Construction of a trans-crustal magma system: Building the Bear Valley Intrusive Suite, southern Sierra Nevada, California

Abstract

The Bear Valley Intrusive Suite (BVIS) in the Southernmost Sierra Nevada exposes a trans-crustal magmatic system that spans emplacement pressures from 3-10 kbars, and was emplaced at all crustal levels between 100.1-101.5 Ma. As such, it represents an unparalleled snapshot of magmatic processes within continental arc crust. In this study we present new field observations combined with whole rock geochemistry that show a fundamental dichotomy within the BVIS. The lower crust of the BVIS is dominantly composed of mafic cumulates that preserve originally shallow to horizontal magmatic fabrics, while the middle and upper crust is dominantly composed of voluminous homogeneous tonalites with steep fabrics. Using a stochastic model of melt fractionation and extraction, we show that these observations strongly constrain the P-T paths along which BVIS magmas must have been emplaced: to create the observed abrupt transition from mafic lower crust to felsic middle and upper crust, evolving melts must cool nearly isobarically in the lower crust before being rapidly emplaced in the upper crust along near-isothermal paths. Our modeling results show that the BVIS magmas must have cooled below 900°C near 7 kbars depth.These modeling results additionally require that the BVIS was emplaced into an unusually warm lower crust. The Sierra Nevada Batholith is typically characterized by felsic crust with low seismic velocities between 6.0-6.5 km/s to at least 30-35 km depth, significantly deeper than the observed transition at ∼28 km depth in the BVIS to mafic cumulates with calculated velocities >7.0 km/s. Given this observation, we conclude that the bulk of the Sierra Nevada Arc magmas must have stalled, cooled and differentiated at greater depths than the BVIS magmas.