MIDDLE TO LATE JURASSIC BACK-ARC MAGMATISM IN THE U.S. CORDILLERA: NEW AGE DATA AND TECTONIC IMPLICATIONS

Back-arc magmatism in the Middle to Late Jurassic is a distinctive feature of the U.S. Cordillera. Earlier magmatism was generally restricted to California, Oregon and westernmost Nevada, consistent with the typical width of magmatic arcs. During the episode of back-arc magmatism, this pattern was disrupted and Jurassic igneous rocks were emplaced as far east as western Utah. Intrusions are most common and vary from metaluminous gabbro to peraluminous granite. Tectonic implications of this magmatic episode have never been clear, partly due to ambiguous or undefined magmatic ages. Where previously dated, ages range from ca. 173-134 Ma (K-Ar), and ca. 165-155 Ma (U-Pb zircon). Most prior zircon ages were obtained using the TIMS method on bulk zircon separates. We collected new samples from most of the back-arc Jurassic igneous bodies, and obtained new U-Pb zircon ages using the SHRIMP-RG (Stanford University) where we could avoid analysis of inherited zircon cores. Our new results indicate that back-arc magmatism in NV-UT was restricted to the interval from 161-156 Ma except in the western back-arc region where we find ages of 165-163 Ma. These results are striking when considered in a more regional context. 1) Back-arc magmatism directly followed a major episode of contractional tectonism in NW Nevada that propogated eastward from the Early to Middle Jurassic (201-165 Ma). Current models suggest that this contractional event resulted from an arc-arc collision. 2) Back-arc magmatism occured over only a brief interval of time across a region that is currently over 300 km in width. 3) Jurassic shortening in NW Nevada and magmatism in the back-arc was followed by regional exhumation, erosion, and tectonic quiescence before the onset of the Cretaceous Sevier orogeny. These observations are incompatable with the model of DeCelles (2004), and are better explained by a model in which arc-arc collision in the Early to Middle Jurassic resulted in foundering and eventual detachment of the previously subducting oceanic lithosphere beneath Nevada. Following slab break-off, influx of hot asthenospheric mantle led to melting of the overlying mantle lithosphere and a brief but widespread pulse of magmatism in the back-arc. Subsequent regional uplift resulted from cessation of subduction and the new (hotter) thermal regime.