BACK-ARC JURASSIC MAGMATISM IN THE NORTHERN GREAT BASIN: A SUMMARY OF U-PB ZIRCON GEOCHRONOLOGY, GEOCHEMISTRY AND STRUCTURAL RELATIONS, WITH IMPLICATIONS FOR TECTONIC MODELS AND YOUNGER GOLD MINERALIZATION

During the Jurassic, magmatism spread anomalously far inboard of the U.S. Cordillera volcanic arc. We now have a nearly complete data set on the geochronology and geochemistry of these back-arc igneous rocks (mostly plutons) that we can use in evaluating their origins. Our compilation includes data from 20 plutons and 2 volcanic units, and spans the region from NW UT to central NV. U-Pb zircon geochronologic data were obtained from the SHRIMP facility at Stanford Univ., and the LA-ICPMS facilities at the Univ. of AZ and Univ. of CA Santa Cruz. These data indicate that back-arc magmatism occurred over a restricted time interval (166-157 Ma), with ages younging from west to east. Petrologic and geochemical analyses of the plutons indicate they are distinct from arc igneous rocks. They are mostly biot/hbl granodiorite-qtz monzonite, except in the Ruby Mtns. and Snake Range where 2-mica granites are found. Plotted values of trace and REE indicate the plutons are K-rich, adakitic, and most similar to post-collisional granites formed by slab failure. Sr-Nd isotopic data plot as only slightly more evolved than bulk earth, implying a magma source from subcontinental mantle with little crustal contribution. This conclusion is supported by the scarcity of inherited zircon cores in most analyzed samples; only in plutons of the Ruby Mtns. and Snake Range are inherited cores abundant. Back-arc magmatism was not associated with any consistent pattern of regional deformation or metamorphism. Some plutons are syntectonic with shortening, some with both shortening and extension, and some (Cortez Mtns) with caldera development, but most are not associated with any deformation except minor strain in contact aureoles. The picture that emerges from these data is of a dynamic event beneath the back arc region, leading to a pulse of mantle-derived magmas that were passively emplaced into overlying crust. We argue that the best explanation was break-off or tear of the subducting slab related to arc collision further west. This model has interesting implications for mineralization, because it would cause an influx of fertile asthenospheric melts that may have enriched the overlying lithosphere in metals (including gold), thus setting the stage for remobilization of these metals to higher crustal levels during the Cenozoic.