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

美国太平洋西北部是一个几百万年来一直受到火山喷发影响的地区。火山活动是地球深部过程的直接结果，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窗口。太平洋西北部的火山活跃期大约在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，有些地方厚达数百英尺，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，研究人员支持两种替代假设。其一是融化是由深达数百英里的“地幔热柱”引起的，该热柱位于地核/地幔边界附近。第二种解释是，熔融发生在地幔较浅的水平，部分是由该地区的活动构造引发的。这一讨论的一个重要方面是，这一巨大的火山活动记录的热异常在时间和空间上是如何表现的，换句话说，火山活动是从哪里开始的，活动是如何随着时间在地理上传播的。就在最近几年，CRB火山活动不仅产生了玄武岩熔岩流，还产生了流纹岩岩浆，这一点变得很明显。这项研究主要讨论了根据放射性年龄测定新发现的CRB火山活动组成部分的时间。它的设计是为了更好地了解CRB火山活动从哪里开始以及如何传播。流纹岩岩浆的火山喷口更直接地指向深处的玄武岩岩浆，而提供玄武岩熔岩流动的裂隙喷发可能发生在距离地球地幔玄武岩岩浆上升到地表很远的地方。这项野外和实验室研究的主要重点是从分布、成分和时间上全面记录俄勒冈州东部与CRB有关的流纹岩火山作用，重点是一系列未被充分研究的中中新世流纹岩中心。这将检验目前关于共同CRB流纹岩火山从麦克德米特火山场开始向北迁移的概念的有效性，以及沿俄勒冈-内华达边界的邻近地区。这反过来将对CRBG泛滥玄武岩中观察到的年龄进程的重要性产生影响。麦克德米特流纹岩传统上被视为与CRB省有关的最古老的流纹岩，早于俄勒冈州更北的那些流纹岩。野外工作，加上地球化学和岩石学数据，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后，将选择一部分样品进行40Ar/39Ar单晶激光聚变和增量加热法的年龄分析。重点将放在流纹岩与CRB熔岩夹层的地层剖面上，以精确地限制CRB火山活动的时间，并注意限制有争议的格兰德-隆德玄武岩的年龄。

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.