American Mineralogist, Volume 74, pages 101-1 12, 1989

Mineralogy of a layered gabbro deformed during magmatic crystallizationowestern Sierra Nevada foothills" California

Rorrnr K. SpmNcrnDepartment of Geology, Brandon University, Brandon, Manitoba R7A 6A9, Canada

ABSTRACT

In the 162 Ma Pine Hill intrusive complex of northern California, a synformal, layeredpyroxenite-gabbro-diorite body, the crystallization sequence and compositional variationof cumulus and intercumulus minerals indicate that magmatic crystallization was char-acteized by a high initial oxygen fugacity, log,ofo,in the range of -4 to -9, which allowedthe early precipitation of abundant titanomagnetite; the oxygen fugacity decreased lessthan two log units with crystallizalion. The magmatic crystallization sequence is ol + aug,ol + aug * mt, ol + aug + mt + pl, ol + aug + mt + pl + opx(invertedfrom pig) andaug + mt + pl + opx (inverted from pig) * qz. Pigeonite crystallization is evidenced by001 exsolution lamellae in orthopyroxene of Enrr. Superimposed upon this sequence areintercumulus minerals, which crystallized from magma trapped within cumulates, andbiotite and calcic amphibole, which largely formed from a subsolidus reaction of a residualhydrous fluid phase with primary minerals. Mineral compositional ranges are plagioclase,Ann, to Anrr; olivine, Fo, to Fooo; augite, CanrMgrFen to CaorMg.rFerr; Ca-poor pyroxene,Ca,Mgr"Fe' to Ca,MgorFeon; Mgl(Mg * Fe * Mn) of calcic amphibole, 0.7 4 to 0.48; andMe/(Me * Fe * Mn) of biotite, 0.78 to 0.45. Prior to complete solidification (>900/ocrystallized), deformation of the complex coincided with an abrupt decrease in Mg/(Mg* Fe) values for the mafic silicates. This compositional discontinuity is attributed to adecrease in oxygen fugacity of the crystallizing magma caused by an exchange between thefluid phase of adjacent country rock and the fluid phase of the magma during deformation.Prior to deformation of the complex, the Mg/(Mg * Fe) values of the mafic silicatesshowed the relationship bio > aug > opx > amph > ol whereas after deformation therelationship is aug > opx = bio * amph > o1. Similarities in mineralogy among maficplutonic rocks of the Pine Hill complex, ophiolites, Alaskan-type complexes, and island-arc plutonic rocks reflect similar conditions of magmatic crystallization. Generation andemplacement of an olivine tholeiite, low in alkalies and high in Ca, within an island-arcenvironment is proposed for the Pine Hill intrusive complex.

INtnonucrroN

Mid-Mesozoic ultramafic to dioritic intrusive com-plexes and their associated volcanic rocks in the KlamathMountains and the western Sierra Nevada, California, arepart of a petrotectonic association in the western NorthAmerican Cordillera (Snoke et al., 1982). These com-plexes are distinguished by (l) dunite, wehrlite, olivine-hornblende clinopyroxenite, olivine gabbro, two-pyrox-ene gabbro and diorite, and diorite to granodiorite, (2) acomplex internal structure, including brecciation and sol-id flow, (3) a systematic variation in mineral composi-tions, (4) chemical variations indicative of calc-alkalinemagmatism, (5) several episodes of magma emplacementat the same locus, and (6) an orogenic tectonic environ-ment. The structure, petrology, and major-element chem-istry of one of these complexes, the Pine Hill intrusivecomplex, has been previously described (Springer, 1980a,1980b). This paper describes the mineralogy of the PineHill intrusive complex and interprets its magmatic and

0003-o04xi 89/0 102-0 I 0 I s02.00 I 0 I

subsolidus crystallization in conjunction with the struc-tural evolution of the complex. Comparison of the PineHill complex with other complexes in the petrotectonicassociation as well as with mafic plutonic rocks from es-tablished igneous associations is discussed.

PrNn Hrr,r, INTRUSIvE coMPLEx

The Middle Jurassic (162Ma, Saleeby, 1982) Pine Hillintrusive complex is a layered igneous body composed ofgabbro with minor pyroxenite and diorite (Springer,1980a, 1980b) (Fig. l). Igneous layering forms an asym-metrical synformal structure with a small central area ofnear-horizontal layering. Primary minerals include oliv-ine, Ca-rich clinopyroxene, Ca-poor pyroxene, plagro-clase, oxide minerals, qrartz, apatite, and sulfide min-erals (Table l). Late-crystallizing minerals are calcicamphibole and biotite. Secondary minerals that are prod-ucts of low-temperature alteration of primary and late-crystallizing minerals constitute up to l00o/o of some gab-

t02 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

Fig. l. Geologic map of the Pine Hill intrusive complex,showing the distribution ofplagioclase composition in the gab-bro. Gabbro types: A: >85 wto/o An, B: 70-85 wt0/0 An, C:55-69 wto/o An, D : <55 wto/o An. The mixed zone is shown bytriangles, and pyroxenite is shown by the dot pattern. Horizon-tally ruled areas are host rocks. Arrows indicate top determina-tion from outcrop study. Wavy lines show discontinuities inmineral composition. The mineral layering orientations are shownby strike-and-dip symbols.

bros (Springer, 1980b). Gabbro is subdivided into fourtypes based on the wto/o An of the plagioclase: A, >An*(180/o); B, Anro to Anr, (400/o); C, An* to Anu, (25o/o);D,<Anr, (90/o). The number in parentheses refers to the por-tion of the complex composed of this tlrpe; pyroxenitemakes up the remaining 80/o of the complex. The mixedzone is characterized by intercalcated lenses of fine-grained, massive gabbro and coarser-grained, massive toslightly foliated gabbro or in some places, by a gabbrobreccia. Plagioclase in the mixed zone gabbro is every-where <Anuo.

A possible model for the structural evolution of thecomplex (Springer, 1980a, I 980b) involves the formationof layered cumulates in a near horizontal sill-like magmachamber. Accumulation of crystallizing minerals by grav-ity settling and magmatic currents plus in situ crystalli-zation (McBirney and Noyes, 1979; In'rne, 1980) pro-duced the "stratigraphic" sequence of p1'roxenite -

interlayered pyroxenite and type A gabbro - type A gab-bro - type B gabbro - type C gabbro - type D gabbro.Subsequent external deformation of this sequence priorto complete solidification of the magma produced large-scale brecciation of the more rigid portions and flowageofthe less rigid portions. Resultant features include iso-clinal folding with axial planes parallel to igneous layer-ing, transposed layering, deformational textures, andmineralogical discontinuities. This deformation also pro-duced the synformal structure of the complex. The inter-nal structure of the complex is not reflected in the struc-ture of the country rock (Springer, 1980b). Adjacentcounty rock has been contact metamorphosed by thecomplex (Springer, 1974; Springer and Craig, 1975).

ANA.r-vrrc.q.L TECHNIeUES

Compositional studies of plagioclase, olivine, pyroxenes, ox-ide minerals, calcic amphibole, and biotite were made utilizingARL EMx electron microprobes at the University of California,Davis, and the Virginia Polytechnic Institute and State Univer-sity. All microprobe data were refined using the correction pro-cedures of Rucklidge and Gasparrini (1969) and/or Bence andAlbee (1968).

TABLE 1. Average modes of pyroxenite and gabbro

OlivineCa-rich clinopyroxeneCa-poor pyroxenePlagioclaseOxide mineralsOuartzApatiteCalcic amphiboleBiotiteSecondary minerals.

1 0 + 1 36 9 + 2 2

< 1< 1

z i J

00

4 + 5< 1

1 5 + 1 7

2 + 21 3 + 5

b 1 5

6 6 + 44 + 1< 1< 11 + 21 + 11 + 2

4 + 32 2 + 5

1 - Z

o J l /

4 + 10

< 13 + 61 + 1< 1

6 + 42 8 + 1 22 + 2

5 5 + 1 34 + 2

0< 1

5 + 71 + 1< 1

1 3 + 81 5 + 1 51 + 1

6 4 + 1 4a + e

00

3 + 4< 11 + 4

Note: Columns are averages and standard deviations for (1) 43 pyroxenites, (2)27 type A gabbro, (3) 33 type B gabbro, (4) 28 type C gabbro, (5)19 type D gabbro.

. Secondary minerals include tremolite-actinolite, clinozoisite-epidote, chlorite, serpentine minerals, talc, iron oxide minerals, sericite, carbonateminerals.

SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

A n o100 90 80 70 60 so 4u ru

Ftg. 2. An-Ab-Or plot of plagioclase compositions from the Pine Hill intrusive complex.

103

To determine the variation of plagioclase composition withina single hand specimen, the chemical composition of plagioclaseseparates was determined by three methods, (l) electron micro-probe, (2) refractive index ofthe plagioclase glass, and (3) X-rayfluorescence analysis. Plagioclase compositions determined bythe three methods are statistically identical. This feature suggeststhat the smaller sample size, i.e., 6-10 spots on 34 grains usedin electron-microprobe analysis versus hundreds ofgrains usedin X-ray fluorescence analysis, adequately reflects the averagecomposition of plagioclase in a single hand specimen. Rapiddetermination of plagioclase composition for this study was madeusing wto/o An : [(f + 0.798)/0.253], where I : 2d(131 - l3l)- 20(l3l - 220). Increasing substitution ofK resulted in lowervalues for l.

To determine outcrop variation in the composition and I ofplagioclase, four hand specimens from an outcrop in each ofthefour gabbro types were selected perpendicular to igneous layer-ing. There is a highly significant difference in I among outcropsfrom the four types but more important, a significant differencewithin a single outcrop. To determine whether the variationwithin a single outcrop could be related to modal layering, sam-pling perpendicular to layering over a 70-m interval with addi-tional sampling with the 70-m interval over 10 m, and over Im, was carried out. Variation in f and plagioclase compositionindicates a top direction consistent with the regional top direc-tion suggested by plagioclase compositions from nearby outcrops(Fie 1).

Mlnrrnar,ocv

Using the compositional range of plagioclase (Fig. 2) asa measure of the degee of magmatic crystallization inthe Pine Hill complex, the compositions of the mafic sil-icates and oxide minerals are discussed relative to thecomposition of their coexisting plagioclase. The relativeproportions of the five mappable units (pyroxenite; gab-bro types A, B, C, and D) suggest that all rock typesresulting from magmatic crystallization are representedwithin the outcrop of the complex.

Plagioclase

Plagioclase occurs as an intercumulus mineral in py-roxenite and as a cumulus mineral in gabbro. Plagioclasegrains in most gabbro have undergone recrystallizationand/or granulation. Plagioclase morphology ranges fromsubhedral elongate grains unaffected by fracturing or re-crystallization through anhedral, but elongate grains tofine-grained equant anhedral grains. Plagioclase defor-mation is generally more common with decreasing an-orthite content of the plagioclase. Mechanical or defor-mational twinning of plagioclase (Smith, 7974, v. 2, p.

345) is common. Similar deformational textures in plu-

tonic igneous rocks have been reported, e.g., Kehlenbeck(1972), Moore (19'73), and Jorgenson (1979).

No compositional differences between deformed andundeformed plagioclase were detected. Compositionalzoning is optically observed only in plagioclases of <Anuo.Exsolution of K-feldspar spindle-shaped blebs, up to 50prm in length, occurs near grain boundaries, near frac-tures, or within twin lamellae in plagioclase of <Anrr.The Fe content of plagioclase,0.04-0.13 wt0/0, increasesslightly with increasing An content.

Olivine

Olivine exhibits a modal decrease from near 20o/o inpyroxenite and type A gabbro to zero percent in gabbrocontaining plagioclase near An3r Gable l). Olivine ex-hibits deformational features such as undulose extinction,deformation lamellae, and mortar texture. Inclusions inolivine are rate, but oriented inclusions of symplectic ironoxide minerals, as described by Moseley (1984), are ob-served in olivine from certain pyroxenites and type Agabbro. Ca-poor pyroxene and vermicular iron oxideminerals are commonly associated with olivine in gabbrowith plagioclase of <Ane.. Olivine with partial rims ofCa-poor pyroxene without vermicular iron oxide min-erals is present in most gabbro, but rarely in pyroxenite.

Olivine shows an apparently continuous Fe-enrich-ment trend from Fo' to Fooo Gable 2) with a sympatheticincrease in MnO. Chemical zoning within oliwine gainsis minor, <2o/oFo variation.

Ca-rich clinopyroxene

Ca-rich clinopyroxene, mainly augite, generally occursas a cumulus mineral. Composite grains in which irreg-ular blebs of Ca-poor pyroxene protrude into or partiallyrim augite grains, are common in gabbro of the Pine Hillcomplex. These composite grains are attributed to gran-ule exsolution ofCa-poor pyroxene from augite (Best andMercy, 1967; Lindsley and Andersen, 1983) rather thanan epitaxic relationship between the two pyroxenes (Tar-ney, 1969). Exsolution lamellae of orthopyroxene, up to40 pm thick and parallel to the 100 plane of the hostaugite, are present in augite ofgabbro with plagioclase ofAnro to Anoo. Irregular blebs of orthopyroxene in opticalcontinuity with associated 100 lamellae are common inaugite. Only in gabbro with plagioclase of An., to An,were two sets of pigeonite exsolution lamellae in augiteobserved, ( l) "00 1" lamellae at an angle of near 106" with

IO4 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

Trele 2. Representative electron-microprobe analyses of olivines

SS147 RKS4O PH58 PH1 20 PH168

sio,Tio,Alro3FeO'MnOMgo

SiTiAIF E

MnNlgCa

38.650.010 0 6

zz Jo0 3 8

39.420.06

1 0 1 . 1 4

0.993

0.0020 4850.0081.5120.002

0.754

37 950.010.04

24 930.44

36.1 70.0s

oo (o

38.370.060.00

26.060.35

37.120 0 3

101 .99

37.360.020.00

29.320.42

33.450.05

100.62

Jb.zc0.020.08

33.960.72

29.050.07

100.15

0 999

0.0020.7830.0171 .1930.002

0.599

34.490.020.06

43.520.83

21.480.08

100.48

0.997

0.0021.0510.0200.9250.002

0.463

33.450.030 . 1 1

46.441.00

18.120 . 1 1

99.26

0 9980 0010.0041 . 1 5 80.0250.8060.004

0.405

Number of ions on the basis of 4 oxygens

l \ 4 9 / ( M g + F e + M n )

1 004

0.0010.5510.0101.4270.002

0.718

0.9950.001

u.cbc0.0081 4340.001

0.714

1.000

0.6560.0101.3340.001

0 667

" Total Fe determined as FeO

the c axis ofaugite and up to 5 pm thick and (2) "001"lamellae at an angle of near I12" with the c axis and upto I prm thick. Exsolution lamellae optically identified infour augites correlated with lamellae in X-ray precessionphotographs of these augites. Exsolution lamellae of pi-geonite in Pine Hill augites correlate with Pig II and PigIII lamellae of Robinson et al. (1977), which nucleate atapproximately 800 and 600 "C, respectively. In Pine Hillaugites, the absence of Pig I and Pig "l00" lamellae, whichnucleate at approximately 1000 and 850 oC, respectively,are attributed to down-temperature migration of pigeon-ite from these lamellae to form irregular grains withinaugite or at its boundaries. Plates and needles of ironoxide minerals in augite are oriented approximately par-allel to the (100) and (001) planes ofthe host augite. Theiron oxide inclusions decrease in abundance with de-creasing anorthite content of the coexisting plagioclase.

Although there has been subsolidus equilibration ofthePine Hill pyroxenes, compositional trends are discernible(Table 3, Fig. 3). Trend A in which En remains relativelyconstant between 42.9 and 39.6 and Wo decreases from49.3 to 41.7 represents the compositions of augite thatdid not coprecipitate with Ca-poor pyroxene. Trend B inwhich En decreases from 42.0 to 34.9 and Wo increasesslightly from 41.7 to 44.2 represents those that did pre-cipitate the Ca-poor pyroxene. The AlrO. content of au-gite shows a decrease, 4.39 to 0.80 wto/o, with increasingFe content ofaugite. TiO, content also decreases, 0.94 to0.22 ulo/o, with increasing Fe content of augite, whereasMnO content shows an increase, 0.13 to 0.66 wt0/0. Al-though NarO and calculated FerO. contents show no trendwith the Fe content of augite, the NarO content increaseswith increasing FerO. content. Correlation of Mg/(Mg +Fe) with anorthite content for coexisting augite and pla-gioclase shows minor Fe enrichment in augite (Fig. a).Plots of Mg/(Mg * Fe) vs. anorthite content for olivineand Ca-poor pyroxene show a similar relationship.

Ca-poor pyroxene

Ca-poor pyroxene occurs as (l) partial rims around ol-ivine, (2) symplectites with vermicular iron oxide min-erals, (3) composite grains with augite, and (4) separatecumulus or intercumulus grains. The first occurrence isobserved in some pyroxenite and most olivine-bearinggabbro; the second, in gabbro containing plagioclase ofAnn, to Anoo; the third, in gabbro containing plagioclaseof Ann, to An.r; the fourth, in gabbro containing plagio-clase of (Anro. Symplectites of Ca-poor pyroxene withvermicular iron oxide minerals occur as single grains ofCa-poor pyroxene enclosing vermicular iron oxide min-erals or as an aggregate ofequant anhedral grains ofCa-poor pyroxene with vermicular iron oxide minerals cross-cutting grain boundaries but not extending beyond theaggregate boundary. These symplectites have formed af-ter the partial rims of Ca-poor pyroxene as these rims areobserved to border aggregates ofCa-poor pyroxene grainswith vermicular iron oxide minerals. Their formation oc-curred prior to the crystallization of calcic amphibole andbiotite as these minerals show a replacement relationshipwith these symplectites.

Ca-poor pyroxenes commonly exhibit exsolution la-mellae of augite up to 0.02 mm thick and parallel to 100.Exsolution lamellae up to 0.02 mm thick and parallel tothe "001" plane ofthe host Ca-poor pyroxene were ob-served in gabbro with plagioclase of Anrr. As texturalfeatures show a gradation from continuous 1 00 or "00 l "lamellae to discontinuous 100 or "00l" lamellae to ran-dom irregular blebs, down-temperature migration ofexsolved augite has largely obliterated most higher-tem-perature exsolution textures (Lindsley and Andersen,1983). Compositional, textural, and single-crystal X-raystudies show that the Ca-poor pyroxene in all four oc-currences is orthopyroxene and that the Ca-poor pyrox-ene coexisting with plagioclase of An' was originally pi-geonite that later inverted to orthopyroxene.

SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

Hd

En FsFig. 3. Compositions of pyroxenes from the Pine Hill intrusive complex plotted on the pyroxene quadrilateral. Crystallization

trends: A, cumulus augite; B, cumulus augite with cumulus Ca-poor pyroxene; C, same as B but augite corrected for exsolution.Solid lines represent the trend for pyroxenes from the Skaergaard intrusion (Wager and Brown, 1967).

105

Although subsolidus exsolution of augite has alteredthe composition of the Ca-poor pyroxenes (Table 3), theircompositions correlate with those of augite (Fig. 3). Al,O.content decreases from l.7l to 0.39 wto/o with decreasingM/@g * Fe), similar to augite. MnO content increaseswith decreasing Mg/(Mg + Fe).

Equilibrium temperatures for ten pairs of cumulus au-gite and Ca-poor pyroxene coexisting with plagioclaseranging from Anro to An., in the complex were calculatedusing various pyroxene geothermometers: 888 + 47 'C

(Wood and Banno, 1913), 919 -r 49 'C (Wells, 1977),912 + 6l oC and 996 + 28'C (low Z and high ?" of thetransfer equation, Kretz, 1982), and a range of 1484 to720'C (exchange equation, Kretz, 1982). Three pairsyielded a range of 950 to 750 "C using the geothermom-eter of Lindsley (1983). These calculated temperaturesare largely subsolidus equilibrium temperatures. Usingthe chemical composition of the coexisting pyroxenes andthe modal percent of exsolved pyroxene in the host py-roxene, a temperature of magmatic crystallization of near1100'C (Lindsley, 1983) was determined. These datasuggest that in the pyroxene quadrilateral (Fig. 3), trendB probably represents a subsolidus trend whereas trendsA and C approximate the magmatic crystallization pathof augite in the complex.

Oxide rninerals

Oxide minerals (titaniferous magnetite and ilmenite)are present in gabbro but only rarely in pyroxenite exceptfor the pyroxenite body at the southern end of the com-plex (Fig. 1). In rhythmically layered gabbro containingplagioclase of >Anrr, layers up to one-third meter thickcontain up to 300/o and, in one instance, l00o/o iron oxideminerals.

Titaniferous magrretite, which occurs as a cumulus andintercumulus mineral as well as vermicular intergrowthswith Ca-poor pyroxene, is always associated with ilmen-ite. Ilmenite occurs as coarse lamellae, fine trellis inter-

growths, and irregular grains adjacent to or within mag-netite, as described by Buddington and Lindsley (1964).All three occurrences, in various combinations, are foundin gabbro with plagioclase of all compositions. The modalratio of ilmenite to magrretite increases with decreasinganorthite content of the coexisting plagioclase. Granulesof ilmenite increase from <10/o of total iron oxide min-

. BUSHVELD

O EMIGRANT GAP

^ GUADALUPE

O sl vlrucerur

@ raoooosX PINE HILL

. BEAR MOUNTAIN

100 80 60 40 20MOL Y" ANORTHITE

Fig.4. Compositions of coexisting Ca-rich clinopyroxene andplagioclase from the Pine Hill intrusive complex compared tothose from other plutonic rocks. Pyroxenes from pyroxenites areassigned an anorthite content of 100. Sources ofdata: Bushveld(Wager and Brown, 1967), Emigrant Gap (James, l97l), Gua-dalupe (Best and Mercy, 1967), St. Vincent (Lewis, 1973b),Troodos (Allen, 1975), Bear Mountain (Snoke et a1., 1981).

uJzi.Il

o

oz)

I

I6O

oL

f

o

106 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

Trele 3. Representative electron-microprobe analyses of pyroxenes

SS147 RSK4O PH58 PH12O JB7 PH1 68 PH115 SS135 PH58

sio,Tio,Al,o3FeO-MnOMgoCaONaro

V]AITiFCMnMgCaNa

tX,Y]

M g / ( M g + F e + M n )

52.33 50.690 4 9 0 5 12.89 3.248.71 8.460.22 0.30

14.52 14.0520.78 20.940 37 't.o7

100.31 99.26

52.55 54.290.22 0.120.80 1.63

12.04 16.560.50 0.34

12.18 27.7020.81 0 580.34 0 00

99.44 101 .23

1 .994 1.9430.006 0.0572.000 2.0000.030 0.0120.006 0.0030.382 0.4960.016 0 .0100 689 1.4770.846 0.0220.025 0.0001.994 2.020

0.634 0.745

44.1 1 135.9 74.019.9 24.9

51.800.764.225.390.14

1 5 1 823.860.30

101 .65

1.8800.1 202.0000.0610.0210.1640.0040.8210.9280 0212.019

0.830

51.390.544.396.020.18

14.8022.520.49

100.33

1.8890 . 1 t 12.0000 0790 0 1 50.1850.0060.8110.8870.0352.018

0.809

47.143.19.8

51.000.583.538.120.19

1 4 . 1 022.000.30

99.82

5 1 . 1 00.482.39

12.420.41

12.7120.330.73

100.57

1.9240.0762.0000.0300.0140.3910.0310.7130.8200.0532.036

0.638

51.060.412.19

12.740.47

12.4220.170.81

100.27

1.9320.0682.0000.0300.0120.4030.0150 7000.8180.0592.037

0.626

Number of ions on the basis ol 6 oxygensSiAI

MgFe

1 9010 0992 0000.0560 0 1 60.2530.0060.7830.8790.0222.016

o.751

1 .935 1.9050.06s 0.0952.000 2.0000 061 0.0490.014 0 .0140.269 0.2660.007 0.0100.800 0.7870.823 0.8430.027 0.0782.001 2.047

0.743 0 740

43.5 44 542.3 41 514.2 14 0

45.940.913.2

4 d . J

42.9o . o

42.6 42.637.1 36.420.3 21.0

- Total Fe determined as FeO

erals in pyroxenite and gabbro with plagioclase of >An85to near 300/o in gabbro with plagioclase near Anrr. Isolatedgrains of ilmenite were not observed. Ilmenite in all oc-currences is optically and chemically homogeneous.Pleonaste occurs as lamellae parallel to the octahedralplanes of magnetite and as irregular grains concentratedat magnetite grain boundaries and within coarse ilmeniteblades. Pleonaste becomes less abundant with decreasinganorthite content of the coexisting plagioclase.

Compositional trends in magnetite with respect to theanorthite content of the coexisting plagioclase are dis-cernible for TiOr, AlrO., FerOr, and FeO (Table 4). TiO,in magnetite increases from < I wto/o in pyroxenite to > I Iwt0/o in gabbro with plagioclase of An*. but drops to < l.5wto/o in gabbro with plagioclase of <An3j. TiO, values fortwo magnetites (FOl3, JD82; Table 4) appear anoma-lously high. McBirney and Noyes (1979) have shown thatthere is sympathetic relationship between the modalabundance and the chemical composition of iron oxideminerals in layered gabbro (i.e., high TiO, and MgO con-

RKS84 JD82 JB7

tent in magnetite from oxide-rich layers). The averagemodal amount of iron oxide minerals in the complex is< 50/o but FO I 3 and JD82 contain - 300/o and I 000/o oxideminerals, respectively. MgO content of these magnetitesand their coexisting ilmenites (Tables 4, 5) are the highestof all magnetite and ilmenite in the complex.

The FerO. content of magnetite, which decreases withdecreasing anorthite content of the coexisting plagioclase,shows an inverse relationship to that of FeO and TiOr,characteristic of the FerTiOo-FerOo series (Lindsl ey, 1 97 6).AlrO, increases with decreasing anorthite content. Be-cause the abundance of exsolution lamellae of pleonastein magnetite decreases with decreasing anorthite content,the AlrO, trend possibly reflects the degree of pleonasteexsolution rather than magmatic fractionation.

Similar to the TiO, content, the FeO and AlrO, con-tents of magnetite from gabbro with plagioclase of <An35are distinctly lower than those in gabbro from the rest ofthe complex, whereas the FerO. content is distinctlyhigher. CrrO, shows a general decrease with decreasing

TABLE 4. Representative electron-microprobe analyses of magnetites and a pleonaste

FO13 FO13.- PH168 RKS18

Tio,Al,03Cr.O.FerO""FeO.MnOMgo

7 . 1 31.940.06

s5.2034.970.512.00

101 .81

0.0558 400.046.17

22.740.24

11.8299.46

3 . 1 20.920.25

62.5833.590.390.31

1 0 1 . 1 6

4 0 90.82028

60.0733.280.610.67

99.82

6.432.340.04

s5.2937.480.41o.12

102.11

1 1 . 0 93.220.09

44.374 1 . 1 6

0.54o.29

100.76

1 1 . 3 53 1 40.04

43.9841 .38

0.65o.22't00.77

1 .340.590.03

66 0132.08

0.450.00

| 00.50. Recalculation of FerO3 and FeO after Carmichael (1967), Anderson (1968), Lindsley and Spencer (1982), and Stormer (1983)

*- Pleonaste.

SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION IO7

TABLE 3-Continued

Number of ions on the basis of 6 oxygens

Calcic amphibole

Calcic amphibole occurs as partial rims around pri-mary minerals, especially pyroxene and iron oxide min-erals, and as irregular patches within pyroxene. In gabbrowith abundant deformational textures, anhedral equantgrains of calcic amphibole are typical. Generally calcicamphibole forms < l0o/o of the mafic minerals in any py-roxenite or gabbro, but in certain gabbros, it constitutesnearly l00o/o of the mafic minerals. Calcic amphibole ex-hibits pleochroism of X : tan, Y: medium brown, andZ : dark brown to reddish brown in most gabbro, but apleochroism of X : colorless, y : pale green, and Z :

medium green in most pyroxenite. Calcic amphibole andbiotite, which are commonly associated in the same rock,appear to have crystallized at the same time. Whethercalcic amphibole and biotite occur together or separatelydepends largely on the bulk composition. For example,in gabbro with plagioclase near Anro, if K'O > 0.55 wt0/0,only biotite is present; if KrO : 0.30-0.55 wto/0, biotiteand calcic amphibole are present; if KrO < 0.30 wt0/0,only calcic amphibole is present.

Using the nomenclature of Leake (1978), the chemicalcompositions of the calcic amphiboles (Table 6, Fig. 5)show a compositional gap between those from gabbrowith plagroclase of <An* and all other calcic amphibolesin the complex. This discontinuity coincides with the firstappearance of quartz. Pargasite, which is not stable withquartz (Gilbert et al., 1982), is replaced by ferro-edenitichornblende. Na + K increases with increasing Fe contentin the calcic amphiboles prior to the crystallization ofq:uartz. The Mg/(Mg + Fe) of calcic amphibole decreaseswith decreasing anorthite content of the coexisting pla-gioclase (Fie. 6).

Biotite

The occurrence and textural features of biotite are sim-ilar to those of calcic amphibole. It exhibits a pleochro-ism of X -- pale brown and Y : Z : dark red brown.Biotite shows a general decrease in Mg/(Mg + Fe) withdecreasing anorthite content of the coexisting plagioclase(Table 7, Fig. 6). Biotite is the most magnesian mineralin gabbro with plagioclase of >Anuo but becomes one ofthe most Fe-rich minerals, second only to olivine, in gab-bro with plagioclase of <Anro. Alumina generally de-creases with decreasing Mg/(Mg * Fe), whereas TiO, in-

PH12O JB7 PH1 68 PH1 15 SS1 35

54.28 52.870 .1 5 0 .141 .58 1 .77

17.70 19.270.41 0.59

25.61 24.070 65 0.800 00 002

100.38 99.s5

' t .967 1.9530.033 0.0472 000 2 0000.034 0.0300.004 0.0040.536 0.59s0.013 0.01 81 .383 1.3250 025 0.0320.000 0.0011 995 2.006

0.716 0.684

1 3 1 671.1 67 I27.6 30 5

51 .54o.201 1 0

24.960.70

19.201.590.01

99.32

5 1 . 1 90 .191 .20

26.1 50.83

1 8 .130.970 0 7

9874

51.440.090.39

30.241 2 5

16.271 . 1 30 0 2

100.82

1.9840.0162.0000.0020.0030.9760.0410.9350.o470.0012.005

0 479

2.447.849.8

1 .9680.0322 0000 0 1 80.0060 7970 0231.0930.0650.0012.002

0.571

1.9750.0252.0000.0300.0060.8440.0271.0430.0400.0051.994

0 545

2.154.14 3 8

3.355.940.8

anorthite content. Although magnetite contains signifi-cant amounts of MgO and MnO, 0.00 to 2.00 wto/o and0.08 to 0.65 wto/0, respectively, no regular variation withanorthite content is discernible. No compositional trendsin ilmenite with respect to the anorthite content of thecoexisting plagioclase are discernible, but the MgO andMnO contents of ilmenite exhibit a direct and inverserelationship, respectively, with those of magnetite.

Using the ulv6spinel and ilmenite components (Car-michael, 1967; Lindsley and Spencer, 1982; Stormer,1983) calculated from the compositions of l2 magnetiteand ilmenite pairs from pyroxenite and all four types ofgabbro and the temperature model of Spencer and Linds-ley (1981), and temperature range of 516 to 648 oC andaIog,;fo, range of -18.4 to -25.6 were determined. Alltextural and compositional features of the oxide mineralsindicate that subsolidus oxidation-exsolution of a pri-mary titanomagnetite produced a titaniferous magnetitervith associated ilmenite and Dleonaste.

TABLE 5, Representative electron-microprobe analyses of ilmenites

FO13 RKS84 PH168 PH1 15 RKS18

Tio,Al,o3Cr.O.FerO..FeO-MnOMgo

51.950.050.005.01

32.421 0 87.41

97.92

53.260.090.031 .92

41 .86

1 .98101 . 61

54 780 0 70 0 01.87

38.002 0 05.18

101 .91

52 .190 . 1 10.001 .85

44.102.340.26

100 75

E 1 7 Q

0 1 10.00Z . J J

43.181 . 1 81 .20

99.96

51 .980 . 1 10.002.62

43.491 .58n o c

100.70

51 .670.070.004.68

40.495.850.02

102.76

- Recalculation of Fe,O3 and FeO after Carmichael (1967), Anderson (1968), Lindsley and Spencer (1982), and Stormer (1983).

108 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION

0.8 I paragas i t i c II hornbleno".unJru paragasite

edenit ichornb lende

+-------]: a 3 0

ferroan-paragasit ichornblende

o60

.56

ferroanparagasite

tr33

f erro-edenit ichornb lende

a quaiz absento quanz present

Si (ca t ions)

Fig. 5. Variation of Mg/(Mg + Fe) with Si for calcic amphiboles in the Pine Hill intrusive complex. Numbers refer to theanorthite content of the coexisting plagioclase.

o4

+ o .

o 4

6.06.26t6.8

creases. Higher values of NarO occur in biotite coexistingwith plagioclase of >Anuo.

Quartz, apatite, and sulfide minerals

Quartz was observed in only three gabbros (Table l),which occur in either the mixed zone or the zone in whichplagioclase compositions are <Ansj. It occurs as anhedralgrains interstitial to primary minerals, except in one gab-bro where it occurs in fractures crosscutting plagioclasegrains. Plagioclase coexisting with quartz is the most so-dic, <Anrr, in the complex.

Apatite first appears in gabbro containing plagioclasenear Anro flable l). It commonly occurs as inclusions ofeuhedral crystals in plagioclase, but can occur as anhedralgrains up to I mm in diameter in some gabbros.

As only trace amounts of sulfide minerals are present

80 60 40MOT O/O ANORTHITE

Fig. 6. Variation of Me/(Mg + Fe) in mafic silicates withplagioclase composition in the Pine Hill intrusive complex.

in the complex, no study of these minerals was under-taken.

PrrnocnNosrs

Crystallization sequence

Using the anorthite content of cumulus plagioclase asa measure of the degree of magmatic crystallization, thestratigraphic succession of primary mineral assemblagesis aug * ol, aug + ol + mt, pl + aug * ol + mt, pl +aug + opx (inverted from pig) + ol + mt, pl t aug *opx (inverted from pig) * mt, pl + aug + opx (invertedfrom pig) + mt + qz. Pigeonite is the crystallizing Ca-poor pyroxene near Enrr, but the compositional limits ofpigeonite crystallization are not known. Superimposed onthese assemblages are Ca-poor pyroxene, iron oxide min-erals, calcic amphibole, and biotite, none of which crys-tallized directly from the main body of magma, but fromisolated intercumulus magma or by a subsolidus reaction.Experimental studies ofbasalts under variable oxygen fu-gacity (Roeder and Osborn , 1966; Presnall, 1966; Osborn,1979) best illustrate the crystallization sequence in thePine Hill complex. Magnetite crystallization, which oc-curs after that of olivine and augite, is closely followed bythat of plagioclase, and results in the simultaneous crys-tallization of olivine, augite, magnetite, and plagioclase.Olivine is totally consumed as plagioclase nears Anoo, sothe common assemblage pl + ol + aug + Ca-poor py-roxene + mt changes to aug + Ca-poor pyroxene + mt+ p l + q z .

Evidence for adcumulus growth during magmatic crys-tallization of the Pine Hill complex is the lack of chemicalzoning in the primary minerals, and the occurrence ofmonomineralic layers of titanomagnetite with <50/o in-terstitial silicate minerals and up to one-third meter thick,and layers of olivine clinoplroxenite with less than 50/ointerstitial plagioclase (Wager et al., 1960; Morse, 1980,p . 2 a $ .

60oI

To

o

o\

t 0 0 L100

OLIVINECa-rich CLINOPYROXENECa-poor PYROXENECaICic AMPHIBOLEBIOTITE

SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION 109

Trele 6. Representative electron-microprobe analyses of calcicamphiboles

Trale 7, Representative electron-microprobe analyses of bio-tites

RKS84 PH58 PH120 SS129 SS135 PH12O JB7 PH168 PH115 SS135

sio,Tio,Alro.FeO.MnOMgoCaONa"OKrO

Number of ions on the basis of 23 oxygens

37.94 37.08 37.81 36.55 36.583.11 3.36 4.50 5 84 4.79

1 6.54 1 6.63 15.17 14.64 13.379.28 11 .67 15.54 18.21 21 .320 02 0.06 0.07 0.11 0j2

18.18 19.01 14.74 12.76 10.390.08 012 0.07 0.05 0.06o.32 0.31 0 06 0 00 0.099.27 8.67 9.41 9.71 9.12

94.74 96.91 97.37 97 .87 95.84

Number of ions on the basis of 22 oxygens5 528 5 342 5.51 0 5.420 5.6122 472 2 658 2.490 2.559 2.3888.000 8.000 8.000 7.979 8.0000.370 0.166 0.174 0.0310.341 0.365 0.493 0.651 0.5521.131 1.406 1.894 2.258 2.7350.003 0.007 0.009 0.014 0.01 63.948 4.081 3.201 2.820 2.3765.793 6.025 5.771 5.743 5.7100.012 0.01 I 0.01 1 0.008 0.0100.091 0.087 0.018 0.0281.723 1 .593 1 .750 1.827 1 .7851.826 1.698 1 .789 1 .835 1.823

43.15 42.942 .15 1 .83

12.63 13.089 .19 9 .410.20 0.08

1 4.99 14.9312.05 12.20257 2 .230 16 1 .08

97.09 97 78

4215 42.05 43.712 67 2.63 1 .90

12.97 12.61 8.8711.44 14.21 20.190.10 0.20 0.33

13 .20 11 .96 94911 .92 11 .40 11 .032.48 2.73 1.671 .38 1 .06 0 .55

98.31 98.85 97.74

sio,Tio,Alro3FeO.MnOMgoCaONa"OKrO

SiAI

tzlAITiFeMnMg

tYlCaNAK

txl

SiAI

tzlAITiFeMnMg

tX, Y]

NAKul

6.299 6.256 6.189 6.209 6.6551.70 f 1 .744 1 .811 1 .791 1 .3458 000 8 000 8.000 8.000 8.000o 472 0 502 0.434 0.404 0.2470.236 0.201 0.295 0.292 0.2181.122 1147 1 .405 1755 2 .5710.025 0.010 0.012 0 025 0.0433.261 3.242 2 889 2.632 2.153s.116 5.102 s.035 5.108 5.2321.885 1.905 1.876 1.804 1.8000.727 0.630 0.706 0.782 0.4930.030 0.200 0 259 0.200 0 1072.642 2.735 2.841 2.786 2.400

0.740 0.737 0.671 0.597 0.452 Mg/(Mg + Fe + Mn) 0]77 0743 0.628 0.s55 0.465M g / ( M g + F e + M n )

. Total Fe determined as FeO . Total Fe determined as FeO.

Crystal sedimentation eventually prohibits effective dif-fusion between the magma body and intercumulus mag-ma, and a certain amount of intercumulus magma istrapped within the cumulates. Partial rims of Ca-poorpyroxene on olivine are attributed to a reaction betweenolivine and trapped intercumulus magma, because therims of Ca-poor pyroxene have formed in pyroxenite andType A gabbro prior to the direct precipitation of Ca-poor pyroxene from the main body of magma. Crystal-lization ofphases directly from the trapped magma, alongwith crystallization of phases resulting from the reactionof this magma with cumulus minerals, would cause de-pletion and concentration of elements within the trappedmagma similar to fractional crystallization of the mainbody of magma. On the basis of textural evidence-suchas irregular patches of hydrous minerals within primaryminerals, lack of primary accessory minerals with hy-drous minerals, and Ca-poor pyroxene with vermiculartitanomagnetite rimmed by Ca-poor pyroxene-the al-teration of olivine to Ca-poor pyroxene and vermiculartitanomagnetite and the formation of calcic amphiboleand biotite probably occurred in equilibrium with an in-tercumulus hydrous fluid phase rather than an intercu-mulus silicate melt. Using calculated temperatures ofcrystallization and similar textures, intercumulus horn-blende in similar gabbros is attributed to a high-temper-ature subsolidus reaction between a hydrous fluid phaseand primary minerals (Otten, 1984; Morrison et al., 1986).

Conditions of crystallization

Magmatic crystallization of the Pine Hill complex oc-curred at a total pressure ofless than 3 kbar on the basis

of contact-metamorphic mineral assemblages (Springer,1974, 1980b). The total pressure can also be estimatedfrom the chronologic sequence of cumulus phases. As thestability of plagioclase is suppressed 250 "C with increas-ing water pressure between 2 and 8 kbar (Yoder and Til-ley, 1962; Holloway and Burnham,1972), a total pres-sure near 2.5 kbar would be indicated for the Pine Hillcomplex.

Under the most oxidizing conditions, i.e., oxygen fu-gacity controlled by the HM (hematite-magnetite) buffer,melting relations of basalts show that titanomagnetite isone of the highest-temperature liquidus phases (Hamil-ton et al., 1964; Holloway and Burnham, 1972; Helz,1973), whereas under a lower oxygen fugacity as con-trolled by the NNO (Ni-NiO) buffer, titanomagnetitecrystallizes after olivine and Ca-rich clinopyroxene. Un-der still lower oxidizing conditions, i.e., the FMQ (fa-yalite-magnetite-quartz) bufer, ilmenite is a high-tem-perature liquidus phase. The Pine Hill complex largelycrystallized under oxidizing conditions between the HMand NNO buffers, i.e., a log'o/o, between -4 and -9

assuming magmatic temperatures near I100 "C. The earlyand continuous crystallization of titanomagnetite and itsabundance, the presence ofsilica- rather than Fe-enrich-ment trend (Springer, 1980b), and the limited Fe enrich-ment in olivine and pyroxene correlate with the resultsof experimental crystallization studies of basalts undersimilar oxidizing conditions (Roeder and Osborn, 1966;Presnall, 1966; Osborn, 1979). The limited Fe enrich-ment in olivine and pyroxene from pyroxenite to gabbrowith plagioclase of >An6o indicates a decrease in oxygenfugacity of one to two log fugacity units (Speidel and Os-

lIO SPRINGER:LAYEREDGABBRODEFORMEDDURINGMAGMATICCRYSTALLIZATION

born, 1967) during crystallization of over 900/o of thecomplex. The high values and range in Mg/(Mg + Fe) forcalcic amphibole and biotite are also consistent with crys-tallization under similar oxidizing conditions (Wones andEugster, 1965:, Helz, 1973). Initially high water acriviryas well as high f, characterize the magmatic crystalliza-tion of ultramafic-mafic rocks in the petrotectonic asso-ciation of the Klamath Mountains and western Sierra Ne-vada (Snoke et al., 1982).

Compositional discontinuities in Mg/(Mg + Fe) for o1-ivine and pyroxene as well as for calcic amphibole andbiotite-all occurring at a plagioclase composition ofnearAnuo-coincide with deformation of the complex. Lackof a similar discontinuity in plagioclase composition sug-gests that a significant temperature decrease did not ac-company this deformation. It is proposed that the initialdeformational event involved fracturing of the Pine Hillcomplex and adjacent country rock such that interactionof the fluid phase in the country rock and crystallizingmagma occurred. This interaction involved a loss of fluidfrom the residual magma and resulted in a lower oxygenfugacity in the magma, reflected by an abrupt decrease inMg/(Mg + Fe) of the mafic silicates. The fine-grainedgabbro of the mixed zone possibly reflects rapid crystal-lization due to the loss of fluid phase from the magma.

The presence of calcic amphibole and biotite in thePine Hill complex indicates that water was present in theparent magma. Melting relations of basalts indicate thatat a total pressure of 3 kbar and under water-saturated(Yoder and Tilley, 1962) as well as water-undersaturated(Prro and 0.6P,*r, Holloway and Burnham, 1972) con-ditions, calcic amphibole would be a liquidus phase; how-ever, at a total pressure of <1.5 kbar, it would be a sol-idus phase. Regardless of the total pressure, if thecrystallizing liquid does not at some time contain theminimum water content (=3.0o/o, Burnham, 1979) re-quired for the coexistence of calcic amphibole and silicatemelt, calcic amphibole would not crystallize from themagma. The absence of primary calcic amphibole as wellas primary biotite in the Pine Hill complex is attributedto a low initial water content in the magma. As the watercontent of trapped intercumulus magma would increasewith crystallizalion, the minimum water content requiredfor the crystallization of calcic amphibole and biotitewould be reached. Whether this intercumulus phase waslargely a silicate melt or hydrous fluid phase during thecrystallization of these hydrous minerals is not known.Prior to deformation of the complex, the Mg/(Mg + Fe)values of the mafic silicates exhibit the relationship bio> aug > opx > amph > ol; after deformation, aug >opx = bio = amph > ol. The relationship aug > amph> bio is common in plutonic igneous rocks in whichcalcic amphibole and biotite crystallized directly from themagma (Gilbert et al., 19821- Speer, 1984) even underoxidizing conditions between the NNO and HM buffercurves (Czamanske and Wones, 1973). A silicate melt-i.e., trapped intercumulus magma-may have been in

equilibrium with crystallizing hydrous minerals after de-formation but not before.

Magma composition

Most ultramafic to dioritic intrusive complexes in theKlamath Mountains and western Sierra Nevada, Califor-nia, have been chbracterized by a multistage differentia-tion-emplacement model involving up to three parentmagmas: (1) a primitive mafic basalt diferentiating todunite, wehrlite, gabbro, diorite, (2) a wet high-Al basaltdifferentiating to hornblende gabbro and diorite and thento tonalite, and (3) a tonalite (Snoke et al., l98l). In con-trast to most complexes in this petrotectonic association,there are neither younger tonalitic rocks nor large amountsofhornblende gabbro and diorite associated with the PineHill intrusive complex. The rocks of Pine Hill complexcorrelate closely with the early ultramafic-gabbroic-dio-ritic rocks of these complexes. This feature suggests thatthe magmas from which the tonalitic rocks and horn-blende gabbro and diorite were derived were commonlyemplaced at the same locus along with the earlier prim-itive mafic basalt, but were generated separately from theprimitive maflc basalt. The dunite-wehrlite-gabbro-diorite association of these complexes is similar to thedunite-wehrlite-gabbro association of the Alaskan-typecomplexes (James, 1971; Snoke et al., 1981, 1982). Char-acteristics of the Pine Hill complex similar to those ofthe Alaskan-type complexes are (l) olivine clinopyrox-enite and clinopyroxenite that are gradational into gabbrowith plagioclase of >Anno; (2) layering of titanomagnetitein clinopyroxenite and gabbro with plagioclase of >Anrr,plus an overall abundance oftitanomagnetite; (3) patchesof pegmatitic clinopyroxenite and gabbro with plagioclaseof >Anr,; (4) igneous layering in gabbro with plagioclaseof >Anrr, produced by pseudosedimentary magmaticprocesses; (5) syncrystallization and subsolidus defor-mation involving flowage and brecciation of cumulates;(6) primary minerals that comprise olivine (Fo, to Foo,),Ca-rich pyroxene, Ca-poor pyroxene (absent in pyrox-enite and gabbro with plagioclase of >Anrr), titanomag-netite (subsolidus oxidation-exsolution to titaniferousmagnetite, ilmenite, and pleonaste), calcic plagioclase, andlate-crystallizing calcic amphibole with low SiO, and highAlrO.; (7) alkali feldspar-+ordierite hornfels facies con-tact metamorphism in adjacent country rock (Taylor,1967); and (8) emplacement into an orogenic tectonic en-vironment. The major difference between the Pine Hillcomplex and the Alaskan-type complexes is the former'slack of dunite, which precludes the presence of chromianspinel and olivine )For,.

Mineralogical similarities between the Pine Hill com-plex and plutonic rocks from island arcs (Aleutian, ksserAntilles) and ophiolites (Troodos) probably indicate thatconditions operating during crystallization of the parentbasaltic magmas were similar. Compositional differencesin parent basaltic magmas exist among these plutonicrocks, but are not reflected in their mineralogy adequately

SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION IIl

to differentiate among them. For example, the mineral-ogy and major-element chemistry of ejected plutonicblocks in the ksser Antilles island arc are similar to thoseof Pine Hill gabbro with plagioclase >Anr,. The chemicalcomposition of the parent basaltic magma for which theseblocks, as well as associated basaltic andesitic volcanicrock, formed (Lewis, 1973a; Arculus and Wills, 1980) ismarkedly distinct from that calculated for the Pine Hillcomplex (Springer, 1980b). The absence of large amountsof silica-rich (i.e., > 500/o SiOr) fractionation products andthe relative amounts of fractionation products in the PineHill complex negate the hypothesis that this complex wasa major source of andesitic magma.

A low-alkali, high-Ca olivine tholeiite similar in com-position to the calculated chemical composition of thePine Hill complex has been proposed as the parent mag-ma for lavas and dikes on the Isle of Skye, Hebrideandikes and sills, and possibly the Cuillins and Rhum lay-ered basic intrusions (Drever and Johnston, 1966; Don-aldson, 1977). This olivine tholeiite, which is possiblyrelated to the early stages in the development of a mid-oceanic ridge (Donaldson, 1977) is possibly correlative tothe primitive mafic basalt of Snoke et al. (1981).

In the context ofthe western Sierra Nevada, the parentmagma for the Pine Hill complex was probably generatedwithin an island arc (Snoke et al., 1982). A similar originis proposed for the gabbrodiorite plutons of the nearbySmartville complex (Beard and Day, 1987), whose ageand mineralogy correlate closely with the Pine Hill com-plex.

Suvrvrlnv

l. Crystallization in the Pine Hill intrusive complexcan be divided into (l) crystallization directly from themagma, (2) crystallization of magma trapped within cu-mulates and closed to the main magma body, and (3) latecrystallization involving a reaction of the primary min-erals with a residual hydrous phase. Cumulus mineralsexhibit the sequence ol + aug, ol + aug + mt, ol + aug+ mt + pl, ol + aug + mt + pl + opx (inverted frompig), aug + mt + pl + opx (inverted from pig) + qz.Intercumulus minerals include plagioclase in pyroxeniteand partial rims of Ca-poor pyroxene on olivine. Latecrystallization resulted in the formation of calcic amphi-bole and biotite.

2. Prior to complete solidification of the Pine Hill mag-ma (> 900/o crystallized), mafic silicates exhibit limited Feenrichment, whereas plagioclase shows a compositionalvariation of Ann, to Anuo. Near plagioclase of Anuo, themafic silicates show an abrupt decrease in Mg/(Mg + Fe),followed by limited Fe enrichment over the plagioclasecomposition range of Anuo to An.r. Mg/(Mg + Fe) valuesfor the mafic silicates prior to crystallization of plagio-clase of Anuo showbio > aug > opx > mph > ol, whereasat plagioclase of <Anuo, the relationship is aug > opx =

amph = bio > ol. This compositional discontinuity inthe mafic silicates coincides with deformation ofthe com-

plex. Fracturing ofthe complex and adjacent country rockallowed the fluid phase in the country rock to gain accessto the crystallizing magma, resulting in a decrease in theoxygen fugacity of the crystallizing magma.

3. Contact-metamorphic mineral assemblages and thecrystallization sequence of primary minerals indicate aload pressure of <3 kbar during magmatic crystalliza-tion. Temperatures of magmatic crystallization were nearI 100'C, based on pyroxene geothermometry. High initialvalues of oxygen fugacity, i.e., between the HM and NNObuffer curves, in the Pine Hill magma allowed the earlyand abundant crystallization of titanomagnetite. A lim-ited Fe-enrichment trend in the mafic silicates and a lim-ited silica-enrichment trend in the magma indicate thatoxygen fugacity probably decreased less than two log unitsduring crystallization of over 900/o of the complex.

4. Comparison of crystallization sequences and mineralvariation in the Pine Hill complex to those in complexesof the Klamath Mountains and western Sierra Nevada,California, petrotectonic association, Alaskan-type com-plexes, ophiolites, and island-arc plutonism shows dis-tinct similarities, largely indicative of similar conditionsof crystallization. The Pine Hill magma, a low-alkali, high-Ca olivine tholeiite, was generated and emplaced in anisland-arc environment.

AcrNowr-nocMENTS

This research was supported by a grant from the National Science andEngineering Research Council ofCanada (48396). I thank M Cameronand A Finnerty for their constructive comments on the initial manu-scnpt A critical review of the manuscript by C Barnes was greatly ap-preciated. The assistance of the Department of Geology, University ofCalifornia, Davis, is gratefully acknowledged

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Mem-rscnrrr RECETvED DscEN{sen 7. 1987MaNuscnrp'r AccEprED Semsussr 19, 1988