MID-MIOCENE SILICIC VOLCANSIM IN THE IDAHO-OREGON-NEVADA REGION AS A WINDOW INTO CONTINENTAL CRUST FORMATION AND MODIFICATION

We report the preliminary results of an investigation designed to address issues of crust formation and modification during intracontinental volcanism in the northwestern United States; specifically within the Idaho-Oregon-Nevada (ION) region. This region is characterized by mid-Miocene silicic volcanism associated with the onset of regional flood basalt volcanism and extension. The silicic volcanism and its continuation through time to the NE (Snake River Plain-Yellowstone) and NW (High Lava Plains) is related to mantle upwelling behind an active magmatic arc, basalt magma intrusion into the crust, melting of heterogeneous lithosphere, and mixing of melts from heterogeneous sources. ION region bimodal volcanism also provides a glimpse into complex magmatic processes occurring within heterogeneous transitional lithosphere at the western boundary of the Wyoming craton. In order to identify and quantify the sources and processes responsible for ION region silicic magmatism we have undertaken an integrated petrologic, geochemical and isotopic investigation of volcanic glass separates complemented by whole rock analyses from five specific eruptive centers selected to represent spatial, temporal (16.5 to 13.5 Ma), and compositional diversity. Pure glass separated from rhyolite flow and ash-flow vitrophyres is utilized in order to facilitate comparisons with data derived from regional fallout tephra and to best represent the pre-eruptive magma compositions.

Major and trace element and Sr, Nd, and Pb isotope data for 23 glass separate - whole rock pairs representing important eruptive units from the ION area silicic centers clearly illustrate the variable imprint of inherited crystals and the role of open system magmatic processes, allow for estimates of the relative proportions of mantle-derived and crustal-derived materials involved in silicic magma evolution, and provide a more detailed picture of the lithospheric architecture within the transition from Proterozoic-Archean to Mesozoic and younger lithosphere. The latter is further enhanced by the combined evaluation of Nd-Hf isotope systematics in the glass separates. Ongoing work is targeting selected eruptive products for mineral chemistry and mineral isotope analyses and crystal isotope stratigraphy studies.