Origin of Basaltic Magmatism in the Tectonically Enigmatic High Lava Plains of Southeastern Oregon

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9725166 Grunder Volcanic rocks of the High Lava Plains (HLP) of central and southeastern Oregon are unique because: (1) they lie at the northwestern margin of the Basin and Range Province; (2) they mirror Late Tertiary and Quaternary volcanism of the Yellowstone-Snake River Plain Province in that silicic volcanism is younger to the west; and (3) they are strongly bimodal, being composed of basalts with oceanic affinities and high-silica rhyolites. Because of these characteristics, the HLP have been idenfied as an outstanding problem in understanding tectonism and volcanism in the western U.S. The tectonic models invoked for the formation of the HLP magmas have been associated with : a) the evolution of the Yellowstone mantle plume; b) an extensional setting related to Late Tertiary development of the Basin and Range Province; and c) a back-arc setting related to the Cascadia subduction zone. In order to discriminate among these models we will investigate a potential age-progressive trend in basaltic volcanism, determine its genetic link to the associated silicic volcanism, evaluate the mantle sources involved in the basaltic magmatism of the HLP, and determine if geographic and temporal geochemical variations are analogous to the Yellowstone hotspot track. These questions will be answered with an integrated field, geochronological and geochemical study of basalts from representative areas in the High Lava Plains. Four areas have been selected for mapping and dating by high precision 40Ar-39Ar methods. The most important samples will also be analyzed for major- and trace-element composition and Sr, Nd, Pb, O and He isotopic ratios.