Petrochemical study of regional/contact metamorphism in metaclastic strata of the central White-Inyo Range, eastern California

Abstract

Sedimentary rocks of the White-Inyo Range have been metamorphosed by episodically emplaced Jurassic–Cretaceous calc-alkaline arc plutons, reflecting stages in the thermotectonic history of this evolving continental margin. A north-northwest–trending anticlinorium is manifested in strata ranging in age from latest Precambrian to middle Paleozoic. Most middle and late Mesozoic (80–180 Ma) intrusive igneous bodies crosscut or deflect the regional fold pattern and incipient axial-plane cleavage in the country rocks—hence granitoid emplacement was largely postkinematic and, accordingly, isogradic surfaces transect structural levels. Retrograde andalusite ± cordierite-bearing metapelitic assemblages and diopside + grossular-bearing calc-silicate wall-rock skarns at plutonic contacts reflect high-temperature thermal maxima in the inner aureoles. Bulk-rock chemistries, mineral parageneses, and compositions of coexisting phases for several noncalcareous, weakly foliated sequences, the uppermost Proterozoic Wyman meta-argillites, the Lower Cambrian Deep Spring and Andrews Mountain metaquartzites, and the Montenegro phyllites, demonstrate that the overall metamorphic zonation is due to advective heat transport by the postkinematic granitoids. The higher grade parageneses do not represent a synchronous thermal event, but instead constitute a composite of local recrystallization episodes attending calc-alkaline pluton emplacement over an ≈100 m.y. interval, overprinting a pervasive, weak, chloritic metamorphism. The latter may have been produced during late Paleozoic and/or early Mesozoic folding. Bulk-rock X-ray fluorescence analyses demonstrate that Wyman meta-argillites are rich in CaO, Al 2 O 3 , Cr, Th, and Sr, and low in SiO 2 , TiO 2 , P 2 O 5 , Na 2 O, and K 2 O, indicating the former presence of carbonate, abundant clays, and minor detrital feldspars and iron oxides. Deep Spring and Andrews Mountain metaquartzite analyses exhibit high concentrations of titania, iron, silica, alkalies, Zr, Nb, and phosphate, but are poor in Al 2 O 3 , CaO, Cr, Th, and Sr, reflecting original protoliths enriched in alkali feldspars, quartz, and magnetite. Montenegro phyllites have compositions intermediate between the other two investigated metasedimentary units; however, they are relatively low in CaO and Sr, and high in Al 2 O 3 , suggesting noncalcareous precursor sediments containing abundant clay minerals, quartz, and alkali feldspars. Metaquartzites and phyllites are more oxidized than the sparsely carbonaceous meta-argillites. Except for a gradual decrease in volatile contents, the analyzed rocks do not exhibit systematic chemical variation with increasing metamorphic grade. Contrasts in phase compositions and proportions reflect protolith chemistries as well as inferred conditions of recrystallization. The areal disposition of visually estimated neoblastic biotite is mapped as isopleths transecting the three investigated sequences. Titanium contents of biotite and white mica, and An contents of plagioclase increase toward the larger, more mafic Barcroft, Cottonwood- Beer Creek, and Joshua Flat plutons, whereas the cation proportions of Si in white micas decrease. Taking into account the ferric iron contents of white micas, Fe 2+ Mg −1 fractionation becomes less pronounced adjacent to these thermal highs. Metamorphic grade west of the range crest is greenschist facies chlorite zone (≈300 °), but rises abruptly on the extreme north adjacent to the Barcroft granodiorite, and gradually increases eastward toward the Beer Creek–Cottonwood and Joshua Flat plutons. An ill-defined metamorphic culmination is directly north of the Papoose Flat granite. Along contacts with plutons, hornfelsic wall rocks approached temperatures of 500–600 °C. On the basis of rare preservation of cordierite and/or andalusite, phase chemistry, and mineralogic thermobarometry, pressures attending regional and thermal recrystallization were about 3 ± 1 kbar. The activity of H 2 O was moderate (2 ± 1 kbar) during recrystallization of carbonate-poor rocks. Compositions of coexisting phases exhibit systematic, sympathetic variations, compatible with a close approach to local chemical equilibrium.