Metamorphic evolution of lower crustal xenoliths from the Mojave Desert, southeastern California: a petrochronologic approach

Abstract

A suite of lower crustal xenoliths brought to the surface in a mid-Tertiary andesitic dike in the North Piute Mountains of the Mojave Desert, southeastern California, preserve fundamental geologic information about the pre-extensional Tertiary lower crust that is otherwise inaccessible for direct study. This xenolith suite consists of aluminous, quartzose, quartzofeldspathic, and mafic rocks, interpreted to represent Paleoproterozoic crustal material that experienced granulite facies metamorphism and anatexis during Mesozoic crustal thickening. The aluminous xenoliths, the primary focus of this study because of their mineralogy and thus suitability for thermobarometric calculations, consist of quartz + garnet + rutile + plagioclase + K-feldspar + biotite + kyanite + sillimanite, and trace zircon, monazite, and apatite. The mineralogy and microstructures are consistent with biotite dehydration melting in the kyanite stability field, significant anatectic melt loss at granulite facies conditions, and minor melt crystallization with kyanite and sillimanite upon cooling. Microstructures and mineral chemistry integrated with phase equilibria modelling suggest a P–T path with peak P of 1.2 to 1.3 GPa, and T of 850 to 870 °C, consistent with a pre-extensional Tertiary lower crustal depth of about 44 to 48 km, followed by decompression. Inheritance and timing of metamorphism are constrained by U–Pb and Lu–Hf isotopes in zircon, and U–Pb and Sm–Nd isotopes in monazite. Zircon cores preserve Archean and Proterozoic isotopic inheritance linked to formation of the Mojave crust. Zircon rims and monazites yield late Cretaceous U–Pb dates consistent with the timing of the Laramide Orogeny and metaluminous to peraluminous granite crystallization in the Mojave. Initial εHfi and εNdi values for zircon rim and monazite are consistent with growth from a melt extracted from an evolved crustal reservoir. The U-Pb dates suggest that decompression following peak P–T can most likely be attributed to crustal thinning after the Laramide Orogeny. Trace element composition of zircon and monazite provide a link between U–Pb dates and prograde garnet growth which in turn are used to establish a cohesive P–T–t history for the Mojave xenolith suite.