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

美国西北太平洋地区是一个数百万年来一直受到火山爆发影响的地区。火山活动是发生在地球内部深处的过程的直接后果，因此，研究火山岩为了解地球动力学行为的基本方面提供了一个重要的窗口。太平洋西北部的一个特别活跃的火山时期是大约1700万到1400万年前。在此期间，俄勒冈州和华盛顿州被哥伦比亚河玄武岩（CRB）的玄武岩熔岩流淹没，数百英尺厚，覆盖面积超过15万平方英里。CRB火山活动的原因已经争论了几十年，研究人员倾向于两种替代假设。一种是熔融是由一个深埋的“地幔柱”引起的，该地幔柱起源于核/幔边界附近数百英里深的地方。第二种解释是，熔融发生在地幔较浅的地方，部分是由该地区的活动构造触发的。在这个讨论中，一个重要的方面是热异常如何在时间和空间上表现出来，换句话说，火山活动从哪里开始，活动如何随着时间的推移在地理上传播。就在最近几年，CRB火山作用不仅产生玄武质熔岩流，而且产生流纹岩岩浆已经变得很明显。这项研究主要解决了新认识的CRB火山作用的放射性年龄测定的基础上的组成部分的时间。它的目的是提供一个更好的了解CRB火山活动开始的地方，以及它是如何传播的。流纹岩岩浆的火山口更直接地指向深处的玄武岩岩浆，而供给玄武岩熔岩流的裂缝喷发可能发生在远离玄武岩岩浆从地幔上升到地表的地方。本野外和实验室研究的主要重点是全面记录俄勒冈州东部与CRB有关的流纹岩火山活动，包括分布、成分和时间，重点是一系列未充分研究的中新世中期流纹岩中心。这将测试目前的概念，共同CRB流纹岩火山活动开始在麦克德米特火山场和邻近地区沿沿着俄勒冈州-内华达州边界向北迁移的有效性。这反过来又会对CRBG洪水玄武岩中观察到的年龄进展的意义产生影响。麦克德米特陨石坑传统上被认为是与CRB省有关的最古老的陨石坑，比俄勒冈州更北的陨石坑要早。野外工作，辅以地球化学和岩相学数据，将在选定的中心建立地层序列。在建立了一个中心演化的概念框架之后，将选择一部分样品通过40 Ar/39 Ar单晶激光熔融和增量加热方法进行年龄分析。重点将放在测年的地层部分，其中流纹岩与CRB熔岩夹层，以精确地限制CRB火山作用的时间，注意限制有争议的年龄的大回旋玄武岩。

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.