PERMIAN MAGMATISM IN THE NORTHERN MOJAVE REGION AND CONSTRAINTS ON THE TIMING AND TECTONICS OF SUBDUCTION INITIATION ALONG THE SOUTHWEST LAURENTIAN MARGIN (Invited Presentation)

Changes in plate motion along the truncated passive margin of southwest Laurentian led to the initiation of subduction in Permian time, previously thought to have occurred at ca. 255 Ma based on early arc intrusive ages and the timing of high pressure metamorphism in ophiolitic assemblages. New geochronology of arc-related plutons extends the record of Cordilleran magmatism to ca. 275 Ma in the northern Mojave region. Ages from 17 intrusive bodies show that early arc magmatism occurred in 3 pulses at 275-270 Ma, 260-250 Ma, and 244-238 Ma. These plutons are intermediate to felsic, calcic to calc-alkalic, metaluminous to weakly peraluminous, and are geochemically consistent with generation in a continental arc setting. The isotopic character of the dated intrusions, however, changes through time from moderately evolved (Sr_i > 0.706, ε_Nd = -3 to -5, ε_Hf (Zrn) = -4 to +4) in the oldest plutons to primitive (Sr_i = 0.703-0.704, ε_Nd = +3 to +4, ε_Hf (Zrn) = +5 to +12) in the Early Triassic plutons. Sr-Nd pluton signatures are consistent with derivation from a 2-component mixture of depleted mantle and local Paleozoic metasedimentary host rocks. The contribution of supracrustal material to melts was up to 40% in 275 – 270 Ma plutons and ≤ 10% in Early Triassic plutons. Results from modeling of the Sr-Nd data precludes involvement of Proterozoic continental crust. Together, these results suggest that subduction nucleated in the Permian in a restricted region of the northern Mojave before migrating north and south to produce the Permo-Triassic arc documented in the broader Transverse Ranges – Mojave – southern Sierra Nevada region. As plate vectors rotated to a convergent orientation in the Permian, peri-allochthonous slivers of outboard oceanic lithosphere covered by off-shelf passive-margin strata were amalgamated to the margin. Those accreted slivers became the heterogeneous upper plate to the nascent subduction system that generated the early arc magmas. Temporal trends in the isotopic systems suggest that immediately following subduction initiation, contractional deformation was dominant in the upper plate, leading to a greater involvement of melt-fertile crustal material in the earliest melts. By Early Triassic time, the upper plate transitioned to an extensional regime, producing more voluminous, mantle-like magmas.