PETROGENESIS OF PLEISTO-HOLOCENE BASALTS FROM NEWBERRY VOLCANO, OREGON

Newberry Volcano (43.7°N, 121.3°W) is a Cascade rear-arc volcano in Oregon that covers nearly 2,000 km² and represents a volume of roughly 200 km³ of erupted volcanics. This composite shield dramatically exceeds the volume of all other Cascade volcanoes, except for Medicine Lake Volcano in California. Newberry began forming about 400 ka with the last eruption ending at 1.3 ka (Donnelly-Nolan et al., 2011). The research goal of this study is to better understand the mantle processes and petrogenetic origin of Newberry Volcano by analyzing mafic lavas from sequential eruptions ranging in age from ~400 ka to 7 ka. 35 samples were collected and geochemical data has been obtained by ICP-MS for major and trace elements. Isotopic analyses of 8 samples were conducted via TIMS and MC-ICPMS. Major element classification on a silica-alkali diagram shows that basalts, trachybasalt, basaltic andesites, basaltic trachyandesites and andesite (SiO₂=48-57%; Na₂O+K₂O=3-6%) provide evidence for crystal fractionation within the Newberry mafic suite and potentially a positive trend in SiO₂ over time. Enrichments in LREE (La/Yb=3-9), LILE (Sr/La=19-60; Ba/La=13-31) and depletions in HFSE (Ba/Ta=147-1064; La/Ta=11-36) indicate a role for subduction related processes and the possible addition of slab-derived fluids and/or sediments to the mantle source. ⁸⁷Sr/⁸⁶Sr (0.7031-0.7036), ¹⁴³Nd/¹⁴⁴Nd (0.5129-0.5130), ²⁰⁶Pb/²⁰⁴Pb (18.82-19.08), ²⁰⁷Pb/²⁰⁴Pb (15.56-15.60) and ²⁰⁸Pb/²⁰⁴Pb (38.43-38.61) indicate a slightly depleted, E-MORB-like asthenospheric source region beneath Newberry. The data presented report that Newberry basalts and basaltic andesites are chemically similar to calc-alkaline basalts from Medicine Lake Volcano (Donnelly-Nolan et al., 2008) and other arc basalts from the central Cascade Range (Bacon et al., 1997) and are most likely produced by rear-arc flux melting of the E-MORB-like asthenospheric wedge from dehydration of the subducting Juan de Fuca Plate.