ORIGIN AND TECTONIC IMPLICATIONS OF ACROSS-STRIKE GEOCHEMICAL VARIATIONS IN THE EOCENE ABSAROKA VOLCANIC PROVINCE, USA

In order to better understand the origin and significance of across-strike geochemical variations in the Eocene Absaroka Volcanic Province (AVP), this study compares compositional data for magmatic rocks from two eruptive centers that define type localities for major stratigraphic subdivisions of the Absaroka Volcanic Supergroup. Geochemical variations for evolved lavas indicate that the calc-alkaline differentiation trend at Washburn volcano results from mixing compositionally diverse mafic and silicic magmas, whereas the shoshonitic trend at the more easterly Sunlight volcano reflects assimilation-fractional crystallization. The mantle source for both volcanoes is, as shown by incompatible trace element contents and radiogenic isotopic ratios of mafic lavas, largely within ancient depleted lithospheric mantle later enriched by a metasomatic event characterized by low Sm/Nd and Rb/Sr ratios. The approximate timing of the enrichment event, as calculated from Nd isotopic data (2.2 – 2.0 Ga), is similar to estimates from other magmatic fields in the Wyoming Province. Large variations in trace element ratios of mafic lavas involving LILE and LREE that are not obviously reflected in isotopic ratios also indicate a recent (< 100 Ma) metasomatic enrichment event. Superficially, the mantle source for Sunlight volcano appears to contain larger amounts of the young metasomatic component, as measured by high LILE/HFSE ratios. Parental magmas at both volcanoes have similar HFSE contents and HFSE/HREE ratios, however, suggesting comparable degrees of source region melting and metasomatism. Rather than variable degrees of metasomatism and partial melting, the geochemical differences between the mafic rocks are best explained by compositional heterogeneity in metasomatic fluids inherited from breakdown of different phases in hydrous peridotite. A consistent geodynamic model to account for generation of AVP magmas involves metasomatism of asthenosphere and subcontinental mantle lithosphere during shallow subduction of the Farallon plate during the Late Cretaceous through Paleocene, followed in the Eocene by foundering of the slab and primary basalt generation through major heating associated with upwelling and influx of hot asthenosphere.