Reconstructing the Physical Record of a Four-Million-Year Volcanic System : Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon

approved: __________________________________________________________________________ Adam J.R. Kent Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. 171 mapped stratigraphic units over an area of 150 km2 reveal four periods of volcanic activity resulting in diverse composition lavas ranging from ~48 to 72 wt% SiO2. Eruptive periods are divided into (1) ~4.0 – 0.78 Ma; (2) 0.78 – 0.3 Ma; (3) 0.3 – 0.02 Ma; and (4) eruptions of the past 13,000 years. Repeated glaciations over the past 800,000 years have sculpted the landscape of the MJA and include the Pleistocene glaciations of Jack Creek (early Wisconsin) and Cabot Creek (late Wisconsin), and the Holocene glaciation of Jefferson Park. Anomalously glassy lava flows, columnar jointing, and streamlined shapes are lithologic evidence of intraglacial eruptions in numerous units, including the andesite of Whitewater Creek (Qawc), andesite and dacite of Park Butte (Qapb; Qdpb), and the basaltic andesite of Whiskey Creek (Qbawh). Mineral compositions and textures are highly variable among the four units. Patchy to oscillatory zoning in plagioclase and dissolution cores and partially resorbed rims in amphiboles indicate pervasive disequilibrium conditions. Feldspar (~An35-98) and amphibole (~1.1-1.5 IVAl) compositions are relatively continuous across a broad range, and pyroxene compositions are typically ~En42-49 and En65-74. Phenocryst assemblages of units Qawc, Qapb, Qdpb, and Qbawh were probed to assess pressure and temperature conditions of pre-eruptive magmas in the MJA. Estimates from amphiboles, feldspars, and pyroxenes indicate temperatures ranging from ~650 to 1100 °C. Pressure estimates indicate crystallization depths of ~3 to 25 km, with the majority of crystallization occurring between ~15 and 25 km. Pyroxene temperatures (Putirka model) are always the highest, (~950-1150 °C) and plagioclase-amphibole pair (HBAS model) temperatures are the lowest (~650-875 °C), with amphibole temperatures (Ridolfi model) falling in between those ranges. Calculated partition coefficients of Sr and Ba from this study range from 1.5 to 6.75 and 0.12 to 1.00, respectively, in close agreement with calculated partition coefficients of Bindeman et al. (1998) and Dohmen and Blundy (2014). Reconstructed Sr concentrations range from 227 to 799 μg/g, which is inconsistent with the melting of a Sr-rich andesite end-member produced at 30-40 km depth as proposed by Conrey et al. (2001). The diverse spread of data reported here suggests complex petrologic mixing processes predominantly occurring in the midto upper crust beneath the MJA and contributes to the understanding of pre-eruptive magmatic conditions in the Cascade volcanic arc. © Copyright by Jennifer DiGiulio November 19, 2015 All Rights Reserved Reconstructing the Physical Record of a Four-Million-Year Volcanic System: Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon by Jennifer DiGiulio