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

南加州和亚利桑那州西南部的晚白垩世至早古近纪的Pelona、OroCopia和Rand片岩代表了北美西南部陆壳之下的一个广泛的俯冲杂岩，在低角度东北向俯冲期间，与拉莱米德造山作用同时代。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。可供研究的片岩领域在2012年显著扩大，当时我们发现了另一处奥罗科皮亚片岩异常遥远的内陆暴露--在大凤凰城以西90公里的墓地山脊。通过对墓地脊片岩的铀-铅碎屑-锆石年龄和Ar-Ar-Ar变质年龄的测定，将揭示Pelona、OroCopia和Rand片岩是在东南方向平行于前俯冲边缘，还是在东北方向垂直于前俯冲边缘而年龄下降。这个问题的解决将允许在两种广泛讨论的构造模式之间进行选择：逐渐向内俯冲侵蚀或平行于俯冲海沟的无地震脊的迁移。此外，墓地山脊折返断裂的野外研究和Ar-Ar研究将有助于了解深俯冲的岩石返回上地壳和地表的过程。墓地山脊的OroCopia片岩中包含大量大洋或大陆边缘地幔橄榄岩的块体和碎片，主要是蛇纹岩化的纯橄榄岩和部分蛇纹化的方辉橄榄岩-橄榄斜辉岩。大部分橄榄岩转化为富阳起岩交代岩，但也有一些含有丰富的斜方辉石和少量残余橄榄石。墓地山脊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，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来源，以及它是如何被合并到奥罗科皮片岩中并与之一起运输的。特别是，我们想知道它是更接近于深海橄榄岩还是地幔楔形(超俯冲)橄榄岩。对墓地山脊交代橄榄岩的研究将揭示低角度俯冲杂岩中壳源(变质)沉积岩和地幔橄榄岩之间的化学作用细节。首先，我们将确定形成墓地山脊俯冲杂岩的岩石的年龄和过程；这些年龄是了解其起源的基础。其次，我们将研究墓地山脊引人注目的橄榄岩的特征和成因，以确定这些大洋岩石是如何运往内陆的。我们的项目将有助于了解北美西南部的低角度俯冲机制。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。更广泛地说，对南加州和亚利桑那州西南部陆上暴露的俯冲复合体的研究将补充来自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。除了研究的科学目标外，该项目还帮助爱荷华州立大学的本科生在研究的各个方面进行培训，包括实地和实验室工作，以及在专业学会会议上展示研究结果。研究结果还将被纳入一个关于北美西部构造的初中生和高中生研讨会，该研讨会将在爱荷华大学怀俄明州的地质野外站授课。该项目涉及与加州大学洛杉矶分校和斯坦福大学在地球化学和同位素分析方面的合作。

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.