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

喀斯喀特弧下地幔的演化：基于熔岩和地幔包体岩石学的俯冲贡献评价

• 批准号: 9419018
• 负责人: William Leeman
• 金额: $ 21.4万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1995
• 资助国家: 美国
• 起止时间: 1995-07-01 至 2000-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9419018&HistoricalAwards=false
• 关键词: Evolution; Cascades; Subarc; Mantle; Evaluation

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.

9419018 Leeman本研究探讨了西北太平洋构造演化与卡斯卡迪亚火山弧下地幔地球化学演化之间的关系。 级联火山作用开始于约40马碰撞海山/海洋岛链（俄勒冈州-华盛顿州海岸山脉）与古近纪俯冲带俯冲向西迁移到现在的位置后不久，这一事件。 从那时到现在，随着胡安·德富卡海脊系统接近大陆，收敛速度下降了五倍，俯冲板块的年龄也有系统地下降。 这些变化应该导致了弧下地幔的渐进和可预测的修改。 计划以两种方式测试这一俯冲模型：首先，研究人员计划描述整个弧的历史中喷发的原始前第四纪熔岩的主要和微量元素和同位素组成。 这将在横跨喀斯喀特山脉的两个时间断面中进行，在这些断面中，照射是最佳的，并且可以进行地质测绘。 第二，研究人员计划完成对来自华盛顿瀑布南部的地幔岩包体的详细岩石学研究。 这些岩石中的一些已经通过渗透钾质硅质熔体和富含CO2的流体而发生了显著的改变。 考虑到现有的岩性范围，并假设原始岩石圈是海洋性的，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。 矿物化学，纹理，流体和熔体包体的包体套件的特点将被彻底记录，以约束的一部分弧下地幔的岩石学演化。