ND-SR-OS SYSTEMATICS OF MANTLE XENOLITHS FROM THE SIERRA NEVADA BATHOLITH: STRUCTURAL AND GEOCHEMICAL EVOLUTION OF A SUBCONTINENTAL LITHOSPHERIC MANTLE

Variable mantle xenolith assemblages from the extinct Mesozoic Sierra Nevada arc provide a probe into the structural and geochemical evolution of the subcontinental lithospheric mantle (SCLM) that underlies the Sierra Nevada Batholith. New Nd-Sr isotope systematic studies on a suite of mantle xenoliths from the Miocene Big Creek and Pleistocene Oak Creek volcanics, combined with published Os isotope studies, show that Sierran SCLM is isotopically heterogeneous. Nd-Sr isotope systematics define three groups of mantle xenoliths: Group A has limited but enriched ⁸⁷Sr/⁸⁶Sr ratios, ranging from 0.7061 to 0.7062, and negative eNd=-18.4 to -22.5; Group B has a wide range of ⁸⁷Sr/⁸⁶Sr from 0.703 to 0.708, and eNd from +8 to -10.5, extending from the mantle array into the Sierra Nevada Batholith domain; and Group C has a relatively narrow range of ⁸⁷Sr/⁸⁶Sr( 0.7065 ~ 0.7078) and positive eNd ( +10 ~ +0.5). Low eNd is generally interpreted to represent ancient LREE enriched SCLM, and thus Group A xenoliths might represent Proterozoic SCLM. However, the Os isotopic compositions of these xenoliths are radiogenic and give Phanerozoic model ages suggesting that either Group A represents ancient SCLM whose Os isotopic signature has been disturbed by "Os metasomatism", or young SCLM which has been recently overprinted by a melt with ancient Nd signatures. Xenoliths in Group B from Big Creek may represent the residues of a Mesozoic partial melting event, associated with the formation of the Sierra Nevada Batholith, while those from Oak Creek, being hot and fertile, represent recent mantle, which replaced the older, cooler and depleted SCLM in response to "delamination" or large-scale extension in Western USA since the Miocene. The radiogenic Sr of Group C might result from contamination of originally depleted mantle xenoliths by Sr enriched components.