An isotopic and petrologic study of an exposure of the deep Sierra Nevada batholith, Tehachapi Mountains, California

Citation

Pickett, David Alan (1991) An isotopic and petrologic study of an exposure of the deep Sierra Nevada batholith, Tehachapi Mountains, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XN66-1508. https://resolver.caltech.edu/CaltechETD:etd-11032005-074035

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Tehachapi Mountains, southernmost Sierra Nevada, California, are composed of variably deformed Cretaceous batholithic intrusive rocks and their high-grade framework. The purpose of this study is to 1) establish stronger constraints on the apparently great depths of exposure, and 2) characterize isotopically and chemically the petrogenesis of the intrusive rocks. The Al contents of hornblendes imply igneous pressures from 3.2 kbar in the extreme north of the study area to around 8 kbar in the south. The southern values are the highest yet determined for the entire Sierra Nevada. The rocks commonly show subsolidus recrystallization and metamorphism producing local, fluid-related, garnet-bearing assemblages. Pressures based on garnet-hornblende-plagioclase-quartz and garnet-biotite-plagioclase-quartz equilibria in these metaigneous rocks and framework rocks (7-8 kbar at 670-760[degrees]C) confirm the igneous results. There is also recorded in one area a lower-pressure event (4 kbar, 600[degrees]C) which apparently occurred after uplift which quickly followed initial crystallization of the magmas. The recognition of deformed quartz-bearing rocks at such great depths implies that the young, warm, deep batholith may represent a site favorable for major delamination of the crust. Initial [...] of the igneous rocks of the main study area generally ranges from 0.7041 to 0.7054, with [...]. Lower [...] and higher [...] values are found in the tonalite of Bear Valley Springs to the north. Combined with Pb and published O results, the isotopic data are most consistent with a model of mixing between mantle derived magmas and metasedimentary rocks of chiefly continental provenance. (Two granites from the older suite deviate from this pattern, probably due to fluid interaction.) However, isotopic ratios do not show coherent variation trends consistent with this model when combined with chemical data. The chemical data suggest that lithologic variation among the intrusive rocks results from fractional crystallization acting on mafic to intermediate parent magmas which obtained their diverse isotopic characteristics at a deeper level (>25-30 km). This study reveals the deep Sierra Nevada batholith to be more heterogeneous and somewhat more mafic than analogous portions of the higher batholith, and suggests that isotopic hybridization of batholith magmas takes place in its deepest levels.

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