EARLY CORDILLERAN ARC DEVELOPMENT RECORDED IN LATE PERMIAN AND EARLY TRIASSIC PLUTONS OF THE NORTHERN MOJAVE DESERT, CALIFORNIA

Late Permian – Early Triassic plutons in the northern Mojave Desert record the onset of subduction-related magmatism that developed following the transition of the southwestern Laurentian margin from a transform to a convergent system. Seventeen new zircon U-Pb ages from the El Paso Mountains and Lane Mountain region of the Mojave Desert record nearly continuous magmatism occurring between ca. 275 and 240 Ma. These ages are older than the earliest arc plutons documented in the southern Sierra Nevada region to the north (256 – 248 Ma; Saleeby and Dunne, 2015) and in the Transverse Ranges to the south (~250 – 210 Ma; Barth and Wooden, 2006). They overlap, however, with Permian arc plutons documented in Sonora, Mexico. Dated plutons are intermediate to felsic in composition, calcic to calc-alkalic, metaluminous to weakly peraluminous, and generally have chemistries consistent with generation in a continental arc setting. Initial whole rock Sr isotopic ratios range from 0.7035 – 0.708 and initial 𝜀Nd values range from +3 – -5. Initial Hf isotope ratios in zircon range from +13 – -3 and correlate well with 𝜀Nd. In all isotopic systems, values are the most evolved in the oldest plutons (275 – 260 Ma), becoming more primitive in the Early Triassic. The overall primitive nature of all isotope systems suggests little to no involvement of Precambrian continental crust and / or miogeoclinal strata derived from cratonic North America. Instead, whole-rock isotope trends indicate magmas are two-component mixtures of depleted mantle and local Paleozoic metasedimentary basement. This is supported by the presence of Proterozoic and Early Paleozoic zircon inheritance recorded in some plutons. In contrast, a 175 Ma pluton adjacent to the Permo-Triassic intrusive suite in the El Paso Mountains has evolved isotope values (Sr_i = 0.7083; 𝜀Nd_i = -9; avg. 𝜀Hf_i = -8). These data suggest that the early arc developed outboard of continental North America but was subsequently juxtaposed against the continental margin by Middle Jurassic time. Regional pluton age patterns suggest that arc magmatism initiated in restricted areas of the southwestern Laurentian margin (northern Mojave, Sonora) and then migrated north and south ultimately becoming a continuous arc by Jurassic time.