METAMORPHIC EVOLUTION OF LOWER CRUSTAL XENOLITHS FROM THE MOJAVE DESERT, CALIFORNIA: A PETROCHRONOLOGIC APPROACH

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 information about the pre-extensional Tertiary lower crust that is otherwise inaccessible for direct study. This 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 main focus of this study, consist of quartz + garnet + rutile + plagioclase + K-feldspar + biotite + kyanite + sillimanite, and trace zircon, monazite, and apatite. Notable microstructures include large, resorbed kyanite blades and protoleucosomes with late growth-zoned sillimanite. Microstructures and mineral chemistry are integrated with phase equilibria modelling to provide insight into the P–T evolution of the Mojave lower crust recorded by the xenoliths. Mineralogy and microstructures are consistent with mica dehydration melting in the kyanite stability field, significant melt loss at peak granulite facies metamorphism, and minor melt crystallization in the sillimanite field upon cooling. The proposed P–T path suggests 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. The presence of late sillimanite suggests that the lower crust experienced decompression following peak P–T.

Inheritance and timing of metamorphism is constrained by U-Pb and Lu-Hf isotopes in zircon, and U-Pb and Sm-Nd in monazite. Zircon cores preserve Proterozoic inheritance linked to formation of the Mojave crust. Zircon rims and monazites yield Cretaceous U-Pb dates consistent with the timing of the Laramide Orogeny and metaluminous to peraluminous granite crystallization in the Mojave. Cretaceous εHf and εNd compositions are consistent with growth from a melt extracted from an evolved crustal reservoir. The dates suggest that decompression following peak P–T can be attributed to crustal thinning after the Laramide Orogeny. Trace element concentrations in zircon and monazite provide a link between U-Pb ages and prograde garnet growth to establish a cohesive P–T–t history for the Mojave xenolith suite.