A Case for Delamination of the Deep Batholithic Crust beneath the Sierra Nevada, California

Abstract

Surface exposures as well as deep-crustal and upper-mantle xenoliths constrain the composition of the lithospheric column beneath the Sierra Nevada mountain range (California) as it resulted from the generation of the Mesozoic Sierra Nevada batholith (SNB). After the cessation of magmatism at ∼80 Ma, the SNB consisted of a ∼30 to 35 km thick granitic crust underlain by a batholithic “root,” a ∼70 km thick sequence of mafic-ultramafic, mainly eclogite-facies cumulate and residues. The deeper root assemblages consist largely of garnet and pyroxenes that precipitated as igneous cumulate phases during the SNB magmatism. The root assemblages were present beneath the SNB as recently as ∼8 to 12 Ma, when they were sampled as xenoliths in fast-ascending magmas erupted through the batholith. Several lines of evidence suggest that the eclogitic root may have disapeared from beneath the SNB since Miocene time, leading to a major change in the lithospheric column. There are no garnet-bearing xenoliths in the Pliocene and Quaternary volcanic rocks; instead, all xenolith lithologies found in the younger volcanic outcrops are peridotitic, have equilibrated at depths between 35 and 70 km, possess locked-in temperatures of ∼1150 to 1200°C, and display an asthenospheric-like adiabatic P-T trend. Some of the Pliocene uppermost-mantle peridotitic xenoliths contain exotic silica-rich glass inclusions that may have originated by partial melting of the eclogitic root. Geophysical evidence suggests that anomalously high seismic velocity may represent eclogitic bodies present at depths of 100 to 200 km beneath the SNB. All of these observations indicate that the “eclogitic” root may have detached and delaminated (sunk) into the underlying mantle, a process compensated by diapiric rise of asthenospheric peridotitic material to the base of the shallow (∼35 km) remnant crust. The delamination hypothesis is consistent with observations documenting the existence of a shallow Moho, a low-velocity, partially molten upper mantle observed today beneath the SNB, a gradual change in Miocene volcanism in the Sierra toward more primitive compositions, and significant late Miocene-Pliocene uplift in the area. If the magmatic arc has indeed lost its root, delamination is an important mechanism in the differentiation of the continental crust at Cordilleran-type margins. The present-day crustal column in the Sierra (the SNB) is a mass extracted from the Earth's mantle predominantly during the Phanerozoic, although not necessarily only during batholithic magmatism. The ∼35 km thick present-day crustal composition of the Sierra Nevada is similar to, or more evolved than, the average continental crust.