Significance of Newly Discovered Subduction Complex, Including Partially Serpentinized Mantle Peridotite, Southwest Arizona

加利福尼亚南部和亚利桑那西南部的晚白垩世至早古近纪Pelona、Orocopia和Rand片岩代表了与Laramide造山运动同期发生的低角度东北俯冲期间北美洲西南部大陆地壳下的一个广泛的俯冲杂岩。人们对低角度俯冲过程的几个重要方面知之甚少，主要是因为只有一小部分俯冲片岩暴露在地表。2012年，当我们发现Orocopia片岩的额外暴露在异常遥远的内陆时，片岩研究领域得到了显著扩大——在大凤凰城郊区以西90公里的墓地岭。测定墓地岭片岩的铀-铅碎屑锆石年龄和氩-氩变质年龄将揭示Pelona片岩、Orocopia片岩和Rand片岩的年龄是否在东南方向平行于前俯冲边缘而减小；或者向东北方向，垂直于前边缘。这个问题的解决将允许在两种被广泛讨论的构造模型之间做出选择：渐进式向内俯冲侵蚀或平行于俯冲海沟的地震脊的迁移。此外，对墓地岭挖掘断层的野外和氩氩研究将有助于理解深俯冲岩石返回上地壳和地表的过程。出乎意料的是，墓地岭的造山角片岩中含有大量的海洋或大陆边缘地幔橄榄岩块和碎片，主要是蛇纹石化的暗岩块和部分蛇纹石化的辉锌矿到橄榄石直辉石质。大部分橄榄岩转化为富含放线石的交代岩，但部分橄榄岩含有丰富的正辉石和少量残余橄榄石。墓地岭保存完好的超镁铁质岩石和矿物在Pelona， Orocopia和Rand片岩中是独一无二的，到目前为止它们在内陆的存在是值得注意的。我们将使用野外、岩石学、地球化学和电子探针技术来确定橄榄岩的起源，以及它是如何与造山岩片岩合并和运输的。特别是，我们想知道它是更像深海橄榄岩还是更像地幔楔（超俯冲）橄榄岩。墓地岭交代橄榄岩的研究将揭示低角度俯冲杂岩中地壳衍生（元）沉积岩与地幔橄榄岩之间化学相互作用的细节。这个项目有两个主要目标。首先，我们将确定形成墓地岭俯冲复合体的岩石年龄和过程；这些年代是理解其起源的基础。其次，我们将研究墓地岭显著的橄榄岩的特征和起源，以确定这些海洋岩石是如何运输到内陆的。我们的项目将有助于了解北美西南部低角度俯冲的机制。特别是，低角度俯冲与落基山脉的形成密切相关，而落基山脉主导着美国西部大部分地区的自然和人文地理。更一般地说，对南加州和亚利桑那州西南部陆地上暴露的俯冲复合体的研究将补充海洋地质学和地球物理学的证据，并进一步了解全球范围内的俯冲作用。除了研究的科学目标之外，该项目还有助于训练爱荷华州立大学的本科生在研究的各个方面，包括实地和实验室工作，以及在专业学会会议上展示研究成果。这项研究的结果也将被纳入一个面向初高中学生的关于北美西部构造的研讨会，该研讨会将在怀俄明州爱荷华大学的地质野外站教授。该项目包括与加州大学圣地亚哥分校和斯坦福大学合作进行地球化学和同位素分析。

The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists of southern California and southwest Arizona represent an extensive subduction complex underplated beneath the continental crust of southwest North America during low-angle northeastward subduction, coeval with the Laramide orogeny. Several important aspects of the low-angle subduction process are poorly understood, mainly because only a small fraction of the subducted schist is exposed at the surface. The field of schists available for study was significantly widened in 2012, when we discovered an additional exposure of Orocopia Schist unusually far inland--at Cemetery Ridge, 90 kilometers west of the outskirts of greater Phoenix. Determination of uranium-lead detrital-zircon ages and argon-argon metamorphic ages for the schist at Cemetery Ridge will reveal whether the Pelona, Orocopia, and Rand Schists decreases in age to the southeast, parallel to the former subduction margin; or to the northeast, perpendicular to the former margin. Resolution of this question will allow a choice between two widely discussed tectonic models: progressive inboard subduction erosion or migration of an aseismic ridge parallel to the subduction trench. Furthermore, field and Argon-Argon study of exhumation faults at Cemetery Ridge will contribute to understanding of processes whereby deeply subducted rocks are returned to the upper crust and surface.Unexpectedly, the Orocopia Schist at Cemetery Ridge includes numerous blocks and fragments of oceanic or continental-margin mantle peridotite, chiefly serpentinized dunite and partially serpentinized harzburgite to olivine orthopyroxenite. Much of the peridotite is converted to actinolite-rich metasomatic rocks, but some contains abundant orthopyroxene and a little relict olivine. The well-preserved ultramafic rocks and minerals at Cemetery Ridge are unique in the Pelona, Orocopia, and Rand Schists, and their presence so far inland is remarkable. We will use field, petrographic, geochemical, and electron microprobe techniques to determine the origin of the peridotite, and how it was incorporated into and transported with the Orocopia Schist. In particular, we want to know whether it more closely resembles abyssal or mantle-wedge (suprasubduction) peridotite. Study of metasomatized peridotite at Cemetery Ridge will reveal details of chemical interactions between crustally derived (meta)sedimentary rocks and mantle peridotite within a low-angle subduction complex.This project has two major goals. First, we will determine the ages of rocks in and processes that formed the subduction complex at Cemetery Ridge; these ages are fundamental to understanding its origin. Second, we will study the character and origin of the remarkable peridotites at Cemetery Ridge, in order to determine how these oceanic rocks were transported so far inland. Our project will contribute to knowledge of mechanisms of low-angle subduction in southwest North America. In particular, low-angle subduction is intimately related to formation of the Rocky Mountains, which dominate the natural and human geography of much of the American West. More generally, study of the subduction complex exposed on-land in southern California and southwest Arizona will complement evidence from marine geology and geophysics and further understanding of subduction on a global scale. In addition to the scientific goals of the research, this project is contributing to the training of undergraduate students from Iowa State University in all aspects of the research, including field and laboratory work, as well as presenting the results of the research at professional society meetings. Results of the research will also be incorporated into a workshop for middle and high school students on the tectonics of western North America that will be taught at the University of Iowa's geology field station in Wyoming. The project involves collaboration with UCSC and Stanford for geochemical and isotopic analyses.