Crustal growth and tectonic evolution of the Mojave crustal province: Insights from hafnium isotope systematics in zircons

Abstract

Coupled U-Pb ages and Hf isotopic ratios in zircons from Proterozoic basement and three siliciclastic cover sequences in southern California provide important 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 crystalline basement of the Mojave crustal province formed from four main components: 1) mantle components ranging from depleted to moderately enriched; 2) metasedimentary framework rocks derived from 2.6 to 2.4 Ga and 2.0 to 1.8 Ga crust; 3) 1.79 to 1.64 Ga intrusive rocks that refl ect mixing of mantle-derived melts and crust; and 4) Mesoproterozoic (1.4 to 1.2 Ga) anorthosite and granitic to syenitic intrusive rocks. 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 derived from Mojave province basement. In contrast, Neoproterozoic quartzites of the Big Bear Group had a distal provenance, either an unexposed, older, western subprovince of the Mojave crustal province lacking ca. 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 refl ect provenance from an integrated transcontinental drainage network delivering sediment to the craton edge and westward into the Cordilleran miogeocline. © 2012 Geological Society of America.