Eclogitic Residues from beneath the Sierra Nevada Batholith; Similarities between the Extraction of Cordilleran and Archean Granitoids

Abstract

Garnet pyroxenites are the most common deep-crustal and upper-mantle xenolith assemblages found in Miocene volcanic rocks erupted through the central part of the Cretaceous Sierra Nevada batholith (SNB) (Ducea and Saleeby, 1996). We propose that the SNB granitoids and the pyroxenite xenoliths are respectively the melt and unmelted residues/cumulates from partial melting of a high-Mg basaltic protolith at depths exceeding 35-40 km. This protolith included 20-30% of material with a preexisting near-surface hydrothermal and/or weathering history ^(18)O =6.5 to 9 in both xenoliths and SNB). We will review many chemical similarities between the SNB granitoids and Archean TTGs, and between the Sierra pyroxenites and eclogitic xenoliths in kimberlites. Modelling shows that if the end-member with elevated ^(18)O is removed, the major and trace element compositions of silicic magmas from modern convergent arcs would be virtually indistinguishable from the Achean TTGs. We suggest that the central Sierra Nevada and other segments of the Cordilleran batholiths have originated by petrologic processes similar to those which led to the extraction of the voluminous granitoids that make up the cratonic nucleii of the continents.