A PRIMARY LITHOSPHERE SECTION FOR THE SIERRA NEVADA BATHOLITH

A primary lithosphere section is reconstructed for the Sierra Nevada batholith based on the convergence of rock types that occurs between the base of its southern oblique crustal section and batholith-related lower crust-mantle xenoliths that were entrained in Miocene lavas erupted through shallow levels of the batholith to the north. The section is best defined for rocks that formed/equilibrated at ca.100 Ma. The shallow-level exposures preserve coeval silicic volcanics while the deepest xenoliths are from ~125 km deep thus rendering an entire lithosphere thickness. The base of the section consists of garnet peridotite which at ~100 to 75 km depths includes lenses of garnet clinopyroxenite ("eclogite"). From ~75 to 45 km such eclogite dominates the section with lenses of garnet and shallower spinel peridotite. Between ~45 to ~35 km depths lies a mixture of subordinate eclogite, garnet granulite and a variety of batholithic gabbroids and granitoids. The upper ~35 km of the section is predominately tonalitic to granodioritic batholithic rock with subordinate wallrock screens. The roof of the batholith is predominately its own silicic volcanic cover, and there is no evidence for the batholith having spread out over a mid-or deep crust regional metamorphic substrate. Steep primary structures pervade the section to shallower than ~10 km, with a complex mixture of steep and flatter intrusions at shallowest levels. The granulitic and eclogitic rocks represent low and high pressure residues, respectively, left from felsic magma production from a polygenetic source consisting of crustal basement and cover strata components, and mantle wedge wet mafic intrusives. Much of the peridotite represents variably metasomatized and depleted asthenosphere hypothesized to have been emplaced into the mantle wedge section by a combination of cornerflow and wet rheologically controlled diapirism. The entire lithosphere including the crust was reconstituted during batholith production. The primary Sierran lithosphere was apparently quenched in this state at the latest Cretaceous onset of the Laramide tectonic regime.