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

晚白垩世至早第三纪Pelona，Orocopia，和兰德Schists南部加州和亚利桑那州西南部代表了一个广泛的俯冲复合体底托在北美西南部的大陆地壳下，在低角度东北俯冲，与Laramide oreal。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。可供研究的片岩领域在2012年得到了显着扩大，当时我们发现了Orocopia片岩的额外暴露，异常遥远的内陆-在大凤凰城郊区以西90公里的墓地岭。墓地岭片岩的铀铅碎屑-锆石年龄和氩-氩变质年龄的测定将揭示Pelona、Orocopia和兰德片岩的年龄是否向东南方向（平行于前俯冲边缘）减小;或东北方向，垂直于前边缘。这个问题的解决将允许两个广泛讨论的构造模型之间的选择：渐进的内侧俯冲侵蚀或迁移的海盆脊平行于俯冲海沟。此外，对墓地岭折返断层的野外和氩-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，墓地岭的奥罗莫皮亚片岩包括大量大洋或大陆边缘地幔橄榄岩的块体和碎片，主要是蛇纹化纯橄榄岩和部分蛇纹化方辉橄榄岩至橄榄石正辉石岩。大部分橄榄岩转化为富含阳起石的交代岩，但也有部分橄榄岩含有丰富的斜方辉石和少量残留橄榄石。墓地岭保存完好的超镁铁质岩石和矿物在佩洛纳、奥罗科皮亚和兰德片岩中是独一无二的，它们在内陆的存在是引人注目的。我们将使用现场，岩相学，地球化学和电子探针技术来确定橄榄岩的起源，以及它是如何被纳入和运输的奥罗科普亚片岩。特别是，我们想知道它是否更接近深海或地幔楔（俯冲）橄榄岩。对墓地岭交代橄榄岩的研究将揭示低角度俯冲复合体中壳源（Meta）沉积岩和地幔橄榄岩之间化学相互作用的细节。首先，我们将确定岩石的年龄和形成墓地岭俯冲复合体的过程;这些年龄是了解其起源的基础。其次，我们将研究公墓岭的橄榄岩的特征和起源，以确定这些大洋岩石是如何被运送到内陆的。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，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，对加州南部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外，该项目还有助于对来自爱荷华州州立大学的本科生进行研究各个方面的培训，包括实地和实验室工作，以及在专业协会会议上展示研究结果。研究结果还将纳入一个为中学生举办的关于北美西部构造的讲习班，该讲习班将在怀俄明州的爱荷华州大学地质野外站教授。该项目涉及与UCSC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.