Geochemical Investigation of Xenoliths From the Central Rio Grande Rift and Colorado Plateau: Constraints on Lithosphere Evolution and Possible Delamination

“该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。“智力优点：该提案寻求资金，以研究中央格兰德河裂谷和科罗拉多高原边缘下方岩石圈地幔的演化。该项目将深入了解与该省的构造和火山演化有关的各种问题，包括法拉隆板块俯冲和脱水在生成格兰德河裂谷熔岩“富集”地幔源方面的作用。然而，这一建议主要是为了回答一个简单的问题：原始元古代时代的岩石圈地幔下的中央格兰德河裂谷和科罗拉多高原边缘积极剥离或对流侵蚀，建议最近的几个地球物理研究？如果正确的话，这些研究表明，岩石圈分层，以前记录下的内华达州，可能比目前认识到的更为普遍，并可能引发的拉伸以及压缩构造制度。橄榄岩捕虏体中Hf和Os同位素变化与熔体贫化程度的矿物学限制的相关性将允许限制格兰德河裂谷和科罗拉多高原捕虏体中记录的熔体贫化时间。此外，橄榄岩单斜辉石微量元素丰度结合Sm-Nd同位素组成将限制在部分科罗拉多高原和格兰德河裂谷观察到的交代叠印的年龄。总之，这些数据将使我们能够区分元古宙，熔融贫化和可能交代岩石圈地幔和年轻，肥沃的软流圈。因此，本研究将为Gao et al.（2004）和Song Helmberger（2007）提出的岩石圈拆沉模型提供直接检验，以解释中央裂谷和高原边缘下的浅地幔中异常低的地震速度以及中原岩石圈下倾斜的板状结构中互补的异常高的速度的存在。在相对较浅的深度（~40-60 km）（以整个区域的橄榄岩捕虏体套件为代表）识别软流圈衍生橄榄岩将支持所提出的拆沉模型，并且与岩石圈响应于伸展的简单被动减薄不一致。与此相反，在这些深度的古岩石圈的识别将建议显着较少的岩石圈去除，并需要重新评估现有的地震层析成像数据的当前解释。通过对大陆裂陷前后岩石圈地幔化学和物理演化的研究，将进一步加深我们对大陆裂陷和大陆裂解的触发机制的认识。更广泛的影响：研究生和本科生将参与该项目的所有方面，从数据采集到解释，以及在国家会议上和通过同行评审的出版物传播结果。本科生参与实地考察和拟议研究的分析方面将允许选定的学生发现课堂外的科学研究的兴奋。一位大学生已经完成了一篇研究我们研究区域捕虏体岩石学的荣誉论文。我们计划通过将更多的本科生纳入研究计划的各个方面来扩大这个初步试点项目的成功。该提案将通过组织以格兰德河裂谷和科罗拉多高原演变为重点的跨学科讲习班，促进跨学科合作和伙伴关系。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: This proposal seeks funding to study the evolution of the lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin. The project will provide insights into a variety of questions related to the tectonic and volcanic evolution of this province, including the role of Farallon plate subduction and dehydration in generation of "enriched" mantle sources of Rio Grande Rift lavas. However, this proposal seeks primarily to answer one simple question: Has the original Proterozoic-age lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin been actively removed by delamination or convective erosion, as suggested by several recent geophysical studies? If correct, these studies suggest that lithosphere delamination, previously recorded beneath the Sierra Nevada, may be more prevalent than currently recognized and may be triggered by extensional as well as compressional tectonic regimes. Correlation of Hf- and Os-isotope variations in peridotite xenoliths with mineralogic constraints on degree of melt depletion will allow constraints on the timing of melt depletion recorded in xenoliths from the Rio Grande Rift and Colorado Plateau. Also, peridotite clinopyroxene trace element abundances combined with Sm-Nd isotope compositions will constrain the age of metasomatic overprints observed in portions of the Colorado Plateau and Rio Grande Rift. Together, these data will allow us to distinguish between Proterozoic, melt-depleted and possibly metasomatized lithospheric mantle and young, fertile asthenosphere. This study will thus provide a direct test of the lithosphere delamination model proposed by Gao et al. (2004) and Song & Helmberger (2007) to explain the presence of anomalously low seismic velocities in the shallow mantle beneath the central rift and plateau margin and complementary, anomalously high velocities within a slab-like structure dipping beneath the Central Plains lithosphere. Identification of asthenosphere-derived peridotite at the relatively shallow depths (~40-60 km) represented by peridotite xenolith suites throughout the region would support the proposed delamination models, and would be inconsistent with simple passive thinning of the lithosphere in response to extension. In contrast, identification of ancient lithosphere at these depths would suggest significantly less lithosphere removal and would require reevaluation of current interpretations of the available seismic tomographic data. Insights gained from this study on the chemical and physical evolution of lithospheric mantle prior to and during continental rifting will expand our understanding of the triggers and mechanisms involved in continental rifting and continental break-up. This remains a fundamental question in plate tectonics.Broader Impacts: Students at the graduate and undergraduate level will participate in all aspects of this project, from data acquisition to interpretation and dissemination of results at national conferences and through peer-reviewed publications. Undergraduate student involvement in fieldwork and analytical aspects of the proposed research will allow the selected student to discover the excitement of scientific research outside the classroom. One undergraduate has already completed an honors thesis examining the petrology of xenoliths from our study area. We plan to extend the success of this initial pilot project by incorporating additional undergraduates into all aspects of the research program. This proposal will foster cross-discipline collaboration and partnerships through organization of interdisciplinary workshops focused on the evolution of the Rio Grande Rift and Colorado Plateau.