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Mineralogy of a Layered Gabbro Deformed During Magmatic

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Mineralogy of a Layered Gabbro Deformed During Magmatic 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, <Anr, (90/o).The number in parenthesesrefers to the por- tion of the complex composed of this tlrpe; pyroxenite makes up the remaining 80/oof the complex. The mixed zone is characterized by intercalcated lenses of fine- grained, massive gabbro and coarser-grained,massive to slightly foliated gabbro or in some places, by a gabbro breccia. Plagioclasein the mixed zone gabbro is every- where <Anuo. A possible model for the structural evolution of the complex (Springer, 1980a, I 980b) involves the formation of layered cumulatesin a near horizontal sill-like magma chamber.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 - interlayeredpyroxenite and type A gabbro - type A gab- bro - type B gabbro - type C gabbro - type D gabbro. Subsequentexternal deformation of this sequenceprior to complete solidification of the magma produced large- scalebrecciation of the more rigid portions and flowage ofthe less rigid portions. Resultant featuresinclude iso- clinal folding with axial planes parallel to igneous layer- ing, transposed layering, deformational textures, and mineralogical discontinuities. This deformation also pro- duced the synformal structure of the complex. The inter- nal structure of the complex is not reflectedin the struc- ture of the country rock (Springer, 1980b). Adjacent county rock has been contact metamorphosed by the complex(Springer, 1974; Springer and Craig, 1975). Fig. l. Geologic map of the Pine Hill intrusive complex, showing the distribution ofplagioclase composition in the gab- ANA.r-vrrc.q.LTECHNIeUES bro. Gabbrotypes: A: >85 wto/oAn, B: 70-85 wt0/0An, C: Compositionalstudies of plagioclase,olivine, pyroxenes, ox- 55-69 wto/oAn, D : <55 wto/oAn. The mixed zone is shown by ide minerals,calcic amphibole, and biotite weremade utilizing triangles,and pyroxenite is shown by the dot pattern. Horizon- ARLEMx electron microprobes at the Universityof California, tally ruled areasare host rocks. Arrows indicate top determina- Davis,and the VirginiaPolytechnic Institute and StateUniver- tion from outcrop study. Wavy lines show discontinuities in sity.All microprobedata were refined using the correctionpro- mineral composition. The mineral layering orientations are shown ceduresof Rucklidgeand Gasparrini(1969) and/or Bence and by strike-and-dip symbols. Albee(1968). 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 <Anuo. that the smallersample size, i.e., 6-10 spotson 34 grainsused Exsolution of K-feldspar spindle-shapedblebs, up to 50 in electron-microprobeanalysis versus hundreds ofgrains used grain near frac- in X-ray fluorescenceanalysis, adequately reflects the average prm in length, occurs near boundaries, composition of plagioclasein a single hand specimen. Rapid tures, or within twin lamellae in plagioclaseof <Anrr. determination of plagioclasecomposition for this study was made The Fe content of plagioclase,0.04-0.13wt0/0, increases - usingwto/o An : [(f + 0.798)/0.253],where I : 2d(131 l3l) slightly with increasingAn content. - 20(l3l - 220). Increasingsubstitution ofK resultedin lower values for l. Olivine To determine outcrop variation in the composition and I of modal decreasefrom near 20o/oin plagioclase,four
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