Date of Award

8-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Matthew Gorring

Committee Member

Tanya Blacic

Committee Member

Stefanie Brachfeld

Abstract

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 mafic magmas. This composite shield dramatically exceeds the volume of all other 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). Proposed tectonic models that explain the origin of the volcano are as follows: 1. Mantle decompression melting from lithospheric extension (Xue and Allen, 2006); 2. Slab window or tear in the lithosphere (Tian and Zhao, 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 whole-rock geochemical data has been obtained by inductively coupled plasma mass spectrometry (ICP-MS) for major and trace elements. Isotopic analyses of eight (8) samples were conducted via TIMS and MC-ICPMS. Petrographic analyses of rock thin sections exhibit olivine and plagioclase phenocrysts throughout the sample majority. Major element classification on a silica-alkali diagram shows that basalts, trachybasalt, basaltic andesites, basaltic trachyandesites and andesite (SiO2=48.2-57.0%; Na2O+K2O=3.2-6.3%) 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.

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