Evolution of the Cascades Subarc Mantle: Evaluation of Subduction Contribution Based on Petrology of Lavas and Mantle Xenoliths

9419018莱曼这项研究探讨了太平洋西北地区构造演化与卡斯卡迪亚火山弧下地幔的地球化学演化之间的关系。大约40 Ma左右，海山/大洋岛链(俄勒冈-华盛顿海岸山脉)与古近纪俯冲带碰撞后开始了级联火山作用，俯冲作用不久后向西迁移到现在的位置。从那时到现在，随着胡安德·富卡海脊系统接近大陆，会聚速度下降了五倍，俯冲板块的年龄也有了系统的下降。这些变化本应导致弧下地幔的渐进式和可预测的变化。计划通过两种方式测试这一俯冲模型：首先，P.I.计划确定整个弧史上喷发的原始前第四纪熔岩的主要、微量元素和同位素组成。这将在两个横跨级联范围的时间横断面上完成，在这些横断面上，曝光是最佳的，并且可以进行地质制图。其次，P.I.计划完成一项详细的岩石学研究，对来自华盛顿瀑布南部的可变交代地幔包体进行研究。其中一些岩石已经被渗入的钾质硅质熔体和富含二氧化碳的流体显著地改变了。鉴于现有岩性的范围，并假设原始岩石圈是海洋的，将有可能直接记录与俯冲有关的质量转移通量的某些方面。包体的矿物化学、结构、流体包裹体和熔体包裹体特征将被彻底记录下来，以限制部分弧下地幔的岩石学演化。

9419018 Leeman This study addresses the relation between tectonic evolution of the Pacific Northwest and geochemical evolution of the mantle below the Cascadia volcanic arc. Cascade volcanism began at about 40 Ma following collision of a seamount/oceanic island chain (Oregon-Washington coast ranges) with the Paleogene subduction zone westward migration of subduction to its present location shortly after this event. Between that time and the present, there was a five-fold decrease in convergence rate as well as a systematic decrease in the age of the subducting plate as the Juan de Fuca ridge system approached the mainland. These variations should have resulted in progressive and predictable modifications of the subarc mantle. It is planned to test this subduction model in two ways: First the P.I.s plan to characterize the major and trace element and isotopic compositions of primitive pre-Quaternary lavas erupted throughout the history of the arc. this will be done in two temporal transects across the Cascade range where exposures are optimal and geologic mapping is available. Second, the P.I.s plan to complete a detailed petrologic study of variably metasomatized mantle xenoliths from the southern Washington Cascades. Some of these rocks have been strikingly modified by infiltrating potassic siliceous melts and CO2-rich fluids. Given the range of lithologies available, and assuming that the original lithosphere was oceanic, it will be possible to document directly certain aspects of subduction-related mass transfer fluxes. The mineral chemistry, textures, and fluid and melt inclusion characteristics of the xenoliths suite will be thoroughly documented to constrain the petrologic evolution of a portion of the subarc mantle.