LOWER CRUSTAL ALUMINOUS XENOLITHS FROM THE MOJAVE DESERT, CALIFORNIA

A suite of lower crust xenoliths exposed in the North Piute Mountains in the Mojave Desert, preserve some complexity of 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 the Mesozoic (Laramide Orogeny).

Representative aluminous xenoliths of this suite consist of quartz + plagioclase + K-feldspar + garnet + biotite + rutile + kyanite, with rare sillimanite. Mineralogy and microstructures were documented using SEM-MLA maps of thin sections and CL intensity maps. Notable microstructures include large, resorbed kyanite blades, rare sillimanite inclusions in the garnet, and protoleucosomes with late sillimanite showing growth zoning in CL. These observations are consistent with biotite dehydration melting in the kyanite stability field and melt crystallization in the sillimanite field. In addition, the good preservation of the peak assemblage is consistent with melt loss during anatexis.

Mineralogy and microstructures along with mineral chemistry were integrated with phase equilibria modelling to infer a P–T path and provide further insight into the melting history of the Mojave lower crust. 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. It is predicted that 25 – 30% anatectic melt was removed from its source if melting began at the water saturated solidus, and only 2 – 3% melt is inferred to have remained in the rocks at the thermal peak. The results are consistent with an extensive anatectic melting event in the lower crust that may be the source of the Cretaceous granites in the region. The presence of late sillimanite suggests that the lower crust experienced decompression following peak P–T. Decompression may have resulted from crustal thinning in the Tertiary period subsequent to the Laramide Orogeny and Cretaceous granitoid emplacement.