FORMATION OF THE MOJAVE CRUSTAL PROVINCE IN A BACK ARC BASIN: DISCERNING TECTONIC PROCESSES WITH PAIRED U-PB-HF ANALYSES FROM DETRITAL AND PLUTONIC ZIRCON

The formation of the Mojave crustal province has been a persistent enigma for tectonic models of the Proterozoic tectonic history of southwestern Laurentia. New paired U-Th-Pb geochronologic and Lu-Hf isotopic analyses of zircon from the oldest metasedimentary rocks and oldest plutonic rocks of the province provide insight into its genesis. Detrital zircon age populations in the Death Valley region are dominated by 1.85 Ga detritus, as well as 2.4-2.7 Ga grains. This detrital zircon signature is similar in metasedimentary samples from across the Mojave province, and we suggest that together these metasedimentary rocks comprise a regionally extensive turbidite basin that we propose to call the Vishnu basin. The Vishnu basin extends from central Arizona to the Transverse Ranges in California, and likely beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. Plutonic rocks that intrude metasedimentary rocks of the Vishnu basin analyzed in this study range in age from 1791 to 1691 Ma. The initial ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of zircon separated from the oldest plutonic rocks yield εHf(t) values ranging from ~-7 to -1 at 1.79 Ga. Based on the U-Pb-Hf systematics of inherited cores and magmatic overgrowths in the oldest plutonic rocks, we interpret them to have been derived, at least in part, from melting of Vishnu basin sediments. Alternatively, a lower crustal substrate similar in age and composition to the crust which provided the source for Vishnu basin detritus cannot be ruled out. Initial ¹⁷⁶Hf/¹⁷⁷Hf ratios become more radiogenic through time, indicating progressive depleted mantle contributions to Mojave province magmatism. This trend culminates with εHf(t) values that range from 0 to +10 at ~1700 Ma, synchronous with peak deformation and metamorphism throughout the province. We interpret these results as recording the formation of a back arc basin system that closed during the Yavapai orogeny.

Vishnu basin detrital zircon age spectra and Hf compositions are similar to those of Australian and/or Antarctic Archean crust which may have been adjacent to southwestern Laurentia at ~1.75 Ga. We favor Nuna configurations which place those cratonic blocks adjacent to Laurentia in the Paleoproterozoic, and posit amalgamation of the Nuna supercontinent by that time.