PETROGENETIC ORIGIN OF MAFIC LAVAS AT NEWBERRY VOLCANO

Newberry Volcano (43.7°N, 121.3°W) is a Cascade rear arc volcano in central Oregon that covers nearly 2,000 km² and represents a volume of roughly 200 km³ of mostly magmas that dramatically exceeds the volume of all Cascade volcanoes, except for Medicine Lake Volcano in Northern California. Newberry began forming about 400 ka with the last eruption ending at 1.3 ka (Donnelly-Nolan et al., 2011). Preexisting tectonic models that explain the origin of Newberry Volcano are as follows: 1. Mantle decompression melting from lithospheric extension (Xue et al., 2006); 2. Slab window or tear in lithosphere (Tian et al., 2012); 3. Subduction-induced counterflow of the Yellowstone plume (Jordan et al., 2004). The research goal of this study is to test these models to better understand the mantle processes and petrogenetic origin of Newberry Volcano by analyzing the mafic lavas from sequential eruptions ranging in age from ~400 ka to 7 ka. Thirty-five (35) samples were collected and preliminary whole-rock geochemical data has been obtained by ICP-MS for major and trace elements. Petrographic analyses of rock thin sections exhibit olivine and plagioclase phenocrysts throughout the sample majority. Major element classification on a silica-alkali diagram shows an alkalic chemical signature with SiO2 content that ranges from 48.2-56.2% and Na2O+K2O ranges from 3.1-6.3%. The normalized REE concentrations of these samples depict parallel patterns similar to those of measured continental flood basalts from the Columbia River Group. Enrichments in LREE [La(18.2-58.0), Ce(17.9-46.6)] and depletions in HREE [Yb(6.8-15.1), Lu(6.4-14.8)] indicate enriched mantle source characteristics. Eu/Eu* ratios of 1.0-2.1 relate to modest negative Eu anomalies that indicate plagioclase crystallization was an important petrogenetic process. The evidence of multiple mantle sources can be explained by rear arc extension from the nearby Cascade arc setting and/or mantle plumes of Columbia River or Yellowstone. In order to further constrain the processes of formation and the mantle source characteristics, Sr-Nd-Pb isotope data may be collected via thermal ionization methods upon continued research.