Mid-Miocene Rhyolites of Eastern Oregon: Constraints for the Timing of Columbia River Basalt Volcanism and Associated Rhyolite Flare-up

Portland State University, Portland OR

Investigators

Abstract

The Pacific Northwest of the United States is an area that has been affected by volcanic eruptions for millions of years. Volcanic activity is a direct consequence of processes that take place in the Earth's deep interior and, thus, studying volcanic rocks provides an important window into understanding fundamental aspects of the dynamic behavior of the earth. A particularly volcanic active period in the Pacific Northwest was from about 17 million to 14 million years ago. During this period, Oregon and Washington states were inundated by the basaltic lava flows of the Columbia River Basalt (CRB, in places hundreds of feet thick and covering in excess of 150,000 square miles. The cause of CRB volcanism has been debated for decades and researchers favor two alternative hypothesis. One is that melting is caused by a deep seated "mantle plume" originating hundreds of miles deep near the core/mantle boundary. The second interpretation is that melting takes place at shallower levels in the Earth's mantle, triggered in part by the active tectonics of the region. One important aspect in this discussion is how the thermal anomaly, as recorded by this voluminous volcanism, is behaving in time and space, in other words where did volcanism start and how did activity spread geographically with time. Just in the last few years, it has become apparent that CRB volcanism did not only produce basaltic lava flows but also rhyolite magmas. This research addresses primarily the timing of the newly recognized component of CRB volcanism based on radiometric age determinations. It is designed to provide a better understanding of where CRB volcanism started and how it spread. Volcanic vents for rhyolite magmas point more directly to a basalt magma at depth while fissure eruptions that fed basalt lava flows can happen long distances away from where basalt magmas from the Earth's mantle rose to the surface. The primary focus of this field and laboratory study is to fully document CRB related rhyolite volcanism of eastern Oregon in terms of distribution, composition, and timing focusing on a series of understudied mid-Miocene rhyolite centers. This will test the validity of current notions of a northward migration of co-CRB rhyolite volcanism starting at the McDermitt volcanic field and adjacent areas along the Oregon-Nevada border. This in turn will have implications for the significance of observed age progression in floodbasalts of the CRBG. McDermitt rhyolites have traditionally been viewed as the oldest rhyolites related to the CRB province, predating those that erupted further north in Oregon. Fieldwork, augmented by geochemical and petrographic data, will establish stratigraphic sequences at selected centers. After a conceptual framework of the evolution of a center has been established, a subset of samples will be selected for age analyses by 40Ar/39Ar single-crystal laser-fusion and incremental heating methods. Emphasis will be placed on dating stratigraphic sections where rhyolites are intercalated with CRB lavas in order to precisely constrain timing of CRB volcanism with attention to constraining the controversial age of the Grande Ronde Basalt.