Collaborative Research: Subduction and Subduction Initiation Processes: Geochemical Investigations of Franciscan High-Grade Blocks and Coherent Metamorphic Rocks

合作研究：俯冲和俯冲起始过程：方济会高品位块体和相干变质岩的地球化学研究

• 批准号: 0635928
• 负责人: Asish Basu
• 金额: $ 12.33万
• 依托单位: University of Rochester
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635928&HistoricalAwards=false
• 关键词: Collaborative; Research; Subduction; Initiation; Processes

The process of subduction, wherein an oceanic plate descends into the Earth's mantle, is believed to have contributed to the geochemical heterogeneity of the mantle, and played a major role in the growth of continents and mountain belts. Many regard the Franciscan Complex of California as the world's best example of a subduction complex--an assemblage of materials that is formed of rocks partly subducted and scraped off the downgoing plate. Accordingly, the Franciscan Complex serves as an ideal field laboratory to investigate processes of subduction, including processes associated with the initiation of subduction. In this study of the geochemistry of Franciscan metamorphic rocks, the team will seek to understand the physical and chemical processes associated with the initiation of subduction and continued subduction and the generation of oceanic crust at spreading ridges above subduction zones. In addition to giving insight into fundamental Earth processes, the project may yield information relevant to the development of certain ore deposits, as well as natural concentrations of some metals that pose environmental hazards.The investigators plan to examine the geochemical characteristics of Franciscan Complex basaltic rocks of different ages and metamorphic grade and use the geochemical data to gain insight into subduction, oceanic crust generation, and associated geochemical processes that spanned over 100 million years. The results will build on a preliminary study that suggest some of the parent rocks of the metamorphosed basalts formed in an island arc or arc-related oceanic spreading center whereas others formed at a mid-ocean ridge (MORB), and that the arc parent rocks may be limited to the oldest and most intensely metamorphosed (high-grade) rocks that are thought to have been metamorphosed at the inception of subduction. None of the samples showed evidence of a continental geochemical signature, indicating the lack of geochemical exchange with fluids derived from continentally-sourced sediments. These results, on a limited number of samples, suggest that a more extensive sampling (over 80 samples from at least 15 localities, spanning 500-km-long section of the paleo subduction zone)may help determine the transition (in time and space) in subduction history between early subduction of mixed arc and mid-ocean ridge generated basalt to subsequent subduction of entirely MORB or ocean island basalt. It is also planned to examine whether younger Franciscan basalts show evidence for a lack of geochemical exchange with fluids derived from subducted continental sediments. This will prove illuminating, since these younger basalts should have been incorporated into the Franciscan at a time when subduction of continental sediments was ongoing. The team will exploit data from recent and ongoing geochronologic studies of the Franciscan Complex in which one of the PIs (JW) is a participant. The new geochronologic data help place the geochemical studies into a tighter temporal framework than is offered by the existing published ages. In addition to the specific issues outlined above, our study will: (1) help illuminate details of the a subduction zone, from subduction inception through 100 million years of subduction history, and the generation of oceanic crust involved in the subduction process; (2) provide insight into geochemical fluxes and arc magma genesis from the geochemistry of deeply subducted volcanic rocks; (3) determine the role of aqueous fluids formed by decomposition of hydrous phases in transporting elements and determining geochemical fluxes within subduction zones. Part of this research will be included in the PhD thesis of a female graduate student at the University of Rochester and an M.S. thesis at Fresno State.

俯冲的过程中，海洋板降落到地球地幔中，被认为促成了地幔的地球化学异质性，并在大陆和山地带的生长中起着重要作用。 许多人认为加利福尼亚州方济各会建筑群是俯冲综合体的世界上最好的例子 - 材料的组合是由岩石部分俯冲并从下游板上刮掉的材料。 因此，方济会综合体是研究俯冲过程的理想现场实验室，包括与俯冲启动相关的过程。在这项关于方济各会变质岩石地球化学的研究中，该小组将寻求了解与俯冲启动和俯冲持续俯冲以及沿俯冲的产生相关的物理和化学过程，并在将山脊扩散到俯冲区以上时产生了海洋壳。 除了深入了解地球过程外，该项目还可能产生与某些矿床发展有关的信息，以及一些构成环境危害的某些金属的自然浓度。该研究人员计划检查Franciscan Complex综合体的地球化学特征跨越了1亿年。结果将基于一项初步研究，该研究表明在岛弧或与弧相关的海洋蔓延中心形成的变形玄武岩的某些家长岩石，而其他人则在海山山脊（Morb）中形成的其他岩石，而弧父岩石可能限于最古老，最强烈的岩石（高度）的岩石，这些岩石可能属于岩石，而这是有限的岩石，这些岩石是曾经是岩石的岩石。俯冲。 这些样品均未显示出大陆地球化学特征的证据，表明缺乏来自大陆采购沉积物的液体的地球化学交换。这些结果在有限的样本上表明，更广泛的抽样（至少15个地区的80多个样品跨越了古俯冲带的500公里长的部分）可能有助于确定俯冲历史（在时间和空间中）在早期俯冲历史中的过渡（在时间和空间）之间的早期俯冲弧形混合弧和中质脊而产生的底层的俯冲到后来的牛仔群，或者是随后的牛皮群的群体。 还计划检查年轻的方济各会玄武岩是否显示出缺乏与俯冲大陆沉积物的液体进行地球化学交换的证据。这将被证明是有启发性的，因为这些年轻的玄武岩应该在正在进行的大陆沉积物俯冲的时候已纳入方济各会。 该团队将利用法兰西斯综合体的最新和正在进行的地球文学研究的数据，其中一个PIS（JW）是参与者。 与现有已发表的年龄相比，新的地质数据有助于将地球化学研究置于更严格的时间框架中。 除了上面概述的具体问题外，我们的研究还将：（1）帮助阐明俯冲带的细节，从俯冲成立到俯冲历史悠久的历史，以及参与俯冲过程的海洋壳的产生； （2）从深俯冲的火山岩地球化学中提供了对地球化学通量和弧形岩浆起源的见解； （3）确定由含水相分解在运输元件和确定俯冲带内的地球化通量中形成的水​​性流体的作用。这项研究的一部分将包括在罗切斯特大学的一位女研究生和硕士学位的博士学位论文中。弗雷斯诺州的论文。