American Mineralogist, Volume 74, pages 1258-1269, 1989
Mineralogy of the La Posta pluton: Implications for the origin of zonedplutons
in the easternPeninsular Rangesbatholith, southernand Baja California
J. P. Cr,rNxnNBEARD.*M. J. War,awnNonn
Department of Geological Sciences,San Diego StateUniversity, San Diego, California 92182, U.S.A.
Ansrucr
The 94 Ma La Posta pluton is located in the eastern zone of the Peninsular Ranges
batholith and is exposed over an area of approximately 1400 km2. It is zoned from a
sphene-hornblende-biotitetonalite rim inward to a core of muscovite-biotite granodiorite
with internal contacts gradational over distancesof several tens to hundreds of meters.
This pluton is texturally distinct from the older synkinematic plutons of the western zone
of the batholith in that it is undeformed and contains sharply euhedral mafic minerals.
Microprobe analysesindicate that hornblendesfrom the easternside of the pluton have
higher Al, and lower Si than those from the western side. This variation in composition
yields geologicallyunreasonabledifferencesin hornblende geobarometryand reflectshornblende equilibration under different thermochemical conditions on either side of the pluton. Compositional variations within individual mineral suites are consistentwith a general model involving inward crystallization of a static magma chamber. However, biotites
in the interior faciesare more oxidized and have abruptly higher Fel(Fe + Mg) than those
in the marginal facies.Thesedata suggestthat the muscovite-bearingcore faciesrepresents
the tail of a diapir that intruded a ballooned magma chamber prior to its complete solidification. A longer residencetime for the tail adjacent to a cratonal source resulted in its
enrichment in, at least, Fe3* and Fe/(Fe + Mg).
INrnooucrroN
The easternside of the Peninsular Rangesbatholith is
dominated by a seriesof large, concentrically zoned plutons of intermediate composition that exhibit distinct
similarities in age, rock type, and geochemistry (Walawenderet al., 1989).The largestof these,the 94 Ma La
Postapluton, is located approximately 65 km eastof San
Diego, California, and straddlesthe international border
(Fig. l). Although the northern portion of this pluton and
the section south of the international border have been
mapped only in reconnaissancefashion, the approximately 750 km'zthat have been studied in detail (Kimzey,
1982;Sinfield, 1983;Clinkenbeard,1987)reveala more
or less single magmatic pulse that crystallized inward to
form a lithologic successionranging from hornblendebiotite tonalite to muscovite-biotite granodiorite. Textural evidenceindicatesthat this pluton experiencedlittle
subsolidus deformation or alteration and thus provides
an excellent opportunity to examine compositional relationships between coexisting silicate minerals that have
retained their primary magmatic character. This paper
describes these compositional relationships and interprets them with respectto various parametersthat affect
cooling magmatic systems.
+ Presentaddress:Division of Mines and Geology, 650-B Bercut Drive, Sacramento.California. U.S.A.
0003-004x/89/
I l l 2-1258$02.00
Gnor,ocrc sETTTNG
The PeninsularRangesbatholith is part of the circumPacific chain of Mesozoic batholiths and extends from
Riverside, California, southward more than 1000 km to
the lat 28"N in Baja California. It consistsof hundreds of
individual plutons that rangein maximum dimension up
to 45 km and in composition from gabbro to granite. The
batholith, as a whole, is calcic in composition (Silver et
al.. 1979\ and can be divided into western and eastern
zones based on geochemical,lithologic, structural, and
geophysicalcriteria.
The western zone is characterizedby smaller plutons
that range in composition from gabbro to monzogranite.
They were emplaced into greenschist-to lower amphibolite-grade host rocks dominated by volcanic and volcanoclastic assemblagesto the west and submarine fan
depositsto the east(Todd et al., 1988).The easternzone
of the batholith is essentiallydevoid of gabbroic plutons
and contains mainly large (hundreds of square kilometers) plutons of intermediate composition that were emplaced into locally migmatitic, sillimanite-bearing metasedimentary rocks. In addition, plutons in the eastern
zone are undeformed and contrast with the synkinematically deformed plutons to the west (Todd and Shaw,
r979).
The boundary between these plutonic zones crudely
coincideswith a sharpgravity contrastthat Oliver (1980)
interpreted as the juxtaposition ofa westernoceaniccrus-
r258
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
1259
.i\
2\
23(
3el
t+
+
+
++
''
-
".
t.
cALtFoRNll --<ugnco
POSTA PLUTON
HB-BTE FACIES
LG BTE FACIES
SM BTE FACIES
MU-BTE FACIES
WESTERN
ZONE
PLUTONIC
ROCKS
METAMORPHIC
ROCKS
SAMPLE
LOCATION
Fig. 1. Geologic map of the La Posta pluton (after Kimzey, 1982; Sinfield, 1983).
tal basementand an eastemcontinental basement.Silver
et al. (1979) showed that this boundary also approximates a "step" in both the emplacementagesof the plutons and their oxygen-isotopecharacter. Plutons in the
westernzone rangein agefrom 120 Ma to 104 Ma with
dtEOless than 8.5Vu,whereasthose in the easternzone
are generallyyoungerthan 100 Ma and have D'EOranging
Walawenderet al. (1989) argued that
from 9 to l2%oo.
although theseregional patterns are generallyvalid, transitional plutons with characteristics of both zones are
presentin the vicinity ofthe ageand isotopic "steps" and
that this boundary representsa transition from mantlederived melts to oceaniccrust-derived melts.
Diamond et al. (1985)and Gastil et al. (1986)have
shown that the plutons of the western zone contain magnetite and ilmenite as their primary opaque phases
whereasthose to the east are devoid of magnetite. This
magnetite-ilmenite "line" approximates the lithologic,
geochemical,and age "steps" described above. Gastil et
al. (1989) have also consideredit to reflect a transition
from mantle-derived to oceaniccrust{erived melts with
the nature ofthe volatile phasereleasedduring anatexis
controlling the presenceor absenceof magnetite.
The La Postapluton is divided into two parts by a large
metasedimentaryscreen(Fig. l) that intemrpts the continuity of the plutonic units. Sillimanite-bearing pelitic
schistsand migmatitic gneissesalong with lesseramounts
of marble, quartzite, and amphibolite make up the bulk
of the metamorphic complex. The metamorphic complex
and adjacentparts of the La Postapluton are intruded by
89-93 Ma garnetiferous two-mica monzogranites. The
western hornblende-biotite and small-biotite facies, and
the easternhornblende-biotite faciesofthe La Posta pluton have zircon U-Pb agesof 93, 94, and 95 Ma (+2
Ma), respectively, and indicate an emplacement age of
approximately 94 Ma (Walawender et al., 1989). This
U-Pb age is consistentwith a four-point Rb-Sr isochron
ageof 92 + 2.8 Ma for the western small-biotite facies.
The continuity of agesacross the pluton along with its
map-unit symmetry and petrographic similarities suggest
that the La Posta pluton was emplaced during a single
intrusive event. In addition, the large-biotite faciesoccurs
in identical pseudostratigraphicposition on either side of
the pluton. That particular facieshas not been recognized
as a map unit in any other La Posta-type pluton studied
to date.
Prrnocupnv
The La Posta pluton is crudely circular and estimated
to be approximately 1400 km'zin total exposedarea.Five
lithologic facies(Fig. 1) have been recognizedwithin the
pluton (Kimzey, 1982).These,from the westerncontact
inward, are the border, hornblende-biotite, large-biotite,
small-biotite, and muscovite-biotite facies.The sequence
is repeatedin reverse order from the muscovite-bearing
core eastward to the exposed edge of the pluton. Here,
the pluton disappearsbeneath a cover of younger allu-
r260
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
The most obvious feature of this unit is the euhedral
characterofthe hornblende, biotite, and sphene(Fig. 2),
which occur as largediscretecrystalsup to 7 mm in length.
Inclusions in the mafic minerals consist of ilmenite, apatite, zircon, and allanite (Clinkenbeard,1987).This textural characteris distinct from that ofthe hornblende and
biotite in the older plutonic rocks to the west, which are
commonly anhedral, charged with euhedral plagioclase
inclusions, and intergrown with one another. Sphenein
the older plutonic rocks is typically anhedral and intergrown with the biotite.
Plagioclaseoccurs as subhedralto euhedral crystalsup
to l0 mm in length that exhibit weak oscillatoryor patchy
zoning. They commonly have cores composedof two or
Fig.2. Photomicrograph
of sample8 from thewestemhorn- more crystalsjoined in a synneusisrelationship; the cores
blende-biotite
facies.Q : quaft4H : hornblende,
S : sphene. are mantled by less calcic rims. Alkali feldspar, where
Idiomorphichomblendeboundaries
againstquartzindicatethat
present, is interstitial and exhibits "gridiron" twinning.
hornblendegrowthhad ceased
by the time quartzbeganto nuLocally, myrmekitic intergrowths occur at the contact becleate.
tween the two feldspars.Quartz exists as discreteor composite grains with weak undulatory extinction and modvium such that neither the host rocks nor the border fa- eratelysuturedboundaries.
ciesis exposed.
Large-biotite facies
Border facies
The sphene-hornblende-biotitetonalite of the hornThe border facies,not shown in Figure l, crops out in
a narrow discontinuousband up to 100 m wide only where
the La Posta pluton is in contact with the older igneous
rocks of the western zone of the batholith. Where the
pluton intrudes metasedimentaryor metaigneousrocks,
the border faciesis absent,and the hornblende-biotite or
large-biotite faciesforms the outermost unit. The border
facies changes inward from a banded marginal zone,
through a gneissic,melanocratic biotite-hornblende diorite into weakly foliated and then nonfoliated hornblende-biotite facies rocks. All contacts are steepto vertical and subparallel to the margin of the pluton. This
sequenceis cut by narrow dikes ofaplite and alkali feldspar megacrysticgranite that form less than l0o/oof the
exposures.
The banded marginal zone rangesup to 50 m in thickness and consists of alternating bands of plagioclase +
quartz and hornblende + biotite, which rangefrom a few
centimetersto more than a meter in thickness.The mafic
minerals are commonly aligned parallel to the banding
but are typically subhedralto euhedralin form. The mafic
diorites are in sharp contact with the banded zone but
changelaterally via an increasein plagioclaseand a loss
of gneissosity into unfoliated hornblende-biotite facies
assemblages.
Hornblende-biotite facies
The hornblende-biotite faciesis the main outer unit of
both the eastern and western sides of the pluton. It is
typically massive, but can exhibit a weak foliation near
and parallel to the margin of the pluton. The dominant
rock type is a sphene-hornblende-biotitetonalite that becomesprogressivelymore enriched in alkali feldspar and
depleted in hornblende inward.
blende-biotite facies grades inward into sphene-hornblende-biotiteand sphene-biotitegranodioriteof the largebiotite faciesby loss ofthe largeeuhedralhornblende and
an increasein both the size and abundanceof the interstitial alkali feldspar. The presenceof large (5-10 mm)
pseudohexagonalbooks of biotite gives this rock a distinctive salt and pepper appearance.Small (l-3 mm) subhedral hornblende crystals are present in the outer portion ofthis facies but decreasein abundancetoward the
core of the pluton. Both biotite and hornblende are otherwise similar to those found in the hornblende-biotite
facies.
Plagioclaseoccursmainly as discretesubhedralcrystals
that average3-4 mm in size but that may reach l0 mm.
Oscillatory zoning is common. Quartz appears to be
slightly coarserin this faciesbut is otherwiseas described
for the hornblende-biotite facies. Alkali feldspar is in
greater abundanceand tends to form oikocrysts up to 3
cm in size that encloseall of the above minerals. As in
the hornblende-biotitefacies,euhedralspheneis the main
accessorymineral.
Srnall-biotite facies
Proceedinginward, the next zone of the La Posta pluton consists of biotite granodiorites that are texturally
distinct from those in the large-biotite facies. The two
facies have a gradational contact on the western side of
the pluton, but this contact on the easternside is at least
in part intrusive (Sinfield,1983).
The small-biotite facies is characterizedby the presence of small (l-4 mm) euhedral to subhedral biotite
crystals.Hornblende is scarceand, where present,occurs
as minute inclusions in biotite or plagioclase.Plagioclase
is presentas l- to 3-mm subhedralcrystals,someof which
t26l
CLINKENBEARDAND WALAWENDER:THE ZONED LA POSTAPLUTON
Trele 1, Averagemodesof rocksin the studyarea
Mineral
W. H
W. LB
n
Quartz
Plagioclase
K-feldspar
Biotite
Hornblende
Muscovite
Sphene
Apatite
Ztcon
Allanite
Opaques
Myrmekite
Sericite
Chlorite
Epidote
3
25.1
55 1
2.7
12.7
2.7
2
29.4
54.4
4.2
8.4
1.9
W. SB
31.5
48.6
I-6
OA
36.1
46.9
8.0
8.1
E. SB
E. LB
E.H
34.5
50.3
4.6
10.1
3
35.8
49.4
3.2
12.2
0.9
3
29.8
45.2
0.8
0.8
tr.
tr.
tr.
Ir.
0.1
0.9
tr.
tr,
tr.
Ir.
tr.
tr.
tr.
Ir.
Ir.
tr.
ff
Ir.
1AA
4.0
0.4
o.4
Ir.
Ir.
Ir.
Ir.
0.4
Ir.
u.c
tr.
0.6
Ir.
Ir.
Ir.
Ir.
0.4
Ir.
0.6
tr.
0.1
Ir.
Ir.
tr.
tr.
o.4
tr.
1.1
tr.
Ir.
Ir.
tr.
tr.
0.3
tr.
tr.
Ir.
tr.
Ir.
3
26.3
49.7
0.2
14.8
8.7
tr.
0.2
Note.'Faciesdesignationasfollows:H=hornblende-biotite,LB:large-biotite,SB:small-biotite,MB:muscovite-biotite,andT:western-zone
tonalite. E. and W. representthe eastern and western sides of the pluton, respectively;n: number of analyses;tr. = trace.
are joined in a synneusisrelationship. Weak oscillatory
zoning is present,but the patchy zoning observedin the
plagioclaseof the hornblende-biotite and large-biotite facies is missing. Quartz is texturally similar to that found
in the other facies but exhibits stronger undulatory extinction. Alkali feldspar typically forms oikocrysts up to
several centimeters in size that have well developed
"gridiron" twinning.
of Ca. Na. and K totaled 13. The ferrous/ferric ratios of
selectedbiotite separateswere determined with the technique ofFritz and Popp (1985).
MrNtur,ocv
Amphibole
Amphibole is found only in the outer zones of the La
Postapluton. It occursin significantamounts in the hornMuscovite-biotite facies
blende-biotite and large-biotite facies but decreasesinThis facies is mainly found in the eastern portion of ward to trace amounts in the innermost portion of the
the La Postapluton where it gradesinto the easternsmall- large-biotite and throughout the small-biotite facies (Tabiotite facies. Locally, however, muscovite-bearing ble l). Hornblende is absent in all muscovite-bearing
granodiorite also occursjust west of the large metasedi- rocks.
Analysesof amphiboles from sevensampleswithin the
mentary screen(Fig. l) in the central part of the study
area. The main rock type is a muscovite-biotite grano- La Posta pluton and four samplesin tonalitic rocks imdiorite that is similar to the small-biotite faciesexceptfor mediately west of the pluton (Fig. l) are reported in Table
the presenceof 1- to 3-mm subhedralmuscovite crystals 2. All samples plot close to the boundary between the
and the absenceofsphene. Basedon the textural criteria magnesio- and ferro-hornblende fields (Fig. 3), but difof Miller et al. (1981),the muscoviteis consideredto be ferencesbetweenthe westernand easternsidesofthe pluprimary, i.e., magmatic in origin. Secondarymuscovite ton are apparent. Amphibole from the western horn(sericite)is presentin trace amounts in the plagioclase.
blende-biotite and large-biotite facies have Si > 7.0,
whereasthose from the easternequivalentshave Si < 6.8.
ANnr,vrrclr, METHoDS
This difference is also observed in the lower Alt of the
Chemical analysesof minerals in polished thin sections amphiboles in the western half of the pluton (Table 2)
were made using the three-channelceueca Msx electron and indicatesthat at the time of amphibole nucleation in
microprobe at the ScrippsInstitute of Oceanography.Op- the melt, conditions of lower a",orartdhigher c^ro, existed
erating conditions were as follows: acceleratingpotential in what is now the easternside of the pluton.
: 15 kV, samplecurrent : 15 nA on brass,spot size :
Hammarstrom and Znn (1986) and Hollister et al.
2 to l0 pm. Standards used for calibration were well- ( I 987) have defined a geobarometerbasedon the Al concharacterized silicates and oxides. Data reduction was tent of hornblende. Constraints on the use of this geoperformed on-line using the program coRREx,which is barometer were summarizedby Hollister et al. (1987)
part of the Msxcon software package.Mineral formulas and include the presenceof magnetite among the opaque
for biotite, muscovite, plagioclase,alkali feldspar,and il- phases;a magmatic assemblageof hornblende, biotite,
menite were calculatedusing the program ceo, also part plagioclase,alkali feldspar, and quartz; and plagioclase
of the Nasxconpackage.Amphibole formulas were cal- rim compositions between An* and Anrr. The hornculatedby using an electronic spreadsheetprogram in the blende-bearingrocks ofthe La Posta pluton have plagiomanner suggestedby Leake (1978). Their ferrous/ferric clase rim compositions between An, and Anrt and the
ratios were varied until the sum ofthe cations exclusive required mineral paragenesiswith the exception of mag-
t262
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
Tlele 2. Averagehornblendecompositionsof rocks in the study area
Unit:
Sample:
n:
sio,
Tio,
Alros
CrrO"
FerO.
FeO
MnO
Mgo
CaO
Naro
KrO
Nio
P"ou
Total
Fel(Fe + Mg)
5l
AI
Sum
AI
Fee*
Ni
Ti
Mg
Fe2+
Mn
Sum
r-e.Mn
Ca
Na
Sum
Na
K
Sum
WH
8
T
T
T
W.H
W.LB
2243512
4745
E.LB
E.H
E.H
19
5
E.H
T
20231239
47784
47.62
1.01
6.74
47.30
1.06
7.13
0.05
47.76
1.01
6.46
0.02
45.54
1.11
8.85
0.05
43.81
1.22
10.24
0.02
44.46
1.08
10.20
0.01
44.25
1.02
10.68
0.02
44.82
1.12
9.64
0.02
44.68
1.21
9.20
0.02
42.93
1.23
10.17
0.02
46.42
1.29
8.81
0.04
18.34
0.60
10.52
11.91
0.98
0.51
0.02
0.10
98.35
0.64
705
0.95
8.00
18.49
0.64
10.22
11.73
1.O2
0.60
0.05
0.10
98.39
0.64
7.00
1.00
8.00
0.24
0.34
0.01
0.01
0.12
2.25
1.95
0.08
0.00
5.00
0.00
0.00
1.86
0.14
2.00
0.15
0.11
0.26
18.69
18.01
0.55
9.73
11.88
1.04
0.86
0.04
0.10
97.76
0.65
6.81
1. 1 9
8.00
0.37
0.25
0.01
0.01
0.13
2.17
2.00
0.07
0.00
5.01
0.00
0.00
1.90
0.10
2.00
0.20
0.16
0.36
18.80
0.47
9.19
12.15
1. 1 6
0.97
18.20
0.44
9.24
11.78
1. 1 5
0.89
0.03
0.10
97.59
0.67
6.67
1.33
8.00
0.47
0.27
0.00
0.00
0.12
2.07
2.01
0.06
0.00
5.00
0.00
0.00
1.89
0.11
2.00
0.22
0.17
0.39
18.53
0.45
9.06
12.00
1. 1 5
1.01
0.02
0.10
98.29
0.67
6.61
1.39
8.00
0.49
0.25
0.01
0.00
0.11
2.02
2.06
0.06
0.00
5.00
0.00
0.00
1.92
0.08
2.00
0.25
0.19
0.44
17.18
0.41
10.76
12.09
1.06
0.71
0.01
0.10
97.92
0.61
6.64
1.36
8.00
17.70
0.43
10.52
12.06
1.11
0.68
0.01
0.10
97.72
0.63
6.65
1.35
8.00
0.32
0.47
0.01
0.00
0.13
2.37
1.66
0.05
0.00
5.01
0.00
0.00
1.92
0.08
2.00
o.22
0.13
0.35
0.46
0.46
0.01
0.00
0.14
2.33
1.74
0.05
0.00
4.99
18.78
0.43
9.60
12.18
1.18
1.10
0.02
0.11
97.75
0.66
6.47
1.53
8.00
0.28
0.42
0.01
0.00
0.14
2.16
1.94
0.06
0.00
5.01
0.00
0.00
't.97
0.03
2.00
0.31
0.21
0.52
16.43
0.39
12.01
11.47
1.16
0.41
0.02
0 . 11
98.56
0.58
6.72
1.28
8.00
0.22
0.76
0.01
0.00
0.14
2.59
1.23
0.05
0.00
5.00
0.00
0.00
1.78
0.22
2.00
0.11
0.08
0.19
o
0.22
0,30
0.00
0.00
0.11
2.32
1.97
0.08
0.00
5.00
0.00
0.00
1.89
0.11
2.00
0.17
0.10
0.27
U.CY
10.48
12.02
0.93
u.c /
0.02
0.09
98.64
0.64
7.06
0.94
8.00
0.19
0.28
0.01
0.00
0.11
2.31
2.03
0.07
0.00
5.00
0.00
0.00
1.90
0.09
1.99
0.18
0.11
0.29
0,11
98,1
3
0.67
6.57
1.43
8.00
0.38
0.28
0.01
0.00
0.14
2.06
2.08
0.06
0.00
5.01
0.00
0.00
1.95
0.05
2.00
0.29
0.19
0.48
0.00
0.00
1.92
0.08
2.00
0.24
0.13
0.37
Note.'Unitabbreviations
as in Table1. Formulas
basedon 23(O,OH);
r : numberof analyses.
netite. However, the data used to establishthe high-pressureend of the geobarometerare from the magnetite-free
Ecstall pluton (Hammarstrom and Zen, 1986) and raise
some question as to the importance of magnetite as a
requirement. Pressureestimatesof synchronous(93 Ma)
and spatially associatedgarnet-muscovite-biotite monzogranites(Fig. l) rangebetween2.T and 3.0 kbar (Clinkenbeard, 1987) and indicate that the La Posta pluton
was emplaced above the 2-kbar minimum pressurerequired by the geobarometer.
Hollister et al. (1987)argued
that only rim compositions of the hornblende crystals
should be used becausethese are likely to have formed
from the late stagesof the melt and therefore in a narrow
rangeof temperatures.Our analyses,part of a largerstudy,
were completed prior to the publication of the above restrictions.Grain traversesand random spot analyses(both
rim and core as seenin thin section) showed little compositional variation within individual grains or from grain
to grain in single samples.The data are reported as sample averagesin Table 2 with the averagevalue of A1, for
each sample applied to the geobarometer(Table 3).
The lack of compositional zoning and constant composition of hornblende with respect to changing rock
(melt?)chemistry (Clinkenbeard, 1987)suggestthat hornblende equilibrated with an intermediate rather than a
late stageof the La Posta melt. On the basis of textural
criteria, Clinkenbeard(1987) also arguedthat hornblende
nucleatedearly in the cooling history of the melt and that
its growth had effectively ceasedbefore the appearanceof
quartz and orthoclase.In addition, the low abundanceof
alkali feldspar in the hornblende-bearingrocks (Table l)
and its interstitial charactermay indicate that the hornblende grains were not in contact with an alkali feldsparsaturated melt. Thus, amphibole may not have equilibrated under the thermal and compositional restrictions
of the geobarometer.
Despite this complicating and unknown factor, a plot
of t4lAlversusAlr Gig. a) for the La Postapluton overlaps
the data shown by Hammarstrom and Zen (1986) for
several plutons from different pressureregimes. Their data
indicate that hornblende from high-pressure (epidotebearing) plutons have Al, > 1.8. Three of four analyses
t263
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
Tlele 3. CalculatedP-f conditions
Sample
no.
o
E
+
o
E
E
750
7.26
sl
Fig. 3. Hornblendeclassification
afterkake (1978).Crosses
represent
hornblendes
from thewesternandeasternfaciesofthe
l,a Postapluton. Solid circlesrepresenthornblendes
from an
oldertonalitebodyadjacentto thewesternmarginofthe pluton.
RemainingfieldsareT : tremolitichornblende,
A : actinolitic
hornblende,
F : ferro-actinolitic
hornblende,
TS : tschermakitic
hornblende,
hornblende.
and FTS : ferro-tschermakitic
from the easternside of the La Posta pluton have Al, >
1.8,whereasthosefrom the westernsidehave Alr < 1.8.
Epidote occurs as an accessorymineral throughout the
La Postapluton exceptin the innermost muscovite-bearing facies. It is typically granular and mantles allanite
where the latter was in contact with biotite. We interpret
this relationship to indicate that epidote is not a primary
magmatic mineral and that its presencedoes not imply
crystallization under high (8 kbar?) pressure.
Results of the hornblende geobarometer(Hollister et
al., I 987)are listed in Table 3 and appearto indicatethat
the western side of the La Posta pluton was emplacedat
considerably shallower levels (1.9 kbar) than the eastern
side (5.2 kbar) and the older tonalitic pluton to the west
(4.5 kbar). This suggestedrelationship implies that major
structural discontinuities exist at the western edgeofthe
pluton and at an unknown central location within the
pluton. Field relationships and radiometric ages(Walawender et al., 1989),however, do not support such an
interpretation. The contact with the older tonalitic pluton
is clearly intrusive, and evidence to support a major
structural feature in either the metasedimentaryscreenor
the interior ofthe pluton is lacking.
Temperature estimates of the La Posta pluton were
basedon the ratAlcontent of the amphibole in the hornblende-biotite and large-biotite facies.This geothermometer(Nabelekand Lindsley, 1985)is designedfor usewith
amphiboles in equilibrium with intermediate composition plagioclase(Anoo-Anur).Amphiboles in this study
coexist with weakly zoned plagioclasewhose compositions fall just below this range (Table 5). The results are
given in Table 3 and indicate that the plagioclase-amphibole equilibration occuffed at lower averagetemperature (740 "C) on the western side ofthe pluton than on
the easternside (763 'C). The lower temperature can be
correlated to the coarsely bracketed lower limit of amphibole stability in a granodiorite system at 2 kbar (Naney, 1983)and appearsto reflectnear-solidusequilibration. Data for the same system at 8 kbar indicate that
Unit
Average
& (kbar) & (kbar)
22
I
4
WH
W.H
W. LB
2.3
1.9
1.6
12
19
20
35
E.H
E. H
E. H
E. LB
5.5
5.4
5.8
4.0
1T4.3
2r5.4
T
23
r
39
11
18
1.9
Average
rfC)
742
738
739
777
757
761
rcc)
763
/JO
789
805
783
787
480
b/u
4.7
3.7
MB
MB
791
Nofe; Unit abbreviationsas in Table 1. Pr was calculatedfrom the algorithmof Hollisteret al. (1987);averagevalue for Alr for each sample
was used in the calculation.Error range based on multipleanalysesin
each sample is 0.7 kbar. Temperaturesfor the hornblende-bearingrocks
were calculated using the algorithm of Nabelek and Lindsley (1985) and
utilizedthe calculatedPr for each sample.Temperaturesfor the muscovitegeothermometerof Green
bearingrocks are from the plagioclase-mus@vite
and Usdansky(1986)and are basedon the rim compositionsot plagioclase.
this limit would migrate to lower temperatures,i.e., show
a negative correlation betweenequilibration temperature
and pressure.The higher pressureand temperature estimatesfor the easternsideof the pluton (763"C,5.2 kbar),
therefore,are in direct conflict with the experimentaldata.
The plagioclase-muscovitegeothermometerof Green and
Usdansky (1986) was used to calculate temperaturesfor
the muscovite-bearingrocks and gave subsolidusconditions (Table 3).
o
Alt
Fig.4. talAlversusAlr for hornblendein the La Postapluton.
The upper line representsthe limiting condition of t41Al: 0
whereasthe lower line is the lower limit of igneousamphiboles
as defined by Leake (1971). The middle line represents the
regressioncurve of Hammarstrom andZnn (1986) and closely
approximatesthe data from this study. Our data yielded a regression fit of t4rAl: 0.64 Alr + 0.23 and a correlation coefrcient
o f 0 . 9 9l .
t264
CLINKENBEARDAND WALAWENDER:THE ZONEDLA POSTAPLUTON
Treu 4, Averagecompositons
of biotiteandmuscovite
Biotite
Unit:
Sample:
ni
sio,
Tio,
W.H
822
45
35.78
3.59
15.52
0.02
n.d.
21.84
0.33
9.50
0.01
0.10
9.07
0.01
0.01
95.78
W.H
W. LB W. SB
MB
4511141832
553454
34.85 35.61
3.64
3.70
14.88 15.23
0.02
0.04
0.46
2.56
21.32 20.28
0.35
0.31
8.97
9.20
0.28
0.02
0.09
0.09
9.04
9.55
0.03
0.02
0.09
93.93 96.70
0.70
0.71
0.72
5.50
5.49
5.48
2.50
2.51
2.52
8.00
8.00
8.00
0.32
0.26
0.24
0.00
0.05
0.29
0.01
0.01
0.01
0.00
0.00
0.00
o.42
0.43
0.43
2 . 1 8 2 . 1 1 2.11
2.81
2.81
2.59
0.04
0.05
0.04
5.78
5.72
5.71
0.00
0.05
0.00
0.03
0.03
0.03
1.78
1.82
1.87
1.81
1.90
1.90
35.80
4.18
14.74
0.02
n.d.
22.63
0.40
8.88
0.03
0.14
9.36
0.01
0.10
96.29
0.72
5.51
2.49
8.00
0.19
0.00
0.01
0.00
0.48
2.04
2.92
0.05
5.69
0.01
0.04
1.84
1.89
MB
34.28
3.15
18.66
0.02
10.19
15.91
0.35
5.72
0.01
0.07
9.69
0.01
0.11
98.17
0.83
5.29
2.71
8.00
0.68
1.15
0.01
0.00
0.37
1.32
2.00
0.05
5.57
0.00
0.02
1.91
1.93
MB
MB
E. LB
13
6
33.5s 33.81 34.32 35.06
3.50
3.33
3.00
3.39
Alr03
17.40 18.72 18.19 16.56
0.01
0.02
0.01
0.04
FerO,
2.64
n.d.
n.d.
6.57
FeO
18.85 21.72 24.50 21.53
MnO
0.33
0.34
0.41
0.27
Mgo
5.98
5.65
5.94
8.77
CaO
0.01
0.01
0.04
Naro
0.08
0.07
0.08
0.09
KrO
9.04
9.04
8.91
8.93
Nio
0.03
0.02
0.02
0.03
Prou
0.01
0.01
0.01
0.02
Total
94.44
96.12 95.s6 95.il
Fe/(Fe + Mg)
0.81
0.80
0.71
0.81
Si
5.31
5.29
5.33
5.46
AI
2.54
2.69
2.71
2.67
Sum
8.00
8.00
8.00
8.00
AI
0.68
0.50
0.56
0.73
Fes
0.00
0.00
0.77
0.31
0.01
0.00
0.01
0.00
Ni
0.00
0.00
0.00
0.00
Ti
0.40
0.41
0.39
0.35
Mg
1.38
2.04
1.41
1.32
Fe2+
2.46
2.82
3.19
2.81
Mn
0.05
0.05
0.04
0.04
Sum
5.68
5.75
5.65
5.6s
0.00
0.00
0.00
0.00
Na
0.03
0.03
0.02
0.03
K
1.80
1.78
1.80
1.80
Sum
1.81
1.82
1.83
1.83
:
Note.'Unitabbreviations
as in Table1. Formulas
arebasedon 22(O,OH);
n numberof analyses.
Micas
Biotite is the most common mafic mineral in all facies
ofthe La Postapluton. It nucleatedafter hornblende and
typically formed discrete subhedral to euhedral crystals.
Lack of fine-grainedbiotite along rims of the hornblende
grains indicates that reaction of hornblende with melt to
produce biotite did not play a role in the evolution of the
melt system.
Smith and Walawender (1987) reported the presence
of significantquantities (up to 670lo)of vermiculite within
biotite separatesfrom the La Posta pluton. However, the
data from that report cannot be either reconciledwith the
analysesreported in this study or replicated in our laboratory. The sample-preparationtechniques employed in
that earlier study appear to favor concentration of vermiculite relative to biotite in the - lO-pm fraction and
led to strongly biased results.
Biotite compositions from l3 samplesof the La Posta
pluton and from 4 samplesof the adjacent tonalitic unit
(Fig. l) are listed in Table 4. Biotites coexistingwith amphibole, including those from the older tonalite to the
west, form a tight cluster near the center of the phlogopite-annite-eastonite-siderophyllitequadrilateral (Fig. 5).
Biotites coexistingwith muscovite, however, form a dis-
E. LB
35
E.H
12
4
35.33 35.86
2.82
3.26
16.49 1 6 . 3 0
0.02
0.04
2.23
n.d.
19.06 20.96
0.32
0.28
9.72
9.54
0.01
0.09
0.28
8.94
9.06
0.01
0.03
0.01
0.01
94.87 95.80
0.68
0.69
5.48
5.47
2.53
2.52
8.00
8.00
0.48
0.42
0.26
0.00
0.01
0.01
0.00
0.00
0.38
0.33
2.20
2.22
2.46
2.68
0.04
0.04
5.78
5.75
0.00
0.00
0.03
0.08
1.77
1.77
1.85
1.80
E.H
19
74
E.H
20
34.74
3.09
16.10
0.03
0.70
19.83
0.26
9.60
0.15
0.07
8.93
0.02
0.01
93.53
0.68
5.45
2.55
8.00
0.43
0.08
0.01
0.00
0.37
2.25
2.60
0.04
5.78
0.02
0.02
1.79
1.83
35.66
3.31
16.59
0.02
4.81
15.95
0.27
9.67
0.01
0.09
9.11
0.01
0.02
95.51
0.69
5.48
2.s2
8.00
0.48
0.55
0.01
0.00
0.38
2.21
2.02
0.04
5.69
0.00
0.03
1.79
1.82
tinct group with higher t4rAland Fel(Fe + Mg). The higher
Al content ofbiotite from the easternversuswesternside
of the pluton is apparent in Figure 6, which depicts a
threefold grouping that is both geographic(west versus
east) and lithologic (muscovite-bearing versus hornblende-bearing).Similar differences have also been reported by de Albuquerque(1973) for a seriesofdiscrete
biotite and biotite-muscovite granodiorite plutons in the
AregosregionofPortugal and by Speer(1981)for biotite
and biotite-cordierite assemblagesin the Cloud Creek
pluton, South Carolina. The latter, however, showed no
significant changein Fe/(Fe + Mg) and has considerable
t+lAl overlap between the two assemblageswhereas the
Portugal 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 increaseduring differentiation.This
ratio increases slightly (0.68-0.72, Table 4) from the
hornblende-bearingrocks on each maryin of the pluton
inward but jumps to 0.81 in the muscovite-bearingcore.
This abrupt changeis not consistentwith in situ fractional crystallization and implies that a compositional discontinuity exists betweenthe core zone ofthe pluton and
its marginal facies.
1265
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
lrcte 4-Continued
s
E
Muscovite
T
1
4
35.06
3.00
16.56
0.02
3.08
16.97
0.33
8.77
0.27
0.13
9.13
0.03
0.01
93.06
0.70
5.46
2.54
8.00
0.50
0.36
0.01
0.00
0.35
2.05
2.21
0.04
TTT
23923
445
35.33
2.82
16.49
o.02
20.84
0.32
9.54
0.01
0.09
9.10
0.01
0.01
95.78
0.69
s.48
2.52
8.00
0.49
35.86
3.26
16.30
0.04
2.73
16 . 1 3
0.28
9.72
0.02
0.19
8.82
0.03
0.01
93.39
0.66
c.5rt
0.01
0.00
0.33
220
2.70
0.04
2.47
8.00
0.49
0.32
0.01
0.00
0.38
2.24
2.08
0.04
J.5Z
c,t I
c.co
0.05
0.04
1.82
1.91
0.00
0.03
1.80
1.83
0.00
0.06
1.74
1.80
34.74
tno
16.10
0.03
19.33
0.26
9.60
0.01
0.15
9.12
0.02
0.01
92.46
u.o/
5.49
2.51
8.00
0.48
0.01
0.00
0.37
2.26
2.55
0.03
-tn
a
0.00
0.0s
MB
11
98
MB
18
45.40
0.79
34.87
45.11
0.89
34.22
2.5
t
a
2.4
(4)Al
2.23
0.02
0.81
0.01
0.39
11.08
2.27
0.01
0.87
0.01
0.37
10.86
95.60
94.60
6.08
1.92
8.00
6.11
1.89
8.00
3.56
?40
0.08
0.16
0.25
0.00
4.08
0.00
0.10
1.89
1.99
0.09
0.18
0.26
0.00
4.09
0.00
0.10
1.88
1.98
"q+
24
'2
Po
.4
.6
"
t.h
FelFe +Mg
planemodifiedfrom Deer
Fig.5. Idealbiotitecompositional
et al. (1963).Crosses
biotitesfrom the
andsoliddotsrepresent
La Postapluton that coexistwith hornblendeand muscovite,
respectively.
Opencirclesare analysesof biotitesthat coexist
with hornblende
from the adjacentoldertonalite.End-members
p : phlogopite,
areE: eastonite,
S : siderophyllite,
andA:
annlte,
also suggestthat the parental melt for the magnetite-free
La Posta pluton was more reduced than the fayalitequartz-magnetitebuffer. In contrast, the data for biotite
from two samplesof the older western tonalite and five
biotite analysesfrom other plutons in the western zone
of the batholith (Larsenand Draisen, 1950)clusterabove
the Ni-NiO buffer (Fig. 7b) and further emphasize the
Biotite was separatedfrom eight samplesof the La Pos- differencesbetweenthe older plutons in the westernzone
ta pluton and two samplesof the older tonalite adjacent ofthe batholith and the younger La Posta-type plutons.
to its western margin (Fig. l). From the microprobe-derived FeO (total Fe) analyses,FeO and FerO, were determined by using the techniqueof Fritz and Popp (1985).
{r)41y'le (ln bte)
The resultsare listed in Table 4. Wonesand Eugster(1965)
determined that biotites crystallizing from a common parental magma under oxygen-bufferedconditions should
plot on a trend with significant increasesin Fe/(Fe + Mg)
but only modest increasesin the ferrous/ferric ratio. Data
from the La Postapluton are plotted on the Fe2+-Fe3+Mg ternary diagram (Fig. 7a) along with the trends for
several oxygen-buffered systems (Wones and Eugster,
I 965).The data are againclearlyseparatedinto two groups
representingbiotite coexisting with hornblende and biotite coexistingwith muscovite.Both groups,however,have
subparallel trends that cut across the trends of oxygenbuffered systems;the most pronounced increasein Fe3+
occursin biotites from the muscovite-biotite faciesin the
Fer
Mg
core of the pluton. These data are not consistent with
crystallization under buffered conditions. They suggest
SymFig. 6. Fet-{t+tA1x l0lMgternarydiagramforbiotite.
that a source more oxidized than the parental melt con- bols as in Fig. 5. Brackets depict samplesfrom the westernand
tributed to the systemand that this contribution is greater eastern sides of the pluton and emphasizetheir differencesin
in the core ofthe pluton than in its outer facies.The data re:Al.
1.84
1.89
CLINKENBEARD AND WALAWENDER: THE ZONED LA POSTA PLUTON
t266
Fe 3*
Fe
3t
TABLE
5, Averageanalysesof feldsparsin the study area
Plagioclase
MB
W.H
W.LB
W. SB
11
224
5
63.e(7.5) 66.9(4.2) 61.8(3.1) 72.4(2.0)
1.5(0.3) 1.4(0.5) 1.0(0.2) 1.2(o.2)
34.6(7.71 31.8(4,6) 37.2(3.31 26.4(2.2)
Plagioclase
E. LB
E. LB
MB
MB
MB
Unit
13
18
32
35
Sample 14
.0) 73.4(1.1) 67.1(3.7) 65.9(2.3)
72.0(1.11 74.1(1
Ab
1.3(0.3) 1.1(0.2) 1.7(0.2) 1.5(0.2) 1.2(0.3)
Or
An
26.9(0.5) 24.7(1.4) 25.1(1.3) 31.4(3.7) 33.0(2.4)
Plagioclase
W. H
Unit
Sample I
Ab
67.0(3.7)
1.2(0.3)
or
An
322(4.0)
H-t,l
Fe
2*
H-M
__
Fe 2*
_--
Mg
Unit
Samole
Ab
or
An
Fig.7. Fe2+-Fe3+-Mg
ternarydiagramfor biotiteafterWones
andEugster(1965).Dashedlinesrepresent
compositional
trends
it
underbufferedconditions;QFM : quartz-fayalite-magnetite,
Ni- SUanm
ple
NiO : nickel-nickeloxide, and HM : hematite-magnetite.Ab
Symbolsasin Fig. 5. (a)Datafrom theLa Postapluton.(b) Data or
An
from plutonsin the westernzoneofthe batholith(seetext).
E.H
T
2
20
65.2(2.2) 59.5(1.6)
1.1(0.2)
1.4(0.2)
33.7(2.2) 39.1(1.7)
T
23
53.9(1.1)
1.0(0.1)
45.1(1.1)
Plagioclase and alkali feldsPal
MB
W. LB
W. SB
T
11
4
6
39
e.6(2.0)
7.0(2.6)
8.0(2.4)
61.1(2.5)
0.6(0.1) e3.0(2.7) 91.s(2.5) 90.3(2.0)
0.1(0'0)
0.0(0.0)
0.1(0.0)
38.3(2.5)
MB
18
7.e(0.8)
92.1(0.8)
0.0(0'0)
E.H
12
62.9(1.5)
1.1(0.2)
36.0(1.6)
E.H
19
62.4(1.7)
1.2(0.3)
36.3(1.8)
ivote: Unit abbreviationsas in Table 1. Number in parenthesesis one
standard deviation.All plagioclasevalueswere calculatedfrom three analyses each of grain center and rim. All alkali feldsparswere calculatedfrom
an averageof five analyses per sample
Miller et al. (198l) summarizedtextural and geochemical criteria for the recognition of primary versussecondary muscovite. Both types occur in the muscovite-biotite
facies of the pluton, but only those meeting the textural
criteria for primary (magmatic) origin were analyzed in
this study. The results are listed in Table 4 and are in
closeagreementwith the "typical" primary muscovite of three samples.These sampleswere from the two outerMiller et al. (1981) in terms of their higher AlrO, and most facies on the west side of the pluton and the adjacent older tonalite. The results are given in Table 6.
TiO, and lower MgO contents.
Opaque minerals that were encounteredin the course of
Feldspars
aralyzing other minerals were examined semiquantitaPlagioclasefeldspar is the most abundant mineral in tively with the energy-dispersivespectrometer (Eos) on
the La Posta pluton. It occurs as subhedral to euhedral the microprobe. All of the opaque grains analyzed in
grains with moderate normal to weak oscillatory zoning samples from the La Posta pluton, including those exand formed throughout the crystallization history of the amined by the eos, were found to be ilmenite. The sampluton. In contrast, alkali feldspar is typically found only ple from the western zone tonalite was found to contain
in the three interior facies of the pluton and occurs as mostly ilmenite, but one grain of the six analyzed was
interstitial fillings or equidimensional poikilitic patches found to be magnetite.This is consistentwith the findings
up to 4 cm in diameter. Calculatedend-member fractions of Gastil et al. (1989) that the magnetite-ilmenite line in
the Peninsular Rangesbatholith coincideswith the westare given in Table 5.
Figure 8 shows the variation in plagioclasecomposi- ern boundary ofthe La Posta pluton.
tion acrossthe pluton. Plagioclasecompositions for the
CoNcr,usroNs
hornblende-biotite, large-biotite, and small-biotite facies
Equilibration of hornblende with plagioclaseof similar
averageapproximately An' and are essentiallyidentical
on either side of the pluton. Plagioclasein the muscovite- compositions on both sides of the pluton at different but
biotite facies,however, showsa rather abrupt decreasein geologicallyreasonabletemperaturessuggestsdifferences
average composition to approximately An$. Core and in the cooling histories of opposite sides of the pluton.
rim compositions are plotted against Fe/(Fe + Mg) for Naney and Swanson(1980) pointed out that undercoolcoexisting biotite (Fig. 9) and indicate that a composi- ing of intermediate-composition melts leads to a more
tional gap exists for both minerals. Alkali feldspar com- rapid nucleation of mafic minerals such as hornblende
positions vary little betweenthe three innermost faciesof and biotite relative to feldspar and quartz. The presence
of mafic mineral-rich rocks in the banded part of the
the pluton.
border faciessuggeststhat the western margin may have
Opaque minerals
undergonemore severeundercooling relative to the eastBecauseof the scarcity of opaqueminerals in the rocks ern margin. Whether this processis responsible for the
of this study, they were quantitatively analyzed in only lower mafic mineral content in the western hornblende-
r26'1
CLINKENBEARDAND WALAWENDER:THE ZONED LA POSTAPLUTON
,rT
,T
I
lI
An
IT
I
'{,
rl
Itt
]I
,,1,,1,,,J.
vvl9V
Sarnple
I
Number
22
13
11 14 1A32
5
36
12 19 20
MB
West
FACIES
Fig. 8. Schematicvariation in plagioclasecomposition from west to east acrossthe pluton. Three rim and core analyses(Table
5) 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 speculativebut cannot be ruled
out.
The hornblende geobarometryfor the two sides of the
La Posta pluton and the older tonalitic body adjacenl to
the western contact (Table 3) overlaps with somewhat
lesspreciseestimatesof P, (2-5 kbar) for the medial zone
of the batholith (Walawenderand Smith, 1980;Berggreen
and Walawender,1977; Detterman, 1984; Eaton, 1982;
Diercks, 1981) that were basedon coexistingmineral assemblagesin both the metamorphic and plutonic rocks.
Other geobarometrictechniqueshave yielded more preciseresults.Taylor et al. (1979)calculatedthat a pressure
of2.1 kbar was required to correct fluid-inclusion closure
temperatures in pegmatites north of the study area to
temperaturesconsistentwith those calculated from their
oxygen-isotopedata. London (1986) estimated an emplacement depth between 2.4 and 2.8 kbar for the same
pegmatite system on the basis of phaseequilibria for the
pocket minerals. Clinkenbeard(1987) used the Ca-exchangegeobarometerofGhent and Stout (1981) on the
93 Ma garnet-bearingmonzogranitenear the center of the
study area (Fig. l) and calculatedPr between2.7 and,3.0
+ 0.5 kbar. K. E. Panish (unpublished data) reported P,
of 2.5 + 0.5 kbar for a similar garnet-bearingmonzogranite locatednear the northern end of the central metamorphic screen (Fig. l). Although the hornblende geobarometry reported in this study brackets these latter
figures, the difference between the western and eastern
sidesof the La Postapluton doesnot fall within the error
estimatesof Hollister et al. (1987). We also argue that
the variation in emplacementdepths (Pr) calculatedfrom
the hornblendegeobarometeris not consistentwith either
the known field and agerelationships,experimentally determined variation in solidus temperature with pressure,
TABLE
6. Averageanalysesof opaqueminerals
Unit:
Sample:
n:6
FeO
Mgo
Tio,
MnO
Total
60
70
ao
00
70
1OO (FelFe + itg)bt€
80
g0
Fe'?*
Fes
Mg
Ti
Mn
Sum
W. LB
4
43.20
0.13
51.23
2.75
97.31
1.846
0.024
0.010
1.994
0.136
4.009
W.H
83939
451
45.41
U.UC
50.29
2.04
97.79
1.857
0.101
0.004
1.949
0.089
4.000
T
51.30
0.15
42.54
1.56
95.55
1.591
0.654
0.012
1.672
0.069
3.998
T
93.30
0.02
93.32
8.004
15.990
0.005
23.999
/Vote:Unitabbreviations
as in Table1. Mineralformulasare basedon
Fig. 9. Average rim and core compositions for plagioclase
6 oxygensfor ilmeniteand 32 oxygensfor magnetite;n : numberof
versus 100 Fe/(Fe + Mg) in coexisting biotite. Symbols as in
analyses.
Fig. 5.
r268
CLINKENBEARDAND WALAWENDER:THE ZONEDLA POSTAPLUTON
or other geobarometersbasedon mineral chemistries.This
discrepancycould be due to equilibration ofhornblende
with an intermediate stage of the La Posta melt rather
than with the alkali feldspar-saturatedstagerequired by
Hollister et al. (1987).Under this condition, the Al, content of amphibole would not reflect a simple pressure
control but would depend upon other variables in the
melt systemsuch as the temperatureof hornblende equilibration. Thus, local perturbationsin cooling historiesmay
also be important in terms of controlling hornblende
compositions.
Unlike hornblende, biotite is found in all facies of the
La Posta pluton so that its compositional variations reflect the entire range of melt compositions during crystallization. Modest but identical increasesin Fe/Ge +
Mg) from the marginal hornblende-biotite facies inward
through the large-biotite and small-biotite faciesare consistent with in situ fractional crystallization. However,
the jump in that ratio (Table 4) combined with a sharp
decreasein plagioclasecomposition (Fig. 8, Table 5) across
a gradational contact into the muscovite-biotite faciesreflectsa compositional discontinuity betweenthe outer facies and the muscovite-bearingcore.
In addition, the ferrous/ferric ratios ofbiotite vary considerably, with the greater degree of variation found in
the muscovite-bearingcore of the pluton (Fig. 7a). The
data form subparallel, quasiJinear arrays that we interpret to representmixing of two componentswith ditrerent
ferrous/ferric ratios. The parental magma is considered
to be the most reduced component since no evidence of
a reducing mechanism, e.g., inclusions of C-bearing
metasedimentarymaterial, has been observed. As such,
the parental magma would have had original oxygen fugacitiesmore reducingthan the QFM buffer (Fig. 7a). The
remaining component in this mixing relationship is more
problematic.
Walawenderet al. (1989)describedanomalousisotopic
patterns (higher D'8Oin qvartz, excessradiogenic Sr, and
higher contents of inherited zircon) in the muscovitebearing cores ofseveral La Posta-type plutons in southern and Baja California and concludedthat they represent
mixing of a parental melt, which was derived primarily
from subducted oceanic crust, and a continental component with an averageage of approximately 1300 Ma.
We would add oxidizing capabilities greaterthan the NiNiO buffer and relatively high Fe/(Fe + Mg) to the aforementioned characteristicsof this continental component.
The parental La Posta melt rose between the older western zone of the batholith and the leading edge of the
North American craton (Gastil, 1975).We suggestthat
during ascent,the portion ofthe rising diapir adjacent to
the deeper (granulite facies?)segmentsof the craton reacted with crustal materials, whereas the remainder of
the diapir rose and expandedinto the overlying metasedimentary apron. Inward crystallization produced the
marginal facies,but prior to the solidification of the smallbiotite facies,the isotopically enrichedand more oxidized
tail of the diapir enteredthe magma chamber and mixed
with the derivative liquid. The resulting hybrid melt could
only undergo limited mixing and diffusion with the alreadypartly solidified outer zonesofthe pluton and cooled
in situ to form the muscovite-bearingcore with its gradational contacts.We would extend this model to include
all of the zoned and lithologically similar La Posta-type
plutons in southern and Baja California.
AcrNowr-BocMENTs
We gratefirlly acknowledgeNSF suppon through gra.ntno. EAR8306663
to Walawender.Ron La Borde and Roy Fujita lent considerableassistance
on the electron microprobe at Scripps Institute ofOceanography. Lawford
Anderson ofUCLA provided his own computer programsfor the various
thermobarometric calculations.We are also indebted to Joan Calhoun,
SDSU, for assistingin the Fe'+/Fe3+determinations ofthe biotite separates, and to Teresa Larson, SDSU, for drafting the figures.The manuscript benefitedfrom reviews by Michael T. Naney, Lincoln S. Hollister,
and David A. Dellinger.
RnrBnrNcns crrnn
Berggreen,R.G, and Walawender, M.J. (1977) Petrography and metamorphism of the Morena Reservoir roof pendant,southernCalifomia.
California Division of Mines and Geology SpecialReport 129,61-66.
Clinkenbeard,J.P. (l 987) Mineralogy, geochemistry,and geochronology
of the La Posta pluton, San Diego and Imperial Counties, California,
215 p. M.S. thesis,San Diego StateUniversity, San Diego, California.
de Albuquerque, C.A.R. (1973) Geochemistry of biotites from granitic
rocks, northem Portugal. Geochimica et Cosmochimica frcta,37, 1779l 802.
Deer, W.A., Howie, H.A., and Zussman, J. (1963) Rock-forming minerals,vol 3 (sheetsilicates),363 p. Longman, Green and Co., London.
Detterman, M E. (1984) Geology of the Metal Mountain district, In-KoPah Mountains, San Diego County, California, 216 p. M.S. thesis,San
Diego State University, San Diego, California.
Diamond, J.L., Knaack, C.M., Gastil, R G., Erskine, B.G., Walawender,
M.J., Marshall, M., and Cameron, G.J. (1985) The magnetite-ilmenite
line in the PeninsularRangesbatholith, southern and Baja California,
USA and Mexico. GeotogicalSociety of America Abstracts with Prog r a m s 1, 7 , 3 5 1 .
Diercks, C. (1981) The structureand petrographyofthe southernportion
of the Jacumba roof pendant, 37 p. UndergraduateResearchReport,
San Dego State University, San Diego, California.
Eaton, S.L. (1982) Petrology of a portion of the SacatoneSprings roof
pendant, In-Ko-Pah Mountains, Califomia, 23 p. Undergradutte ResearchReport, San Diego StateUniversity, San Diego, California.
Fritz, S.F., and Popp, R.K. (1985) A single-dissolutiontechniquefor determining FeO and FerO, in rock and mineral samples.American Mineralogist,70, 961-968.
Gastil, R.G. (1975) Plutonic zonesin the PeninsularRangesof southem
California and northem Baja Califomia. Geology, 3,361-363.
Gastil, R.G., Damond, J., and Knaack, C. (1986)The magnetite-ilmenite
line in peninsular California. GeologicalSociety ofAmerica Abstracts
with Programs,18, 109.
Gastil, G., Diamond, J., Knaack, C., Walawender, M., Marshall, M.,
Boyles-Reaber,C., Chadwick, 8., and Erskine, B. (1989) The problem
of the magnetite-ilmeniteboundary in southem and Baja Califomia.
GeologicalSocietyof America Memoir 174, in press.
Ghent, 8.D., and Stout, M.Z. (1981) Geobarometryand geothermometry
Contributions to
assemblages.
of plagioclase-biotite-garnet-muscovite
Mineralogy and Petrology,76, 92-97.
Green, N.L., and Usdansky, S.I. (1986) Toward a practical plagioclasemuscovite thermometer.American Mineralogist, 7l, ll09-lll7 .
Hammarstrom, J.M., and Z,en,E-an. (1986) Aluminum in hornblende:
An empirical igneousgeobarometer.American Mineralogist, 7 | , 12971313.
Hollister, L.S., Grissom, G.C., Peters,E.K., Stowell, H.H., and Sisson,
CLINKENBEARDAND WALAWENDER:THE ZONED LA POSTAPLUTON
V B. (1987) Confirmation of the empincal correlation of Al in hornblende with pressureof solidification of calc-alkalineplutons. American Mineralogist, 72, 231-239.
Kimzey, J A. (l 982) Petrology and geochemistryof the La Posta $anodiorite,8l p. M.S. thesis,San Diego State University, San Diego, Califomia.
Larsen,E.S.,Jr, and Draisen,W.M. (1950)Compositionof the minerals
in the rocks of the southern California batholirh. 18th Intemational
GeologicalCongress(London), Part 2, 66-79.
kake, B.E. (1971)On aluminous and edenitichornblendes.Mineralogical
Magazine,38, 389-407
-(1978)
Nomenclature of amphiboles.American Mineralogist, 63,
1023-1052
London, D.A. (1986) Formation of tourmaline-rich gem pocketsin miarolitic pegmatites.American Mineralogist, 7 l, 396405.
Miller, C F., Stoddard,8.F., Bradfish,L.J., and Dollase,W. (1981)Composition of plutonic muscovite: Genetic implications. Canadian Mineralogist, 19, 25-34.
Nabelek, C R., and Lindsley, D H. (1985) Tetrahedral aluminum in amphibole: A potential thermometer for some mafic rocks. Geological
Societyof America Abstracls with Programs, 17, 673.
Naney, M.T (1983) Phase equilibria of rock-forming ferro-magnesian
silicatesin granitic systems.American Journal of Science,283, 993I033.
Naney, M T., and Swanson,S.E (l 980) The effectof Fe and Mg on crystallization in granitic systems.American Mineralogist, 65, 639-653.
Oliver, H.W. (1980) Interpretation of the gravity map of Califomia and
its continental region. California Division of Mines and Geology Bulletin 205, 52 p.
Silver,L.T., Taylor,H P, Jr., and Chappell,B. (1979)Somepetrological,
geochemical,and geochronologicalobservationsof the Peninsular Ranges
batholith near the international border of the U.S.A. and Mexieo. In
P L Abbott and V.R. Todd, Eds., Mesozoic crystalline rocks, p 83I l0 Department of Geological Sciences,San Diego State University,
San Diego, California.
Sinfield,L. (1983)The geologyofthe eastemfaciesofthe La Postapluton,
r269
44 p UndergraduateResearchReport SanDiego StateUniversity, San
Diego, California.
Smith, B.M., and Walawender,M.J (1987)Stableisotoperelationsamong
rocks and minerals ofthe I-a Posta pluton, eastemPeninsular Ranges
batholith, southemCalifornia. GeologicalSocietyof America Abstracts
with Programs, 19, 848
Speer,J.A (l 98 l) Petrology of cordierite- and almandine-bearinggranitoid plutons of southem Appalachian Piedmont, U.S.A. Canadian
Mineralogist,19, 3546.
Taylor, B.E., Foord, E.E., and Friedrichsen,H. (1979) Stableisotope and
fluid inclusion studies of gem-bearinggranitic pegmatite-aplitedikes,
San Diego County, California. Contributions to Mineralogy and Petrology,68, 187-205.
Todd, V.R., and Shaw, S E. (1979) Structural, metamorphic, and intrusive framework of the Peninsular Rangesbatholith in southern San
Diego County, California. In P.L. Abbott and V.R. Todd, Eds., Mesozoic crystalline rocks, p 177-232 Department of Geological Sciences,San Diego State University, San Diego, California.
Todd, V R., Erskine,B.R., and Monon, D.M. (1988)Metamorphicand
tectonic evolution of the northem PeninsularRangesbatholith, southem California. In W.G. Ernst, Ed., Metamorphism and crustal evolution of the western United States,p. 894-937. Prentice-Hall, Englewood Cliffs, New Jersey.
Walawender,M J., and Smith, T.E. (1980) Geochemicaland perrological
evolution of the basic plutons of the Peninsular Ranges batholith,
southernCalifomia. Journal of Geology, 88, 233-242.
Walawender,M.J., Gastil, R.G., Clinkenbeard,J.P., McCormick, W.V.,
Eastman,B.G., Wemicke, R.S., Wardlaw, M.S., Gunn, S.H., and Smith,
B M. (1989) Origin and evolution ofthe zoned La Posta-typeplutons,
eastem Peninsular Ranges batholith, southern and Baja California.
GeologicalSocietyof America Memoir 174, in press.
Wones,D.R, and Eugster,H.P. (1965)Stability of biotite: Experiment,
theory, and application. American Mineralogist, 50, 1228-1272.
Mnruscnrsr R.EcErvED
Sesrelrsen 30, 1988
MeNuscnrpr AcCEPTED
Jurv 26, 1989