PETROGENETIC EVOLUTION DURING CORDILLERAN OROGENESIS: CONSTRAINTS FROM THE NORTH AMERICAN CORDILLERA

Petrogenic and thermochronologic study of Jurassic–Eocene retroarc plutons in the North American Cordilleran hinterland provides key constraints on the timing and style of magmatism and upper-crustal deformation in the Sevier–Laramide orogen. Results from spatially isolated sites across the hinterland support the general existence of high-magnitude Cretaceous and Eocene exhumation in the hinterland, but quantitative constraints on Cretaceous–Eocene cooling histories are limited. We present preliminary results from trace and rare earth element geochemistry, zircon and apatite U-Pb geochronology, and (U-Th)/He thermochronology of Jurassic–Eocene plutonic samples collected across a broad E–W transect of the Nevada–Utah hinterland (n=11). This approach helps constrain magmatic sources and differentiation histories of plutons that represent emplacement into a range of structural depths and during discrete orogenic phases. A combination of high- and low-temperature geothermometric and thermochronologic constraints provide quantitative estimates of mid- to upper-crustal unroofing in the hinterland. Preliminary geochemical results of Jurassic–Cretaceous plutons reveal: (1) broadly similar whole rock and trace element geochemistry suggesting correlative petrogenic emplacement histories, (2) trace element variations are often related to proximal porphyry- and skarn-type alteration, and (3) the degree of alumina saturation is indicative of peraluminous plutons formed through partial melting of metasedimentary rocks. New apatite U-Pb geochronology and (U-Th)/He thermochronology indicates three prominent phases of exhumation during the Late Cretaceous, Paleogene, and Miocene. The preponderance of apatite grains with Cretaceous–Paleogene (U-Th)/He cooling ages are consistent with recent hypotheses of significant early, pre-Basin-and-Range, exhumation in the North American Cordilleran hinterland. This project will ultimately incorporate data from ongoing analyses and historical datasets to build time-space models of petrogenesis and upper-crustal exhumation for a broad swath of the hinterland, with the ultimate goal of improving geodynamic models of Cordilleran orogenesis.