CRUSTAL GROWTH AND TECTONIC EVOLUTION OF THE MOJAVE CRUSTAL PROVINCE: INSIGHTS FROM HAFNIUM ISOTOPE SYSTEMATICS IN ZIRCONS

Coupled U-Pb ages and Hf isotopic ratios in zircons from Proterozoic basement rocks and three siliciclastic cover sequences in southern California provide insights into the formation of the southern Mojave crustal province and its incorporation into southwestern Laurentia. Hafnium isotopic ratios measured in >800 zircons coupled with new and previously reported U-Pb ages suggest that the Mojave crustal province was formed from multiple components. Geochemistry of amphibolites and zircons with high initial Hf ratios in intrusive rocks record partial melting of both depleted and enriched mantle beneath the early crust of the province. Metasedimentary upper crustal framework rocks were derived from older, 2.6 to 2.4 Ga and 2.0 to 1.8 Ga crust. Intrusive rocks with ages of 1.79 to 1.64 Ga formed from crystallization of magmas generated by mixing between melts of depleted and enriched mantle and the pre-existing crust. Initial Hf isotopic ratios of detrital zircons in siliciclastic cover sequences suggest varying degrees of insularity of the Mojave province during assembly of southwestern Laurentia. The Mesoproterozoic Pinto Mountain Group appears entirely locally derived from within Mojave province basement. In contrast, Neoproterozoic quartzites of the Big Bear Group had a distal provenance, either derived from an unexposed, older, western subprovince of the Mojave crustal province lacking ~1.7 Ga magmatic rocks, or from a distinctive Paleo- and Mesoproterozoic basement province far to the east within Laurentia. Zircons in latest Neoproterozoic to Cambrian quartzites reflect provenance from an integrated transcontinental drainage network delivering sediment to the craton edge and westward into the Cordilleran miogeocline.