JURASSIC BACK-ARC MAGMATISM IN THE EASTERN GREAT BASIN: A PRODUCT OF SLAB-TEAR DUE TO LOCALIZED ARC COLLISION AT THE PLATE BOUNDARY

Metamorphic core complexes of eastern Nevada (Ruby Mtns., Snake Range) reflect extension of crust with a long and complicated history. Jurassic magmatism is an important, but poorly understood, element of this history. Jurassic plutons of the core complexes form part of an anomalous zone of back-arc magmatism that extends from central Nevada to western Utah, far inland of the convergent boundary of the western Cordillera. Although wide in E-W extent, the Jurassic back-arc magmatic belt is spatially-restricted in N-S extent from 38.5°-42° N latitude. Most elements of this belt consist of plutons, although some volcanic rocks are found in the Cortez Range. Intermediate to felsic compositions are characteristic. We have obtained U-Pb zircon geochronology from most of the back-arc igneous rocks (24 sites), using laser-ablation ICPMS (Arizona LaserChron Center) and SHRIMP-RG (Stanford-USGS). Our new age data indicate that back-arc magmatism is narrowly restricted in time to the interval 166-158 Ma, with ages becoming generally younger from west to east. We have also obtained geochemical data on all the dated samples; these data indicate a common tendency towards adakitic compositions, and derivation from sub-continental lithospheric mantle. Finally, the back-arc igneous province is associated with multiple indicators of regional high heat flow, including syn-intrusive metamorphism and deformation. Collectively, these characteristics suggest that a narrow zone of the back-arc was subjected to influx of hot asthenospheric mantle, resulting in localized and short-lived melting of the overlying lithosphere, in response to development of a tear in the underlying subducting slab. Development of a slab tear beneath the NE Great Basin in the late Middle-early Late Jurassic provides intriguing limits on possible models for Jurassic tectonics along the Cordilleran margin, on terrane locations, and on the geometry and evolution of offshore microplates. Future studies are needed to evaluate how this back-arc magmatic pulse affected the crust and influenced subsequent tectonism in the region. Detailed age mapping of individual zircons for inherited cores is planned, and will provide information about the crust forming basement for the igneous rocks.