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

俯冲过程，其中一个海洋板块下降到地球的地幔，被认为是促成了地幔的地球化学不均一性，并在大陆和山脉带的生长中发挥了重要作用。许多人认为加利福尼亚的方济各会杂岩是世界上俯冲杂岩的最好例子--俯冲杂岩是由部分俯冲并从下沉的板块上刮下来的岩石形成的物质集合。因此，方济各会建筑群是研究俯冲过程的理想野外实验室，包括与俯冲开始相关的过程。在这项对方济各会变质岩的地球化学研究中，该小组将试图了解与俯冲开始和持续俯冲有关的物理和化学过程，以及在俯冲带上方的扩张海脊上形成洋壳的过程。除了提供对地球基本过程的洞察外，该项目还可能产生与某些矿床的发展相关的信息，以及一些构成环境危害的金属的自然浓度。研究人员计划研究不同年龄和变质等级的方济各会复杂玄武岩的地球化学特征，并利用这些地球化学数据来洞察俯冲、洋壳形成和相关的跨越1亿多年的地球化学过程。这一结果将建立在一项初步研究的基础上，该研究表明，变质玄武岩的一些母岩形成于岛弧或与弧有关的大洋扩张中心，而另一些母岩形成于大洋中脊(MORB)，弧母岩可能仅限于最古老和最强烈变质(高级)的岩石，这些岩石被认为是在俯冲开始时变质的。没有一个样本显示出大陆地球化学特征的证据，这表明缺乏与来自大陆来源的沉积物的流体的地球化学交换。这些结果在数量有限的样品上表明，更广泛的采样(来自至少15个地点的80多个样品，跨越500公里长的古俯冲带段)可能有助于确定俯冲历史上从早期混合弧和大洋中脊生成的玄武岩向随后完全俯冲的MORB或洋岛玄武岩的过渡(在时间和空间上)。它还计划检查更年轻的方济各会玄武岩是否显示出与俯冲大陆沉积物中的流体缺乏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大陆沉积物俯冲进行的时候被并入方济各会。该小组将利用最近和正在进行的方济各会建筑群的地质年代学研究的数据，其中一名PI(JW)是参与者。新的地质年代学数据有助于将地球化学研究置于比现有出版年代提供的更紧密的时间框架中。除上述具体问题外，我们的研究将：(1)有助于阐明从俯冲开始到俯冲1亿年历史的整个俯冲带过程的详细情况，包括俯冲过程中洋壳的形成；(2)从深俯冲火山岩的地球化学出发，深入了解地球化学通量和弧岩浆的成因；(3)确定水相分解形成的水流体在俯冲带内元素的搬运和确定地球化学通量方面的作用。这项研究的一部分将被包括在罗切斯特大学一名女研究生的博士论文和弗雷斯诺州立大学的一篇硕士论文中。

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.