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

大洋板块下降到地幔的俯冲过程被认为对地幔的地球化学不均匀性有贡献，并在大陆和山带的形成中起了重要作用。许多人认为加利福尼亚的方济各复合体是世界上最好的俯冲复合体——一种由部分俯冲并从下行板块上刮下来的岩石组成的物质组合。因此，方济各杂岩可以作为一个理想的野外实验室来研究俯冲过程，包括与俯冲开始有关的过程。在对方济各变质岩的地球化学研究中，研究小组将试图了解与俯冲开始和持续俯冲以及在俯冲带上方伸展脊形成海洋地壳有关的物理和化学过程。除了深入了解地球的基本过程外，该项目还可能提供与某些矿床的开发有关的资料，以及对环境造成危害的某些金属的自然浓度。研究人员计划研究不同年龄和变质等级的方济各复杂玄武岩的地球化学特征，并利用地球化学数据深入了解跨越1亿年的俯冲、海洋地壳生成和相关的地球化学过程。结果将建立在初步研究的基础上，该研究表明，变质玄武岩的一些母岩形成于岛弧或与弧相关的海洋扩张中心，而另一些母岩形成于海中脊（MORB），并且弧母岩可能仅限于最古老和最强烈的变质（高级）岩石，这些岩石被认为是在俯冲开始时变质的。所有样品都没有显示出大陆地球化学特征的证据，这表明地球化学与来自大陆沉积物的流体缺乏交换。在有限数量的样品上，这些结果表明，更广泛的采样（来自至少15个地点的80多个样本，跨越500公里长的古俯冲带剖面）可能有助于确定俯冲史上从早期混合弧和洋中脊生成的玄武岩俯冲到随后完全MORB或洋岛玄武岩俯冲的转变（在时间和空间上）。它还计划检查年轻的方济各玄武岩是否显示出与俯冲大陆沉积物产生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是在大陆沉积物俯冲进行的时候被纳入方济各会的。该小组将利用方济会复合体最近和正在进行的地质年代学研究的数据，其中一位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龄提供的更严格的时间框架中。除上述具体问题外，我们的研究还将：(1)有助于阐明a俯冲带从俯冲开始到1亿年俯冲历史的细节，以及俯冲过程中洋壳的生成；(2)从深俯冲火山岩的地球化学特征出发，揭示地球化学通量和弧岩浆成因；(3)确定含水相分解形成的含水流体在俯冲带内输运元素和确定地球化学通量中的作用。该研究的部分内容将被纳入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.