Fragments of the mantle and crust from beneath the Sierra Nevada batholith: Xenoliths in a volcanic pipe near Big Creek, California

摘要

A Miocene volcanic pipe that cuts Cretaceous granodiorite is exposed in the central Sierra Nevada near the town of Big Creek, California. The volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent, resulting in a "pseudo-andesitic" composition. The conduit cut through the entire thickness of the Sierra Nevada batholith, as well as the underlying lower crust and upper mantle, and the magma entrained a diverse, polygenetic assemblage of igneous and metamorphic xenoliths, including peridotites, eclogites and granulites, schists and hornfels, and gabbroids and granitoids. Some xenoliths of the high-grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the Sierra Nevada. Garnet granulites, the most abundant rock type of the mafic high-grade metamorphic assemblage, and eclogites are restricted to the central area of the pipe. These xenoliths are essentially bimineralic, consisting of colorless to pale green clinopyroxene and garnet; eclogites contain brilliant emerald green clinopyroxene. The clinopyroxenes, which have a broad range of compostions [Mg/ (Mg + Fe) = 0.45-0.90], have a >20% jadeite component in eclogites. Garnets also have a range of compositions [Mg/(Mg + Fe) = 0.30-0.70], with those from eclogites being the most Fe rich. One of the xenoliths contains Ca-rich garnet (grs + adr > 90%). Feldspathic granulites contain plagioclase of An_45-70, and some lack garnet. Hornblende occurs in some granulites, and a few amphibolites are present. Likely protoliths are ocean-floor basalts for the eclogites and Precambrian limestone for the grossular-clinopyroxene rock. The Big Creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western Sierra Nevada.