The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

Abstract

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.