Metamorphism of mafic dikes from the central White-Inyo Range, eastern California

Abstract

Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote, and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular, and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks. Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that took place sporadically over portions of an approximately 100 million year interval.