American Mineralogist, Volume 74, pages 101-112, 1989
Mineralogy of a layered gabbro deformedduring magmatic crystallizationo western Sierra Nevada foothills" California
Rorrnr K. SpmNcrn Departmentof Geology,Brandon University, Brandon, Manitoba R7A 6A9, Canada
ABSTRACT In the 162 Ma Pine Hill intrusive complex of northern California, a synformal, layered pyroxenite-gabbro-dioritebody, the crystallization sequenceand compositional variation of cumulus and intercumulus minerals indicate that magmatic crystallization was char- acteized by a high initial oxygenfugacity, log,ofo,in the rangeof -4 to -9, which allowed the early precipitation of abundant titanomagnetite; the oxygen fugacity decreasedless than two log units with crystallizalion. The magmatic crystallization sequenceis ol + aug, ol + aug * mt, ol + aug + mt + pl, ol + aug + mt + pl + opx(invertedfrom pig) and aug + mt + pl + opx (inverted from pig) * qz. Pigeonite crystallization is evidenced by 001 exsolution lamellae in orthopyroxene of Enrr. Superimposedupon this sequenceare intercumulus minerals, which crystallized from magma trapped within cumulates, and biotite and calcic amphibole, which largely formed from a subsolidusreaction of a residual hydrous fluid phasewith primary minerals. Mineral compositional rangesare plagioclase, Ann, to Anrr; olivine, Fo, to Fooo;augite, CanrMgrFento CaorMg.rFerr;Ca-poor pyroxene, Ca,Mgr"Fe' to Ca,MgorFeon;Mgl(Mg * Fe * Mn) of calcic amphibole, 0.74 to 0.48; and Me/(Me * Fe * Mn) of biotite, 0.78 to 0.45. Prior to complete solidification (>900/o crystallized), deformation of the complex coincided with an abrupt decreasein Mg/(Mg * Fe) values for the mafic silicates. This compositional discontinuity is attributed to a decreasein oxygen fugacity of the crystallizing magma causedby an exchangebetween the fluid phaseof adjacentcountry rock and the fluid phaseof the magma during deformation. Prior to deformation of the complex, the Mg/(Mg * Fe) values of the mafic silicates showed the relationship bio > aug > opx > amph > ol whereasafter deformation the relationship is aug > opx = bio * amph > o1. Similarities in mineralogy among mafic plutonic rocks of the Pine Hill complex, ophiolites, Alaskan-type complexes,and island- arc plutonic rocks reflect similar conditions of magmatic crystallization. Generation and emplacementof an olivine tholeiite, low in alkalies and high in Ca, within an island-arc environment is proposed for the Pine Hill intrusive complex.
INtnonucrroN subsolidus crystallization in conjunction with the struc- Mid-Mesozoic ultramafic to dioritic intrusive com- tural evolution of the complex. Comparison of the Pine plexesand their associatedvolcanic rocks in the Klamath Hill complex with other complexesin the petrotectonic Mountains and the westernSierra Nevada, California, are associationas well as with mafic plutonic rocks from es- part of a petrotectonic associationin the western North tablished igneousassociations is discussed. American Cordillera (Snoke et al., 1982). These com- plexes are distinguished by (l) dunite, wehrlite, olivine- PrNn Hrr,r, INTRUSIvEcoMPLEx hornblende clinopyroxenite, olivine gabbro, two-pyrox- The Middle Jurassic(162Ma, Saleeby,1982) Pine Hill ene gabbro and diorite, and diorite to granodiorite, (2) a intrusive complex is a layered igneousbody composedof complex internal structure,including brecciation and sol- gabbro with minor pyroxenite and diorite (Springer, id flow, (3) a systematic variation in mineral composi- 1980a,1980b) (Fig. l). Igneouslayering forms an asym- tions, (4) chemical variations indicative of calc-alkaline metrical synformal structure with a small central area of magmatism, (5) severalepisodes of magma emplacement near-horizontal layering. Primary minerals include oliv- at the same locus, and (6) an orogenic tectonic environ- ine, Ca-rich clinopyroxene, Ca-poor pyroxene, plagro- ment. The structure,petrology, and major-element chem- clase, oxide minerals, qrartz, apatite, and sulfide min- istry of one of these complexes, the Pine Hill intrusive erals (Table l). Late-crystallizing minerals are calcic complex, has beenpreviously described(Springer, 1980a, amphibole and biotite. Secondaryminerals that are prod- 1980b).This paper describesthe mineralogyof the Pine ucts of low-temperature alteration of primary and late- Hill intrusive complex and interprets its magmatic and crystallizing minerals constitute up to l00o/oof some gab- 0003-o04xi89/0 102-0 I 0 I s02.00 I 0I t02 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION
bros (Springer, 1980b). Gabbro is subdivided into four types basedon the wto/oAn of the plagioclase:A, >An* (180/o);B, Anro to Anr, (400/o);C, An* to Anu, (25o/o);D,
TABLE1. Average modes of pyroxenite and gabbro
Olivine 10+13 13+8 6+4 4+3 2+2 Ca-rich clinopyroxene 69+22 15+15 28+12 22+5 13+5 Ca-poor pyroxene <1 1+1 2+2 1-Z b15 Plagioclase <1 64+14 55+13 oJl / 66+4 Oxideminerals ziJ a+e 4+2 4+1 4+1 Ouartz 0 0 0 0 <1 Apatite 0 0 <1 <1 <1 Calcicamphibole 4+5 3+4 5+7 3+6 1+2 Biotite <1 <1 1+1 1+1 1+1 Secondaryminerals. 15+17 1+4 <1 <1 1+2 Note: Columns are averages and standard deviationsfor (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, carbonate minerals. SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION 103
Ano 100 90 80 70 60 so 4u ru
Ftg. 2. An-Ab-Or plot of plagioclasecompositions from the Pine Hill intrusive complex.
To determine the variation of plagioclasecomposition within 345) is common. Similar deformational textures in plu- a singlehand specimen,the chemical composition of plagioclase tonic igneousrocks have been reported, e.g.,Kehlenbeck separateswas determined by three methods, (l) electron micro- (1972),Moore (19'73),and Jorgenson(1979). probe, (2) refractive index ofthe plagioclaseglass, and (3) X-ray No compositional differencesbetween deformed and fluorescenceanalysis. Plagioclasecompositions determined by undeformed plagioclase were detected. Compositional methods statisticallyidentical. This featuresuggests the three are zoning is optically observedonly in plagioclasesof Trele 2. Representativeelectron-microprobe analyses of olivines SS147 RKS4O PH58 PH120 PH168 sio, 38.65 3795 38.37 37.36 Jb.zc 34.49 33.45 Tio, 0.01 0.01 0.06 0.02 0.02 0.02 0.03 Alro3 006 0.04 0.00 0.00 0.08 0.06 0.11 FeO' zz Jo 2493 26.06 29.32 33.96 43.52 46.44 MnO 038 0.44 0.35 0.42 0.72 0.83 1.00 Mgo 39.42 36.17 37.12 33.45 29.05 21.48 18.12 0.06 0.0s 003 0.05 0.07 0.08 0.11 101.14 oo (o 101.99 100.62 100.15 100.48 99.26 Number of ions on the basis of 4 oxygens Si 0.993 1 004 0.995 1.000 0 999 0.997 0 998 Ti 0.001 0 001 AI 0.002 0.001 0.002 0.002 0.004 FE 0 485 0.551 u.cbc 0.656 0.783 1.051 1.158 Mn 0.008 0.010 0.008 0.010 0.017 0.020 0.025 Nlg 1.512 1.427 1 434 1.334 1.193 0.925 0.806 Ca 0.002 0.002 0.001 0.001 0.002 0.002 0.004 l\49/(Mg+Fe+Mn) 0.754 0.718 0.714 0 667 0.599 0.463 0.405 " Total Fe determinedas FeO the c axis ofaugite and up to 5 pm thick and (2) "001" Ca-poor pyroxene lamellae at an angle of near I12" with the c axis and up Ca-poor pyroxene occurs as (l) partial rims around ol- prm to I thick. Exsolution lamellae optically identified in ivine, (2) symplectites with vermicular iron oxide min- four augitescorrelated with lamellae in X-ray precession erals, (3) composite grains with augite, and (4) separate photographs of these augites.Exsolution lamellae of pi- cumulus or intercumulus grains. The first occurrenceis geonite in Pine Hill augitescorrelate with Pig II and Pig observed in some pyroxenite and most olivine-bearing III lamellae of Robinson et al. (1977), which nucleate at gabbro; the second, in gabbro containing plagioclaseof approximately 800 and 600 "C, respectively.In Pine Hill Ann, to Anoo;the third, in gabbro containing plagioclase augites,the absenceof Pig I and Pig "l00" lamellae,which gabbro plagio- oC, of Ann, to An.r; the fourth, in containing nucleateat approximately 1000 and 850 respectively, clase of (Anro. Symplectites of Ca-poor pyroxene with are attributed to down-temperaturemigration of pigeon- vermicular iron oxide minerals occur as single grains of ite from these lamellae to form irregular grains within Ca-poor pyroxene enclosingvermicular iron oxide min- augite or at its boundaries. Plates and needles of iron erals or as an aggregateofequant anhedral grains ofCa- oxide minerals in augite are oriented approximately par- poor pyroxenewith vermicular iron oxide minerals cross- allel to the (100)and (001) planesofthe host augite.The cutting grain boundaries but not extending beyond the iron oxide inclusions decreasein abundance with de- aggregateboundary. These symplectiteshave formed af- creasinganorthite content of the coexisting plagioclase. ter the partial rims of Ca-poor pyroxene as theserims are Although there has been subsolidusequilibration ofthe observedto border aggregatesofCa-poor pyroxenegrains Pine Hill pyroxenes,compositional trends are discernible with vermicular iron oxide minerals. Their formation oc- (Table 3, Fig. 3). Trend A in which En remains relatively curred prior to the crystallization of calcic amphibole and constant between 42.9 and 39.6 and Wo decreasesfrom biotite as theseminerals show a replacementrelationship 49.3 to 41.7 representsthe compositions of augite that with these symplectites. did not coprecipitatewith Ca-poor pyroxene. Trend B in Ca-poor pyroxenes commonly exhibit exsolution la- which En decreasesfrom 42.0 to 34.9 and Wo increases mellae of augite up to 0.02 mm thick and parallel to 100. slightly from 41.7 to 44.2 representsthose that did pre- Exsolution lamellae up to 0.02 mm thick and parallel to cipitate the Ca-poor pyroxene. The AlrO. content of au- the "001" plane ofthe host Ca-poor pyroxenewere ob- gite shows a decrease,4.39 to 0.80 wto/o,with increasing served in gabbro with plagioclaseof Anrr. As textural Fe content ofaugite. TiO, content also decreases,0.94 to featuresshow a gradation from continuous 100 or "00 l " 0.22 ulo/o,with increasingFe content of augite, whereas lamellaeto discontinuous100 or "00l" lamellaeto ran- MnO content showsan increase,0.13 to 0.66 wt0/0.Al- dom irregular blebs, down-temperature migration of though NarO and calculatedFerO. contentsshow no trend exsolved augite has largely obliterated most higher-tem- with the Fe content of augite, the NarO content increases perature exsolution textures (Lindsley and Andersen, with increasingFerO. content. Correlation of Mg/(Mg + 1983).Compositional, textural, and single-crystalX-ray Fe) with anorthite content for coexisting augite and pla- studies show that the Ca-poor pyroxene in all four oc- gioclase shows minor Fe enrichment in augite (Fig. a). currencesis orthopyroxene and that the Ca-poor pyrox- Plots of Mg/(Mg * Fe) vs. anorthite content for olivine ene coexistingwith plagioclaseof An' was originally pi- and Ca-poor pyroxene show a similar relationship. geonite that later inverted to orthopyroxene. SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION 105 Hd En Fs Fig. 3. Compositions of pyroxenesfrom 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 representthe trend for pyroxenesfrom the Skaergaardintrusion (Wagerand Brown, 1967). Although subsolidus exsolution of augite has altered growths, and irregular grains adjacent to or within mag- the composition of the Ca-poor pyroxenes(Table 3), their netite, as describedby Buddington and Lindsley (1964). compositions correlatewith those of augite (Fig. 3). Al,O. All three occurrences,in various combinations, are found contentdecreases from l.7l to 0.39 wto/owith decreasing in gabbro with plagioclaseof all compositions.The modal M/@g * Fe), similar to augite. MnO content increases ratio of ilmenite to magrretiteincreases with decreasing with decreasingMg/(Mg + Fe). anorthite content of the coexisting plagioclase.Granules Equilibrium temperaturesfor ten pairs of cumulus au- of ilmenite increasefrom <10/oof total iron oxide min- gite and Ca-poor pyroxene coexisting with plagioclase rangingfrom Anro to An., in the complex were calculated using various pyroxene geothermometers:888 + 47 'C -r 'C (Wells, uJ . BUSHVELD (Wood and Banno, 1913), 919 49 1977), z 912 + 6l oC and 996 + 28'C (low Z and high ?"of the i.Il O EMIGRANT GAP o transfer equation, Kretz, 1982), and a range of 1484 to ^ GUADALUPE (exchange pairs 720'C equation, Kretz, 1982). Three o O sl vlrucerur geothermom- z yielded a range of 950 to 750 "C using the ) @ raoooos eter of Lindsley (1983). These calculatedtemperatures X PINE HILL I are largely subsolidus equilibrium temperatures. Using . BEAR MOUNTAIN the chemical composition of the coexistingpyroxenes and I 6 the modal percent of exsolved pyroxene in the host py- O roxene,a temperatureof magmatic crystallization of near o 1100'C (Lindsley, 1983) was determined.These data L f suggestthat in the pyroxene quadrilateral (Fig. 3), trend o B probably representsa subsolidustrend whereastrends A and C approximate the magmatic crystallization path of augitein the complex. Oxide rninerals Oxide minerals (titaniferous magnetite and ilmenite) are presentin gabbro but only rarely in pyroxenite except for the pyroxenite body at the southern end of the com- 100 80 60 40 20 plex (Fig. 1). In rhythmically layered gabbro containing MOL Y" ANORTHITE plagioclaseof >Anrr, layers up to one-third meter thick Fig.4. Compositionsof coexistingCa-rich clinopyroxeneand contain up to 300/oand, in one instance, l00o/oiron oxide plagioclasefrom the Pine Hill intrusive complex compared to minerals. those from other plutonic rocks. Pyroxenesfrom pyroxenitesare Titaniferous magrretite,which occursas a cumulus and assignedan anorthite content of 100. Sourcesofdata: Bushveld intercumulus mineral as well as vermicular intergrowths (Wager and Brown, 1967), Emigrant Gap (James, l97l), Gua- with Ca-poor pyroxene, is always associatedwith ilmen- dalupe (Best and Mercy, 1967), St. Vincent (Lewis, 1973b), ite. Ilmenite occurs as coarselamellae, fine trellis inter- Troodos(Allen, 1975),Bear Mountain (Snokeet a1.,1981). 106 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION Trele 3. Representativeelectron-microprobe analyses of pyroxenes SS147 RSK4O PH58 PH12O JB7 PH168 PH115 SS135 PH58 sio, 51.80 51.39 51.00 52.33 50.69 51.10 51.06 52.55 54.29 Tio, 0.76 0.54 0.58 049 051 0.48 0.41 0.22 0.12 Al,o3 4.22 4.39 3.53 2.89 3.24 2.39 2.19 0.80 1.63 FeO- 5.39 6.02 8.12 8.71 8.46 12.42 12.74 12.04 16.56 MnO 0.14 0.18 0.19 0.22 0.30 0.41 0.47 0.50 0.34 Mgo 1518 14.80 14.10 14.52 14.05 12.71 12.42 12.18 27.70 CaO 23.86 22.52 22.00 20.78 20.94 20.33 20.17 20.81 0 58 Naro 0.30 0.49 0.30 0 37 't.o7 0.73 0.81 0.34 0 00 101.65 100.33 99.82 100.31 99.26 100.57 100.27 99.44 101.23 Number of ions on the basis ol 6 oxygens Si 1.880 1.889 1 901 1.935 1.905 1.924 1.932 1.994 1.943 AI 0.120 0.1t1 0 099 0.06s 0.095 0.076 0.068 0.006 0.057 V] 2.000 2.000 2 000 2.000 2.000 2.000 2.000 2.000 2.000 AI 0.061 0 079 0.056 0 061 0.049 0.030 0.030 0.030 0.012 Ti 0.021 0 015 0 016 0.014 0.014 0.014 0.012 0.006 0.003 FC 0.164 0.185 0.253 0.269 0.266 0.391 0.403 0.382 0.496 Mn 0.004 0.006 0.006 0.007 0.010 0.031 0.015 0.016 0.010 Mg 0.821 0.811 0.783 0.800 0.787 0.713 0 700 0 689 1.477 Ca 0.928 0.887 0.879 0.823 0.843 0.820 0.818 0.846 0.022 Na 0 021 0.035 0.022 0.027 0.078 0.053 0.059 0.025 0.000 tX,Y] 2.019 2.018 2.016 2.001 2.047 2.036 2.037 1.994 2.020 Mg/(Mg+Fe+Mn) 0.830 0.809 o.751 0.743 0 740 0.638 0.626 0.634 0.745 4d.J 47.1 45.9 43.5 44 5 42.6 42.6 44.1 1 1 Mg 42.9 43.1 40.9 42.3 415 37.1 36.4 35.9 74.0 Fe o.o 9.8 13.2 14.2 140 20.3 21.0 19.9 24.9 - Total Fe determinedas FeO erals in pyroxenite and gabbro with plagioclaseof >An85 tent in magnetite from oxide-rich layers). The average to near 300/oin gabbro with plagioclasenear Anrr. Isolated modal amount of iron oxide minerals in the complex is grains of ilmenite were not observed.Ilmenite in all oc- < 50/obut FO I 3 and JD82 contain - 300/oand I 000/ooxide currences is optically and chemically homogeneous. minerals, respectively.MgO content of these magnetites Pleonaste occurs as lamellae parallel to the octahedral and their coexistingilmenites (Tables4, 5) arethe highest planes of magnetite and as irregular grains concentrated of all magnetite and ilmenite in the complex. at magnetitegrain boundariesand within coarseilmenite The FerO. content of magnetite,which decreaseswith blades.Pleonaste becomes less abundant with decreasing decreasinganorthite content of the coexistingplagioclase, anorthite content of the coexisting plagioclase. shows an inverse relationship to that of FeO and TiOr, Compositional trends in magnetite with respectto the characteristicof the FerTiOo-FerOoseries (Lindsl ey, 197 6). anorthite content of the coexisting plagioclase are dis- AlrO, increaseswith decreasinganorthite content. Be- cernible for TiOr, AlrO., FerOr, and FeO (Table 4). TiO, causethe abundanceof exsolution lamellae of pleonaste in magnetiteincreases from < I wto/oin pyroxenite to > I I in magnetitedecreases with decreasinganorthite content, wt0/oin gabbro with plagioclaseof An*. but drops to < l.5 the AlrO, trend possibly reflects the degreeof pleonaste wto/oin gabbro with plagioclaseof TABLE4. Representativeelectron-microprobe analyses of magnetitesand a pleonaste FO13 FO13.- RKS84 JD82 JB7 PH168 RKS18 Tio, 7.13 0.05 3.12 409 6.43 11.09 11.35 1.34 Al,03 1.94 5840 0.92 0.82 2.34 3.22 314 0.59 Cr.O. 0.06 0.04 0.25 028 0.04 0.09 0.04 0.03 FerO"" s5.20 6.17 62.58 60.07 s5.29 44.37 43.98 66 01 FeO. 34.97 22.74 33.59 33.28 37.48 41.16 41.38 32.08 MnO 0.51 0.24 0.39 0.61 0.41 0.54 0.65 0.45 Mgo 2.00 11.82 0.31 0.67 o.12 o.29 o.22 0.00 101.81 99.46 101.16 99.82 102.11 100.76 't00.77 | 00.50 . Recalculationof FerO3and FeO after Carmichael(1967), Anderson (1968), Lindsley and Spencer(1982), and Stormer(1983) *- Pleonaste. SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION IO7 TABLE3-Continued Calcic amphibole partial rims around pri- PH12O JB7 PH168 PH115 SS135 Calcic amphibole occurs as mary minerals, especiallypyroxene and iron oxide min- 54.28 52.87 51.54 51.19 51.44 patcheswithin pyroxene.In gabbro 0.15 0.14 o.20 0.19 0.09 erals,and as irregular 1.58 1.77 110 1.20 0.39 with abundant deformational textures, anhedral equant 17.70 19.27 24.96 26.15 30.24 grains of calcic amphibole are typical. Generally calcic 0.41 0.59 0.70 0.83 125 py- 25.61 24.07 19.20 18.13 16.27 amphibole forms < l0o/oof the mafic minerals in any 0 65 0.80 1.59 0.97 1.13 roxenite or gabbro, but in certain gabbros,it constitutes 0 00 002 0.01 007 002 nearly l00o/oof the mafic minerals. Calcic amphibole ex- 100.38 99.s5 99.32 9874 100.82 hibits pleochroism of X : tan, Y: medium brown, and Number of ions on the basis of 6 oxygens 't.967 1.953 1.968 1.975 1.984 Z : dark brown to reddish brown in most gabbro, but a 0.033 0.047 0.032 0.025 0.016 pleochroism of X : colorless, y : pale green, and Z : 2 000 2 000 2 000 2.000 2.000 greenin most pyroxenite. Calcic amphibole and 0.034 0.030 0 018 0.030 0.002 medium 0.004 0.004 0.006 0.006 0.003 biotite, which are commonly associatedin the samerock, 0.536 0.59s 0 797 0.844 0.976 appear to have crystallized at the same time. Whether 0.013 0.018 0 023 0.027 0.041 1.383 1.325 1.093 1.043 0.935 calcic amphibole and biotite occur together or separately 0 025 0.032 0.065 0.040 0.o47 depends largely on the bulk composition. For example, 0.000 0.001 0.001 0.005 0.001 in gabbro with plagioclasenear Anro, if K'O > 0.55 wt0/0, 1 995 2.006 2.002 1.994 2.005 only biotite is present; if KrO : 0.30-0.55 wto/0,biotite 0.716 0.684 0.571 0 545 0 479 and calcic amphibole are present; if KrO < 0.30 wt0/0, 13 16 3.3 2.1 2.4 amphibole is present. 71.1 67I 55.9 54.1 47.8 only calcic 27.6 305 40.8 438 49.8 Using the nomenclature of Leake (1978), the chemical compositions of the calcic amphiboles (Table 6, Fig. 5) show a compositional gap between those from gabbro with plagroclaseof TABLE5, Representativeelectron-microprobe analyses of ilmenites FO13 RKS84 PH168 PH115 RKS18 Tio, 51.95 53.26 5478 52.19 E1 7Q 51.98 51.67 Al,o3 0.05 0.09 007 0.11 011 0.11 0.07 Cr.O. 0.00 0.03 000 0.00 0.00 0.00 0.00 FerO.. 5.01 1.92 1.87 1.85 Z.JJ 2.62 4.68 FeO- 32.42 41.86 38.00 44.10 43.18 43.49 40.49 MnO 108 200 2.34 1.18 1.58 5.85 Mgo 7.41 1.98 5.18 0.26 1.20 noc 0.02 97.92 101.61 101.91 10075 99.96 100.70 102.76 - Recalculationof Fe,O3and FeO after Carmichael(1967), Anderson (1968), Lindsley and Spencer(1982), and Stormer(1983). 108 SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION 0.8 I paragasitic I I hornbleno".unJru paragasite edenitic hornblende o o60 4 +o. .56 ferroan- paragasitic hornblende ferroan paragasite +------]: a30 tr33 o4 f erro-edenitic hornblende a quaiz absent o quanzpresent 6.8 6t 6.2 6.0 Si (cations) Fig. 5. Variation of Mg/(Mg + Fe) with Si for calcic amphiboles in the Pine Hill intrusive complex. Numbers refer to the anorthite content of the coexistingplagioclase. creases.Higher values of NarO occur in biotite coexisting in the complex, no study of these minerals was under- with plagioclaseof >Anuo. taken. Quartz, apatite, and sulfide minerals Quartz was observed in only three gabbros (Table l), PrrnocnNosrs which occur in either the mixed zone or the zone in which Crystallization sequence plagioclase OLIVINE isolated intercumulus magma or by a subsolidusreaction. Ca-rich CLINOPYROXENE Experimental studiesofbasalts under variable oxygenfu- Ca-poor PYROXENE gacity(Roeder and Osborn, 1966;Presnall, 1966; Osborn, CaICic AMPHIBOLE 1979) best illustrate the crystallization sequencein BIOTITE the o Pine Hill complex. Magnetite crystallization, which oc- I T 60 curs after that of olivine and augite,is closelyfollowed by o that of plagioclase,and results in the simultaneous crys- tallization of olivine, augite, magnetite, and plagioclase. o Olivine is totally consumed as plagioclasenears Anoo,so o\ the common assemblagepl + ol + aug + Ca-poor py- roxene + mt changesto aug + Ca-poor pyroxene + mt +pl+qz. Evidencefor adcumulus growth during magmatic crys- tallization of the Pine Hill complex is the lack of chemical zoning in the primary minerals, and the occurrence of t00L monomineralic layers of titanomagnetite with <50/oin- 100 80 60 40 terstitial silicate minerals and up to one-third meter thick, O/O MOT ANORTHITE and layers of olivine clinoplroxenite with less than 50/o Fig. 6. Variation of Me/(Mg + Fe) in mafic silicates with interstitial plagioclase(Wager et al., 1960;Morse, 1980, plagioclasecomposition in the Pine Hill intrusive complex. p.2a$. SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION 109 Trele 6. Representativeelectron-microprobe analyses of calcic Trale 7, Representativeelectron-microprobe analyses of bio- amphiboles tites RKS84 PH58 PH120 SS129 SS135 PH12O JB7 PH168 PH115 SS135 sio, 43.15 42.94 4215 42.05 43.71 sio, 37.94 37.08 37.81 36.55 36.58 Tio, 2.15 1.83 2 67 2.63 1.90 Tio, 3.11 3.36 4.50 5 84 4.79 Alro. 12.63 13.08 12.97 12.61 8.87 Alro3 16.54 16.63 15.17 14.64 13.37 FeO. 9.19 9.41 11.44 14.21 20.19 FeO. 9.28 11.67 15.54 18.21 21.32 MnO 0.20 0.08 0.10 0.20 0.33 MnO 0 02 0.06 0.07 0.11 0j2 Mgo 14.99 14.93 13.20 11.96 949 Mgo 18.18 19.01 14.74 12.76 10.39 CaO 12.05 12.20 11.92 11.40 11.03 CaO 0.08 012 0.07 0.05 0.06 Na"O 257 2.23 2.48 2.73 1.67 Na"O o.32 0.31 0 06 0 00 0.09 KrO 0 16 1.08 1.38 1.06 0.55 KrO 9.27 8.67 9.41 9.71 9.12 97.09 97 78 98.31 98.85 97.74 94.74 96.91 97.37 97.87 95.84 Numberof ions on the basis of 23 oxygens Numberof ions on the basis of 22 oxygens Si 6.299 6.256 6.189 6.209 6.655 Si 5 528 5 342 5.510 5.420 5.612 AI 1.70f 1.744 1.811 1.791 1.345 AI 2 472 2 658 2.490 2.559 2.388 tzl 8 000 8 000 8.000 8.000 8.000 tzl 8.000 8.000 8.000 7.979 8.000 AI o 472 0 502 0.434 0.404 0.247 AI 0.370 0.166 0.174 0.031 Ti 0.236 0.201 0.295 0.292 0.218 Ti 0.341 0.365 0.493 0.651 0.552 Fe 1.122 1147 1.405 1755 2.571 Fe 1.131 1.406 1.894 2.258 2.735 Mn 0.025 0.010 0.012 0 025 0.043 Mn 0.003 0.007 0.009 0.014 0.016 Mg 3.261 3.242 2 889 2.632 2.153 Mg 3.948 4.081 3.201 2.820 2.376 tX, Y] s.116 5.102 s.035 5.108 5.232 tYl 5.793 6.025 5.771 5.743 5.710 1.885 1.905 1.876 1.804 1.800 Ca 0.012 0.01I 0.011 0.008 0.010 NA 0.727 0.630 0.706 0.782 0.493 NA 0.091 0.087 0.018 0.028 K 0.030 0.200 0 259 0.200 0 107 K 1.723 1.593 1.750 1.827 1.785 ul 2.642 2.735 2.841 2.786 2.400 txl 1.826 1.698 1.789 1.835 1.823 Mg/(Mg+Fe+Mn) 0.740 0.737 0.671 0.597 0.452 Mg/(Mg + Fe + Mn) 0]77 0743 0.628 0.s55 0.465 . Total Fe determinedas FeO . Total Fe determinedas FeO. Crystal sedimentationeventually prohibits effectivedif- of contact-metamorphic mineral assemblages(Springer, fusion between the magma body and intercumulus mag- 1974, 1980b).The total pressurecan also be estimated ma, and a certain amount of intercumulus magma is from the chronologic sequenceof cumulus phases.As the trapped within the cumulates. Partial rims of Ca-poor stability of plagioclaseis suppressed250 "C with increas- pyroxene on olivine are attributed to a reaction between ing water pressurebetween 2 and 8 kbar (Yoder and Til- olivine and trapped intercumulus magma, becausethe ley, 1962;Holloway and Burnham,1972), a total pres- rims of Ca-poor pyroxenehave formed in pyroxenite and sure near 2.5 kbar would be indicated for the Pine Hill Type A gabbro prior to the direct precipitation of Ca- complex. poor pyroxene from the main body of magma. Crystal- Under the most oxidizing conditions, i.e., oxygen fu- lization ofphasesdirectly from the trapped magma,along gacity controlled by the HM (hematite-magnetite)buffer, with crystallization of phasesresulting from the reaction melting relations of basalts show that titanomagnetite is of this magma with cumulus minerals, would causede- one of the highest-temperatureliquidus phases(Hamil- pletion and concentration of elementswithin the trapped ton et al., 1964; Holloway and Burnham, 1972; Helz, magma similar to fractional crystallization of the main 1973), whereas under a lower oxygen fugacity as con- body of magma. On the basis of textural evidence-such trolled by the NNO (Ni-NiO) buffer, titanomagnetite as irregular patchesof hydrous minerals within primary crystallizesafter olivine and Ca-rich clinopyroxene. Un- minerals, lack of primary accessoryminerals with hy- der still lower oxidizing conditions, i.e., the FMQ (fa- drous minerals, and Ca-poor pyroxene with vermicular yalite-magnetite-quartz)bufer, ilmenite is a high-tem- titanomagnetite rimmed by Ca-poor pyroxene-the al- perature liquidus phase. The Pine Hill complex largely teration of olivine to Ca-poor pyroxene and vermicular crystallized under oxidizing conditions between the HM titanomagnetite and the formation of calcic amphibole and NNO buffers, i.e., a log'o/o, between -4 and -9 and biotite probably occurred in equilibrium with an in- assumingmagmatic temperaturesnear I100 "C. The early tercumulus hydrous fluid phase rather than an intercu- and continuous crystallization of titanomagnetite and its mulus silicate melt. Using calculated temperatures of abundance,the presenceofsilica- rather than Fe-enrich- crystallization and similar textures, intercumulus horn- ment trend (Springer, 1980b),and the limited Fe enrich- blende in similar gabbrosis attributed to a high-temper- ment in olivine and pyroxene correlate with the results ature subsolidusreaction between a hydrous fluid phase of experimental crystallization studies of basalts under and primary minerals(Otten, 1984;Morrison et al., 1986). similar oxidizing conditions (Roeder and Osborn, 1966; Presnall, 1966; Osborn, 1979).The limited Fe enrich- Conditions of crystallization ment in olivine and pyroxene from pyroxenite to gabbro Magmatic crystallization of the Pine Hill complex oc- with plagioclaseof >An6o indicates a decreasein oxygen curred at a total pressureofless than 3 kbar on the basis fugacity of one to two log fugacity units (Speideland Os- lIO SPRINGER:LAYEREDGABBRODEFORMEDDURINGMAGMATICCRYSTALLIZATION born, 1967) during crystallization of over 900/oof the equilibrium with crystallizing hydrous minerals after de- complex. The high values and rangein Mg/(Mg + Fe) for formation but not before. calcic amphibole and biotite are also consistentwith crys- tallization under similar oxidizing conditions (Wonesand Magma composition Eugster, 1965:,Helz, 1973). Initially high water acriviry Most ultramafic to dioritic intrusive complexesin the as well as high f, characterizethe magmatic crystalliza- Klamath Mountains and westernSierra Nevada, Califor- tion of ultramafic-mafic rocks in the petrotectonic asso- nia, have been chbracterizedby a multistage differentia- ciation of the Klamath Mountains and westernSierra Ne- tion-emplacement model involving up to three parent vada (Snokeet al., 1982). magmas: (1) a primitive mafic basalt diferentiating to Compositional discontinuities in Mg/(Mg + Fe) for o1- dunite, wehrlite, gabbro, diorite, (2) a wet high-Al basalt ivine and pyroxene as well as for calcic amphibole and differentiating to hornblende gabbro and diorite and then biotite-all occurring at a plagioclasecomposition ofnear to tonalite,and (3) a tonalite (Snokeet al., l98l). In con- Anuo-coincide with deformation of the complex. Lack trast to most complexesin this petrotectonic association, of a similar discontinuity in plagioclasecomposition sug- there are neither younger tonalitic rocks nor large amounts geststhat a significant temperature decreasedid not ac- ofhornblende gabbro and diorite associatedwith the Pine company this deformation. It is proposed that the initial Hill intrusive complex. The rocks of Pine Hill complex deformational event involved fracturing of the Pine Hill correlate closely with the early ultramafic-gabbroic-dio- complex and adjacent country rock such that interaction ritic rocks of these complexes.This feature suggeststhat of the fluid phase in the country rock and crystallizing the magmas from which the tonalitic rocks and horn- magma occurred.This interaction involved a loss of fluid blende gabbro and diorite were derived were commonly from the residual magma and resulted in a lower oxygen emplaced at the same locus along with the earlier prim- fugacity in the magma, reflectedby an abrupt decreasein itive mafic basalt,but were generatedseparately from the Mg/(Mg + Fe) of the mafic silicates. The fine-grained primitive maflc basalt. The dunite-wehrlite-gabbro- gabbro of the mixed zone possibly reflects rapid crystal- diorite association of these complexes is similar to the lization due to the loss of fluid phasefrom the magma. dunite-wehrlite-gabbro association of the Alaskan-type The presenceof calcic amphibole and biotite in the complexes(James, 1971; Snoke et al., 1981,1982). Char- Pine Hill complex indicates that water was presentin the acteristics of the Pine Hill complex similar to those of parent magma. Melting relations of basalts indicate that the Alaskan-type complexes are (l) olivine clinopyrox- at a total pressureof 3 kbar and under water-saturated enite and clinopyroxenite that are gradational into gabbro (Yoder and Tilley, 1962) as well as water-undersaturated with plagioclaseof >Anno;(2) layering of titanomagnetite (Prro and 0.6P,*r, Holloway and Burnham, 1972) con- in clinopyroxenite and gabbro with plagioclaseof >Anrr, ditions, calcic amphibole would be a liquidus phase;how- plus an overall abundanceoftitanomagnetite; (3) patches ever, at a total pressureof <1.5 kbar, it would be a sol- of pegmatitic clinopyroxenite and gabbrowith plagioclase idus phase. Regardless of the total pressure, if the of >Anr,; (4) igneouslayering in gabbro with plagioclase crystallizing liquid does not at some time contain the of >Anrr, produced by pseudosedimentarymagmatic minimum water content (=3.0o/o,Burnham, 1979) re- processes;(5) syncrystallization and subsolidus defor- quired for the coexistenceof calcic amphibole and silicate mation involving flowage and brecciation of cumulates; melt, calcic amphibole would not crystallize from the (6) primary minerals that comprise olivine (Fo, to Foo,), magma. The absenceof primary calcic amphibole as well Ca-rich pyroxene, Ca-poor pyroxene (absent in pyrox- as primary biotite in the Pine Hill complex is attributed enite and gabbro with plagioclaseof >Anrr), titanomag- to a low initial water content in the magma. As the water netite (subsolidus oxidation-exsolution to titaniferous content of trapped intercumulus magma would increase magnetite,ilmenite, and pleonaste),calcic plagioclase, and with crystallizalion, the minimum water content required late-crystallizingcalcic amphibole with low SiO, and high for the crystallization of calcic amphibole and biotite AlrO.; (7) alkali feldspar-+ordierite hornfels facies con- would be reached.Whether this intercumulus phasewas tact metamorphism in adjacent country rock (Taylor, largely a silicate melt or hydrous fluid phase during the 1967); and (8) emplacementinto an orogenictectonic en- crystallization of these hydrous minerals is not known. vironment. The major difference between the Pine Hill Prior to deformation of the complex, the Mg/(Mg + Fe) complex and the Alaskan-type complexesis the former's values of the mafic silicates exhibit the relationship bio lack of dunite, which precludesthe presenceof chromian > aug > opx > amph > ol; after deformation, aug > spineland olivine )For,. opx = bio = amph > ol. The relationship aug > amph Mineralogical similarities between the Pine Hill com- > bio is common in plutonic igneous rocks in which plex and plutonic rocks from island arcs (Aleutian, ksser calcic amphibole and biotite crystallizeddirectly from the Antilles) and ophiolites (Troodos) probably indicate that magma (Gilbert et al., 19821-Speer, 1984) even under conditions operating during crystallization of the parent oxidizing conditions between the NNO and HM buffer basaltic magmaswere similar. Compositional differences curves(Czamanske and Wones, 1973).A silicatemelt- in parent basaltic magmas exist among these plutonic i.e., trapped intercumulus magma-may have been in rocks, but are not reflectedin their mineralogy adequately SPRINGER: LAYERED GABBRO DEFORMED DURING MAGMATIC CRYSTALLIZATION IIl to differentiate among them. For example, the mineral- plex. Fracturing ofthe complex and adjacentcountry rock ogy and major-element chemistry of ejected plutonic allowed the fluid phasein the country rock to gain access blocks in the ksser Antilles island arc are similar to those to the crystallizing magma, resulting in a decreasein the of Pine Hill gabbro with plagioclase>Anr,. The chemical oxygen fugacity of the crystallizing magma. composition of the parent basalticmagma for which these 3. Contact-metamorphic mineral assemblagesand the blocks, as well as associatedbasaltic andesitic volcanic crystallization sequenceof primary minerals indicate a rock, formed (Lewis, 1973a;Arculus and Wills, 1980)is load pressure of <3 kbar during magmatic crystalliza- markedly distinct from that calculated for the Pine Hill tion. Temperaturesof magmatic crystallization were near complex (Springer,1980b). The absenceof large amounts I 100'C, basedon pyroxenegeothermometry. High initial of silica-rich (i.e., > 500/oSiOr) fractionation products and valuesof oxygenfugacity, i.e., betweenthe HM and NNO the relative amounts of fractionation products in the Pine buffer curves, in the Pine Hill magma allowed the early Hill complex negatethe hypothesisthat this complex was and abundant crystallization of titanomagnetite. A lim- a major sourceof andesiticmagma. ited Fe-enrichmenttrend in the mafic silicatesand a lim- A low-alkali, high-Ca olivine tholeiite similar in com- ited silica-enrichment trend in the magma indicate that position to the calculated chemical composition of the oxygenfugacity probably decreasedless than two log units Pine Hill complex has been proposedas the parent mag- during crystallization of over 900/oof the complex. ma for lavas and dikes on the Isle of Skye, Hebridean 4. Comparison of crystallization sequencesand mineral dikes and sills, and possibly the Cuillins and Rhum lay- variation in the Pine Hill complex to those in complexes eredbasic intrusions(Drever and Johnston,1966; Don- of the Klamath Mountains and western Sierra Nevada, aldson, 1977).This olivine tholeiite, which is possibly California, petrotectonic association,Alaskan-type com- related to the early stagesin the development of a mid- plexes, ophiolites, and island-arc plutonism shows dis- oceanicridge (Donaldson, 1977)is possibly correlative to tinct similarities, largely indicative of similar conditions the primitive mafic basaltof Snokeet al. (1981). of crystallization.The Pine Hill magma,a low-alkali, high- In the context ofthe westernSierra Nevada, the parent Ca olivine tholeiite, was generatedand emplaced in an magma for the Pine Hill complex was probably generated island-arc environment. within an island arc (Snokeet al., 1982).A similar origin AcrNowr-nocMENTS is proposed for the gabbrodiorite plutons of the nearby Smartville complex (Beard and Day, 1987),whose age This researchwas supportedby a grant from the National Scienceand (48396). and mineralogy correlate closely with the Pine Hill com- EngineeringResearch Council ofCanada I thank M Cameron and A Finnerty for their constructive comments on the initial manu- plex. scnpt A critical review of the manuscript by C Barneswas greatly ap- preciated.The assistanceof the Department of Geology, University of Suvrvrlnv California, Davis, is gratefully acknowledged l. 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(1964) Iron-titanium oxide min- mafic silicatesshow an abrupt decreasein Mg/(Mg + Fe), eralsand syntheticequivalents. Journal ofPetrology, 5, 310-357. Burnham, C W (1979) The importance of volatile constituents.In H.S. followed by limited Fe enrichment over the plagioclase Yoder, Jr., Ed, The evolution ofthe igneousrocks. Princeton Univer- composition rangeof Anuoto An.r. Mg/(Mg + Fe) values sity Press,Princeton, New Jersey. for the mafic silicates prior to crystallization of plagio- Carmichael,I S E (1967) the iron-titanium oxides ofsalic volcanic rocks claseof Anuoshowbio > aug > opx > mph > ol, whereas and their associatedferromagnesian silicates. Contributions to Miner- Petrology,14,3644. plagioclase Donaldson, CH (1977) Petrology of anorthite-bearing gabbroic anor- Robinson,Peter, Ross, M., Nord, G.L., Jr., Sm1th,J R, and Jaffe,H.W. thosite dykes in northwest Skye Journal ofPetrology, 18, 595-620. 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