0.64

7.001.008.00

0.240.340.010.010.122.251.950.080.005.00

0.000.001.860 . 1 42.00

0 . 1 50 . 1 10.26

18.69U.CY

10.4812.020.93u.c /0.020.09

98.64

0.64

7.060.948.00

0 . 1 90.280.010.000 . 1 12.312.030.070.005.00

0.000.001.900.091.99

0.180 . 1 10.29

45.541 . 1 18.850.05

18.010.559.73

1 1.881.040.860.040.10

97.76

0.65

6.811 . 1 98.00

0.370.250.010.010.132 . 1 72.000.070.005.01

0.000.001.900 . 1 02.00

0.200.160.36

43.811.22

10.240.02

18.800.479 . 1 9

12.151 . 1 60.97

0 , 1 198,1 3

0.67

6.571.438.00

0.380.280.010.000.142.062.080.060.005.01

0.000.001.950.052.00

0.290.190.48

44.461.08

10.200.01

18.200.449.24

11.781 . 1 50.890.030.10

97.59

0.67

44.251.02

10.680.02

18.530.459.06

12.001 . 1 51.010.020.10

98.29

0.67

44.821 . 1 29.640.02

1 7 . 1 80.41

10.7612.091.060.710.010 . 1 0

97.92

0.61

6.641.368.00

0.320.470.010.000.132.371.660.050.005.01

0.000.001.920.082.00

o.220.130.35

17.700.43

10.5212.061 . 1 10.680.010 . 1 0

97.72

0.63

6.651.358.00

0.460.460.010.000.142.331.740.050.004.99

0.000.001.920.082.00

0.240.130.37

44.681 .219.200.02

47.761 .016.460.02

42.93 46.421.23 1 .29

10.17 8 .810.02 0.04

18.78 16.430.43 0.399.60 12.01

12.18 11.471 . 1 8 1 . 1 61 .10 0.410.02 0.020.11 0 .1 1

97.75 98.56

0.66 0.58

6.47 6.721 .53 1.288.00 8.00

0.28 0.220.42 0.760.01 0.010.00 0.000.14 0 .142.16 2.591.94 1 .230.06 0.050.00 0.005.01 5.00

0.00 0.000.00 0.00't .97 1.780.03 0.222.00 2.00

0.31 0.110.21 0.080.52 0.19

Fel(Fe + Mg)

5 l

AISum

7 0 50.958.00

0.220,300.000.000 . 1 12.321.970.080.005.00

0.000.001.890 . 1 12.00

0.170.100.27

6.67 6.611.33 1.398.00 8.00

0.47 0.490.27 0.250.00 0.010.00 0.000.12 0 .112.07 2.022.01 2.060.06 0.060.00 0.005.00 5.00

0.00 0.000.00 0.001.89 1 .920.11 0.082.00 2.00

0.22 0.250.17 0 .190.39 0.44

Note.' Unit abbreviations as in Table 1. Formulas based on 23(O,OH); r : number of analyses.

netite. However, the data used to establish the high-pres-sure end of the geobarometer are from the magnetite-freeEcstall pluton (Hammarstrom and Zen, 1986) and raisesome question as to the importance of magnetite as arequirement. Pressure estimates of synchronous (93 Ma)and spatially associated garnet-muscovite-biotite mon-zogranites (Fig. l) range between2.T and 3.0 kbar (Clin-kenbeard, 1987) and indicate that the La Posta plutonwas emplaced above the 2-kbar minimum pressure re-quired by the geobarometer. Hollister et al. (1987) arguedthat only rim compositions of the hornblende crystalsshould be used because these are likely to have formedfrom the late stages of the melt and therefore in a narrowrange of temperatures. Our analyses, part of a larger study,were completed prior to the publication of the above re-strictions. Grain traverses and random spot analyses (bothrim and core as seen in thin section) showed little com-positional variation within individual grains or from grainto grain in single samples. The data are reported as sam-ple averages in Table 2 with the average value of A1, foreach sample applied to the geobarometer (Table 3).

The lack of compositional zoning and constant com-position of hornblende with respect to changing rock(melt?) chemistry (Clinkenbeard, 1987) suggest that horn-blende equilibrated with an intermediate rather than alate stage of the La Posta melt. On the basis of texturalcriteria, Clinkenbeard (1987) also argued that hornblendenucleated early in the cooling history of the melt and thatits growth had effectively ceased before the appearance ofquartz and orthoclase. In addition, the low abundance ofalkali feldspar in the hornblende-bearing rocks (Table l)and its interstitial character may indicate that the horn-blende grains were not in contact with an alkali feldspar-saturated melt. Thus, amphibole may not have equili-brated under the thermal and compositional restrictionsof the geobarometer.

Despite this complicating and unknown factor, a plotof t4lAl versus Alr Gig. a) for the La Posta pluton overlapsthe data shown by Hammarstrom and Zen (1986) forseveral plutons from different pressure regimes. Their dataindicate that hornblende from high-pressure (epidote-bearing) plutons have Al, > 1.8. Three of four analyses


Tlele 3. Calculated P-f conditions

t263

sl

Fig. 3. Hornblende classification after kake (1978). Crossesrepresent hornblendes from the western and eastern facies ofthel,a Posta pluton. Solid circles represent hornblendes from anolder tonalite body adjacent to the western margin ofthe pluton.Remaining fields are T : tremolitic hornblende, A : actinolitichornblende, F : ferro-actinolitic hornblende, TS : tschermakitichornblende, and FTS : ferro-tschermakitic hornblende.

from the eastern side of the La Posta pluton have Al, >1.8, whereas those from the western side have Alr < 1.8.Epidote occurs as an accessory mineral throughout theLa Posta pluton except in the innermost muscovite-bear-ing facies. It is typically granular and mantles allanitewhere the latter was in contact with biotite. We interpretthis relationship to indicate that epidote is not a primarymagmatic mineral and that its presence does not implycrystallization under high (8 kbar?) pressure.

Results of the hornblende geobarometer (Hollister etal., I 987) are listed in Table 3 and appear to indicate thatthe western side of the La Posta pluton was emplaced atconsiderably shallower levels (1.9 kbar) than the easternside (5.2 kbar) and the older tonalitic pluton to the west(4.5 kbar). This suggested relationship implies that majorstructural discontinuities exist at the western edge ofthepluton and at an unknown central location within thepluton. Field relationships and radiometric ages (Wala-wender et al., 1989), however, do not support such aninterpretation. The contact with the older tonalitic plutonis clearly intrusive, and evidence to support a majorstructural feature in either the metasedimentary screen orthe interior ofthe pluton is lacking.

Temperature estimates of the La Posta pluton werebased on the ratAl content of the amphibole in the horn-blende-biotite and large-biotite facies. This geothermom-eter (Nabelek and Lindsley, 1985) is designed for use withamphiboles in equilibrium with intermediate composi-tion plagioclase (Anoo-Anur). Amphiboles in this studycoexist with weakly zoned plagioclase whose composi-tions fall just below this range (Table 5). The results aregiven in Table 3 and indicate that the plagioclase-am-phibole equilibration occuffed at lower average temper-ature (740 "C) on the western side ofthe pluton than onthe eastern side (763 'C). The lower temperature can becorrelated to the coarsely bracketed lower limit of am-phibole stability in a granodiorite system at 2 kbar (Na-ney, 1983) and appears to reflect near-solidus equilibra-tion. Data for the same system at 8 kbar indicate that

Averagercc)

763

791

Nofe; Unit abbreviations as in Table 1. Pr was calculated from the al-gorithm of Hollister et al. (1987); average value for Alr for each samplewas used in the calculation. Error range based on multiple analyses ineach sample is 0.7 kbar. Temperatures for the hornblende-bearing rockswere calculated using the algorithm of Nabelek and Lindsley (1985) andutilized the calculated Pr for each sample. Temperatures for the muscovite-bearing rocks are from the plagioclase-mus@vite geothermometer of Greenand Usdansky (1 986) and are based on the rim compositions ot plagioclase.

this limit would migrate to lower temperatures, i.e., showa negative correlation between equilibration temperatureand pressure. The higher pressure and temperature esti-mates for the eastern side of the pluton (763"C,5.2 kbar),therefore, are in direct conflict with the experimental data.The plagioclase-muscovite geothermometer of Green andUsdansky (1986) was used to calculate temperatures forthe muscovite-bearing rocks and gave subsolidus condi-tions (Table 3).

A l t

Fig.4. talAl versus Alr for hornblende in the La Posta pluton.The upper line represents the limiting condition of t41Al : 0whereas the lower line is the lower limit of igneous amphibolesas defined by Leake (1971). The middle line represents theregression curve of Hammarstrom andZnn (1986) and closelyapproximates the data from this study. Our data yielded a regres-sion fit of t4rAl : 0.64 Alr + 0.23 and a correlation coefrciento f 0 .99 l .

Sampleno. Unit

Average& (kbar) & (kbar) rfC)

oE+oE

E

742738739

777757761/ J O

789805783787

480b/u

1 . 92 2 W H 2 . 3I W . H 1 . 94 W. LB 1 .6

12 E . H 5 .519 E. H 5.420 E. H 5.835 E. LB 4.0

1 T 4 . 32 r 5 . 4

23 T 4.739 r 3.7

1 1 M B1 8 M B

7 5 0 7 . 2 6

o

t264 CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON

Treu 4, Average compositons of biotite and muscovite

Biotite

E . H E . H19 207 4

E. LB35

W. LB W. SB MB MB MB MB4 5 1 1 1 4 1 8 3 25 5 3 4 5 4

E. LB1 36

E . H1 24

Unit :Sample:ni

W . H W . H8 2 24 5

sio,Tio,Alr03

FerO,FeOMnOMgoCaONaroKrONioProu

Total

SiAI

Sum

AIFes

N iTiMgFe2+Mn

Sum

NaK

Sum

Fe/(Fe + Mg)

35.78 34.853.59 3.64

15.52 14.880.02 0.02n.d. 0.46

21.84 21.320.33 0.359.50 8.970.01 0.280.10 0.099.07 9.040.01 0.030.01

95.78 93.93

0.70 0.71

5.50 5.492.50 2.518.00 8.00

0.32 0.260.00 0.050.01 0.010.00 0.00o.42 0.432 . 1 8 2 . 1 12.81 2.810.04 0.055.78 5.72

0.00 0.050.03 0.031.78 1 .821.81 1 .90

35.61 35.803.70 4.18

15.23 14.740.04 0.022.56 n.d.

20.28 22.630.31 0.409.20 8.880.02 0.030.09 0.149.55 9.360.02 0.010.09 0.10

96.70 96.29

0.72 0.72

5.48 5.512.52 2.498.00 8.00

0.24 0.190.29 0.000.01 0.010.00 0.000.43 0.482.11 2.042.59 2.920.04 0.055.71 5.69

0.00 0.010.03 0.041.87 1 .841 .90 1.89

34.28 33.5s3.15 3.39

18.66 17 .400.02 0.01

10.19 6.5715.91 18.850.35 0.345.72 5.980.01 0.010.07 0.089.69 8.910.01 0.020.11 0 .01

98.17 96.12

0.83 0.81

5.29 5.312.71 2.698.00 8.00

0.68 0.561.15 0 .770.01 0.000.00 0.000.37 0.401.32 1 .412.00 2.460.05 0.055.57 5.65

33.81 34.323.50 3.33

18.72 18.190.04 0.012.64 n.d.

21.72 24.500.41 0.275.65 5.940.04 0.010.07 0.088.93 9.040.02 0.030.01 0.02

95.s6 95.il

0.81 0.80

5.29 5.332.71 2.678.00 8.00

0.73 0.680.31 0.000.01 0.000.00 0.000.41 0.391.32 1.382.82 3.190.05 0.045.6s 5.68

35.06 35.333.00 2.82

16.56 16.490.02 0.02n.d. 2.23

21.53 19.060.33 0.328.77 9.54

0.010.09 0.099.04 8.940.03 0.010.01 0.01

94.44 94.87

0.71 0.69

0.00 0.000.03 0.031 .80 1.771 .83 1.80

35.86 34.74 35.663.26 3.09 3.31

16.30 16 .10 16 .590.04 0.03 0.02n.d. 0.70 4.81

20.96 19.83 15.950.28 0.26 0.279.72 9.60 9.67

0.15 0.010.28 0.07 0.099.06 8.93 9.110.03 0.02 0.010.01 0.01 0.02

95.80 93.53 95.51

0.68 0.68 0.69

5.482.s28.00

0.480.550.010.000.382.212.020.045.69

0.00 0.02 0.000.08 0.02 0.031.77 1 .79 1 .791.85 1 .83 1.82

5.46 5.47 5.48 5.452.54 2.53 2.52 2.558.00 8.00 8.00 8.00

0.50 0.48 0.42 0.430.00 0.26 0.00 0.080.01 0.01 0.01 0.010.00 0.00 0.00 0.000.35 0.33 0.38 0.372.04 2.20 2.22 2.252.81 2.46 2.68 2.600.04 0.04 0.04 0.045.75 5.78 5.75 5.78

0.00 0.00 0.00 0.000.02 0.03 0.03 0.021 .91 1 .80 1.78 1 .801.93 1 .83 1 .81 1 .82

Note.' Unit abbreviations as in Table 1. Formulas are based on 22(O,OH); n : number of analyses.

Micas

Biotite is the most common mafic mineral in all faciesofthe La Posta pluton. It nucleated after hornblende andtypically formed discrete subhedral to euhedral crystals.Lack of fine-grained biotite along rims of the hornblendegrains indicates that reaction of hornblende with melt toproduce biotite did not play a role in the evolution of themelt system.

Smith and Walawender (1987) reported the presenceof significant quantities (up to 670lo) of vermiculite withinbiotite separates from the La Posta pluton. However, thedata from that report cannot be either reconciled with theanalyses reported in this study or replicated in our labo-ratory. The sample-preparation techniques employed inthat earlier study appear to favor concentration of ver-miculite relative to biotite in the - lO-pm fraction andled to strongly biased results.

Biotite compositions from l3 samples of the La Postapluton and from 4 samples of the adjacent tonalitic unit(Fig. l) are listed in Table 4. Biotites coexisting with am-phibole, including those from the older tonalite to thewest, form a tight cluster near the center of the phlogo-pite-annite-eastonite-siderophyllite quadrilateral (Fig. 5).Biotites coexisting with muscovite, however, form a dis-

tinct group with higher t4rAl and Fel(Fe + Mg). The higherAl content ofbiotite from the eastern versus western sideof the pluton is apparent in Figure 6, which depicts athreefold grouping that is both geographic (west versuseast) and lithologic (muscovite-bearing versus horn-blende-bearing). Similar differences have also been re-ported by de Albuquerque (1973) for a series ofdiscretebiotite and biotite-muscovite granodiorite plutons in theAregos region ofPortugal and by Speer (1981) for biotiteand biotite-cordierite assemblages in the Cloud Creekpluton, South Carolina. The latter, however, showed nosignificant change in Fe/(Fe + Mg) and has considerablet+lAl overlap between the two assemblages whereas thePortugal study documents a continuum in terms of Fe/(Fe + Mg) rather than distinct jumps.

Wones and Eugster (1965) argued that the Fe/(Fe +Mg) of biotites should increase during differentiation. Thisratio increases slightly (0.68-0.72, Table 4) from thehornblende-bearing rocks on each maryin of the plutoninward but jumps to 0.81 in the muscovite-bearing core.This abrupt change is not consistent with in situ fraction-al crystallization and implies that a compositional dis-continuity exists between the core zone ofthe pluton andits marginal facies.

lrcte 4-Continued

Muscovite

35.063.00

16.560.023.08

16.970.338.770.270.139.130.030.01

93.06

0.70

5.462.548.00

0.500.360.010.000.352.052.210.04J.5Z

0.050.041.821 . 9 1

20.840.329.540.010.099 . 1 00.010.01

95.78

0.69

s.482.528.00

0.49

0.010.000.332202.700.04c , t I

0.000.031.801.83

35.863.26

16.300.042.73

1 6 .130.289.720.020.198.820.030.01

93.39

0.66

c.5rt

2.478.00

0.490.320.010.000.382.242.080.04c . c o

0.000.061.741.80

34.74t n o

1 6 . 1 00.03

19.330.269.600.010.159.120.020.01

92.46

u.o/

5.492.518.00

0.48

0.010.000.372.262.55

0.03a -tn

45.400.79

34.87

2.230.020.810.010.39

1 1 . 0 8

95.60

45.110.89

34.22

2.270.010.870.010.37

10.86

94.60

6 . 1 11.898.00

3.56

0.090.180.260.004.09

0.000 . 1 01.881.98

0.080.160.250.004.08

0.000.101.891.99

Biotite was separated from eight samples of the La Pos-ta pluton and two samples of the older tonalite adjacentto its western margin (Fig. l). From the microprobe-de-rived FeO (total Fe) analyses, FeO and FerO, were de-termined by using the technique of Fritz and Popp (1985).The results are listed in Table 4. Wones and Eugster (1965)determined that biotites crystallizing from a common pa-rental magma under oxygen-buffered conditions shouldplot on a trend with significant increases in Fe/(Fe + Mg)but only modest increases in the ferrous/ferric ratio. Datafrom the La Posta pluton are plotted on the Fe2+-Fe3+-Mg ternary diagram (Fig. 7a) along with the trends forseveral oxygen-buffered systems (Wones and Eugster,I 965). The data are again clearly separated into two groupsrepresenting biotite coexisting with hornblende and bio-tite coexisting with muscovite. Both groups, however, havesubparallel trends that cut across the trends of oxygen-buffered systems; the most pronounced increase in Fe3+occurs in biotites from the muscovite-biotite facies in thecore of the pluton. These data are not consistent withcrystallization under buffered conditions. They suggestthat a source more oxidized than the parental melt con-tributed to the system and that this contribution is greaterin the core ofthe pluton than in its outer facies. The data

1265

s

ta

" q +

Po '2 .4 .6

" t .h

FelFe +Mg

Fig. 5. Ideal biotite compositional plane modified from Deeret al. (1963). Crosses and solid dots represent biotites from theLa Posta pluton that coexist with hornblende and muscovite,respectively. Open circles are analyses of biotites that coexistwith hornblende from the adjacent older tonalite. End-membersare E: eastonite, S : siderophyllite, p : phlogopite, and A:annlte,

also suggest that the parental melt for the magnetite-freeLa Posta pluton was more reduced than the fayalite-quartz-magnetite buffer. In contrast, the data for biotitefrom two samples of the older western tonalite and fivebiotite analyses from other plutons in the western zoneof the batholith (Larsen and Draisen, 1950) cluster abovethe Ni-NiO buffer (Fig. 7b) and further emphasize thedifferences between the older plutons in the western zoneofthe batholith and the younger La Posta-type plutons.

Fer

Fig. 6. Fet-{t+tA1 x l0lMgternarydiagramforbiotite. Sym-bols as in Fig. 5. Brackets depict samples from the western andeastern sides of the pluton and emphasize their differences inre:Al.


E

MB MB1 1 1 89 8

T T T2 3 9 2 34 4 5

T14

35.332.82

16.49o.02

2 . 5

2.4( 4 ) A l

2 4

6.081.928.00? 4 0

0.000.0s1 .841 .89

Mg

{r)41 y' le ( ln bte)

H-t,l

H-M __

_--

t266

Fe 3*

Fe 2 *

Fe 2*

Fig. 7. Fe2+-Fe3+-Mg ternary diagram for biotite after Wonesand Eugster (1965). Dashed lines represent compositional trendsunder buffered conditions; QFM : quartz-fayalite-magnetite, Ni-NiO : nickel-nickel oxide, and HM : hematite-magnetite.Symbols as in Fig. 5. (a) Data from the La Posta pluton. (b) Datafrom plutons in the western zone ofthe batholith (see text).

Miller et al. (198 l) summarized textural and geochem-ical criteria for the recognition of primary versus second-ary muscovite. Both types occur in the muscovite-biotitefacies of the pluton, but only those meeting the texturalcriteria for primary (magmatic) origin were analyzed inthis study. The results are listed in Table 4 and are inclose agreement with the "typical" primary muscovite ofMiller et al. (1981) in terms of their higher AlrO, andTiO, and lower MgO contents.

Feldspars

Plagioclase feldspar is the most abundant mineral inthe La Posta pluton. It occurs as subhedral to euhedralgrains with moderate normal to weak oscillatory zoningand formed throughout the crystallization history of thepluton. In contrast, alkali feldspar is typically found onlyin the three interior facies of the pluton and occurs asinterstitial fillings or equidimensional poikilitic patchesup to 4 cm in diameter. Calculated end-member fractionsare given in Table 5.

Figure 8 shows the variation in plagioclase composi-tion across the pluton. Plagioclase compositions for thehornblende-biotite, large-biotite, and small-biotite faciesaverage approximately An' and are essentially identicalon either side of the pluton. Plagioclase in the muscovite-biotite facies, however, shows a rather abrupt decrease inaverage composition to approximately An$. Core andrim compositions are plotted against Fe/(Fe + Mg) forcoexisting biotite (Fig. 9) and indicate that a composi-tional gap exists for both minerals. Alkali feldspar com-positions vary little between the three innermost facies ofthe pluton.

Opaque minerals

Because of the scarcity of opaque minerals in the rocksof this study, they were quantitatively analyzed in only


Fe 3 t TABLE 5, Average analyses of feldspars in the study area

Unit W. HSample IAb 67.0(3.7)or 1.2(0.3)An 322(4.0)

Unit MBSample 14Ab 72.0(1.11Or 1.3(0.3)An 26.9(0.5)

PlagioclaseW . H W . L B2 2 463.e(7.5) 66.9(4.2)1.5(0.3) 1.4(0.5)

34.6(7.71 31.8(4,6)

Plagioclase

W. SB MB5 1 1

61.8(3.1) 72.4(2.0)1 .0(0.2) 1.2(o.2)

37.2(3.31 26.4(2.2)

Mg

MB MB E. LB E. LB18 32 13 3574.1(1 .0) 73.4(1.1) 67 .1(3.7) 65.9(2.3)1 .1(0.2) 1.7(0.2) 1 .5(0.2) 1.2(0.3)

24.7(1.4) 25.1(1.3) 31.4(3.7) 33.0(2.4)

PlagioclaseU n i t E . H E . H E . H T TSamole 12 19 20 2 23Ab 62.9(1.5) 62.4(1.7) 65.2(2.2) 59.5(1.6) 53.9(1.1)or 1.1(0.2) 1.2(0.3) 1.1(0.2) 1.4(0.2) 1.0(0.1)An 36.0(1.6) 36.3(1.8) 33.7(2.2) 39.1(1.7) 45.1(1.1)

Plagioclase and alkali feldsPalUni t T W. LB W. SB MB MBSample 39 4 6 11 18Ab 61.1(2.5) 7.0(2.6) 8.0(2.4) e.6(2.0) 7.e(0.8)or 0.6(0.1) e3.0(2.7) 91 .s(2.5) 90.3(2.0) 92.1(0.8)An 38.3(2.5) 0.0(0.0) 0.1(0.0) 0.1(0'0) 0.0(0'0)

ivote: Unit abbreviations as in Table 1. Number in parentheses is onestandard deviation. All plagioclase values were calculated from three anal-yses each of grain center and rim. All alkali feldspars were calculated froman average of five analyses per sample

three samples. These samples were from the two outer-most facies on the west side of the pluton and the adja-cent older tonalite. The results are given in Table 6.Opaque minerals that were encountered in the course ofaralyzing other minerals were examined semiquantita-tively with the energy-dispersive spectrometer (Eos) onthe microprobe. All of the opaque grains analyzed insamples from the La Posta pluton, including those ex-amined by the eos, were found to be ilmenite. The sam-ple from the western zone tonalite was found to containmostly ilmenite, but one grain of the six analyzed wasfound to be magnetite. This is consistent with the findingsof Gastil et al. (1989) that the magnetite-ilmenite line inthe Peninsular Ranges batholith coincides with the west-ern boundary ofthe La Posta pluton.

CoNcr,usroNs

Equilibration of hornblende with plagioclase of similarcompositions on both sides of the pluton at different butgeologically reasonable temperatures suggests differencesin the cooling histories of opposite sides of the pluton.Naney and Swanson (1980) pointed out that undercool-ing of intermediate-composition melts leads to a morerapid nucleation of mafic minerals such as hornblendeand biotite relative to feldspar and quartz. The presenceof mafic mineral-rich rocks in the banded part of theborder facies suggests that the western margin may haveundergone more severe undercooling relative to the east-ern margin. Whether this process is responsible for thelower mafic mineral content in the western hornblende-

CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON r26'1

,T

lI,rTI I T

r l

IttI

' { , ]IAn

,,1,,1,,,J.Sarnp le

N u m b e r1 3 3 65I 2 2

v v l 9 V

1 1 1 4 1 A 3 2 1 2 1 9 2 0

West

M B

FACIES

Fig. 8. Schematic variation in plagioclase composition from west to east across the pluton. Three rim and core analyses (Table5) are shown for each sample. Facies symbols are H : hornblende-biotite, LB : large-biotite, SB : small-biotite, and MB :muscovite-biotite.

biotite facies (Table l) is speculative but cannot be ruledout.

The hornblende geobarometry for the two sides of theLa Posta pluton and the older tonalitic body adjacenl tothe western contact (Table 3) overlaps with somewhatless precise estimates of P, (2-5 kbar) for the medial zoneof the batholith (Walawender and Smith, 1980; Berggreenand Walawender, 1977; Detterman, 1984; Eaton, 1982;Diercks, 198 1) that were based on coexisting mineral as-semblages in both the metamorphic and plutonic rocks.Other geobarometric techniques have yielded more pre-cise results. Taylor et al. (1979) calculated that a pressureof2.1 kbar was required to correct fluid-inclusion closuretemperatures in pegmatites north of the study area totemperatures consistent with those calculated from theiroxygen-isotope data. London (1986) estimated an em-placement depth between 2.4 and 2.8 kbar for the same

60 70 ao 00 70 80 g0

1OO (Fe lFe + i tg )b t€

Fig. 9. Average rim and core compositions for plagioclaseversus 100 Fe/(Fe + Mg) in coexisting biotite. Symbols as inFig. 5.

pegmatite system on the basis of phase equilibria for thepocket minerals. Clinkenbeard (1987) used the Ca-ex-change geobarometer ofGhent and Stout (1981) on the93 Ma garnet-bearing monzogranite near the center of thestudy area (Fig. l) and calculated Pr between 2.7 and,3.0+ 0.5 kbar. K. E. Panish (unpublished data) reported P,of 2.5 + 0.5 kbar for a similar garnet-bearing monzo-granite located near the northern end of the central meta-morphic screen (Fig. l). Although the hornblende geo-barometry reported in this study brackets these latterfigures, the difference between the western and easternsides of the La Posta pluton does not fall within the errorestimates of Hollister et al. (1987). We also argue thatthe variation in emplacement depths (Pr) calculated fromthe hornblende geobarometer is not consistent with eitherthe known field and age relationships, experimentally de-termined variation in solidus temperature with pressure,

TABLE 6. Average analyses of opaque minerals

Unit : W. LBSample: 4n : 6

W . H T T8 3 9 3 94 5 1

FeOMgoTio,MnO

Total

Fe'?*FesMgTiMn

Sum

43.200 . 1 3

51.232.75

97.31

1.8460.0240.0101.9940.1364.009

45.41U.UC

50.292.04

97.79

1.8570.1010.0041.9490.0894.000

51.30 93.300 . 1 5

42.54 0.021.56

95.55 93.32

1.591 8.0040.654 15.9900.0121.672 0.0050.0693.998 23.999

/Vote: Unit abbreviations as in Table 1. Mineral formulas are based on6 oxygens for ilmenite and 32 oxygens for magnetite; n : number ofanalyses.

r268

or other geobarometers based on mineral chemistries. Thisdiscrepancy could be due to equilibration ofhornblendewith an intermediate stage of the La Posta melt ratherthan with the alkali feldspar-saturated stage required byHollister et al. (1987). Under this condition, the Al, con-tent of amphibole would not reflect a simple pressurecontrol but would depend upon other variables in themelt system such as the temperature of hornblende equil-ibration. Thus, local perturbations in cooling histories mayalso be important in terms of controlling hornblendecompositions.

Unlike hornblende, biotite is found in all facies of theLa Posta pluton so that its compositional variations re-flect the entire range of melt compositions during crys-tallization. Modest but identical increases in Fe/Ge +Mg) from the marginal hornblende-biotite facies inwardthrough the large-biotite and small-biotite facies are con-sistent with in situ fractional crystallization. However,the jump in that ratio (Table 4) combined with a sharpdecrease in plagioclase composition (Fig. 8, Table 5) acrossa gradational contact into the muscovite-biotite facies re-flects a compositional discontinuity between the outer fa-cies and the muscovite-bearing core.

In addition, the ferrous/ferric ratios ofbiotite vary con-siderably, with the greater degree of variation found inthe muscovite-bearing core of the pluton (Fig. 7a). Thedata form subparallel, quasiJinear arrays that we inter-pret to represent mixing of two components with ditrerentferrous/ferric ratios. The parental magma is consideredto be the most reduced component since no evidence ofa reducing mechanism, e.g., inclusions of C-bearingmetasedimentary material, has been observed. As such,the parental magma would have had original oxygen fu-gacities more reducing than the QFM buffer (Fig. 7a). Theremaining component in this mixing relationship is moreproblematic.

Walawender et al. (1989) described anomalous isotopicpatterns (higher D'8O in qvartz, excess radiogenic Sr, andhigher contents of inherited zircon) in the muscovite-bearing cores ofseveral La Posta-type plutons in south-ern and Baja California and concluded that they representmixing of a parental melt, which was derived primarilyfrom subducted oceanic crust, and a continental com-ponent with an average age of approximately 1300 Ma.We would add oxidizing capabilities greater than the Ni-NiO buffer and relatively high Fe/(Fe + Mg) to the afore-mentioned characteristics of this continental component.The parental La Posta melt rose between the older west-ern zone of the batholith and the leading edge of theNorth American craton (Gastil, 1975). We suggest thatduring ascent, the portion ofthe rising diapir adjacent tothe deeper (granulite facies?) segments of the craton re-acted with crustal materials, whereas the remainder ofthe diapir rose and expanded into the overlying metase-dimentary apron. Inward crystallization produced themarginal facies, but prior to the solidification of the small-biotite facies, the isotopically enriched and more oxidizedtail of the diapir entered the magma chamber and mixed


with the derivative liquid. The resulting hybrid melt couldonly undergo limited mixing and diffusion with the al-ready partly solidified outer zones ofthe pluton and cooledin situ to form the muscovite-bearing core with its gra-dational contacts. We would extend this model to includeall of the zoned and lithologically similar La Posta-typeplutons in southern and Baja California.

AcrNowr-BocMENTs

We gratefirlly acknowledge NSF suppon through gra.nt no. EAR8306663to Walawender. Ron La Borde and Roy Fujita lent considerable assistanceon the electron microprobe at Scripps Institute ofOceanography. LawfordAnderson ofUCLA provided his own computer programs for the various

thermobarometric calculations. We are also indebted to Joan Calhoun,SDSU, for assisting in the Fe'+/Fe3+ determinations ofthe biotite sepa-rates, and to Teresa Larson, SDSU, for drafting the figures. The manu-script benefited from reviews by Michael T. Naney, Lincoln S. Hollister,and David A. Dellinger.

RnrBnrNcns crrnn

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Mnruscnrsr R.EcErvED Sesrelrsen 30, 1988MeNuscnrpr AcCEPTED Jurv 26, 1989


Documents

Mineralogy of the La Posta pluton: Implications for the origin of zoned plutons in the eastern