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

9725166俄勒冈州中部和东南部熔岩平原砂砾火山岩的独特之处在于：(1)它们位于盆地和山脉省的西北部边缘；(2)它们反映了黄石-蛇河平原省晚第三纪和第四纪的火山作用，因为硅质火山活动向西较年轻；(3)它们是强烈的双峰式火山岩，由具有洋性的玄武岩和高硅质流纹岩组成。由于这些特征，HLP已被认为是理解美国西部构造和火山作用的一个突出问题。HLP岩浆形成的构造模式与以下几个方面有关：a)黄石地幔热柱的演化；b)与盆地和山脉省晚第三纪发展有关的伸展环境；以及c)与卡斯卡迪亚俯冲带有关的弧后环境。为了区分这些模型，我们将研究玄武岩火山作用潜在的年龄递进趋势，确定其与伴生的硅质火山作用的成因联系，评估HLP玄武岩岩浆活动所涉及的地幔来源，并确定地理和时间地球化学变化是否类似于黄石热点地区。这些问题将通过对高熔岩平原代表性地区的玄武岩进行综合野外、年代学和地球化学研究来回答。用高精度的40Ar-39Ar方法对四个地区进行了填图和测年。还将分析最重要的样品的常量和微量元素组成以及锶、钕、铅、氧和氦的同位素比率。

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.