Multi-Mineral Geochronology and REE Geochemistry of High- and Ultrahigh-Pressure Metamorphic Rocks: North Qaidam Terrane, Northwest China

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Intellectual merit. Ultrahigh-pressure (UHP) metamorphism (i.e., at depths 80 km) is a consequence of continental subduction and collision. Investigation of such rarely exposed rocks can provide new insights into processes of continental collision, growth, and mountain building, as well as crust/mantle interaction. An outstanding question regarding UHP metamorphism concerns how these rocks return to the surface while preserving their high P/T mineral assemblages. Critical to addressing this question is a better understanding of the exhumation rates. To attain such information, this project focuses on well exposed eclogite-facies rocks of the North Qaidam Mountains of China. U-Pb geochronology of zircon, monazite and titanite will be coupled with pressure, temperature, and mineral paragenesis (based on REE geochemistry, mineral inclusion analysis, and thermobarometry) to evaluate ages of peak and retrograde metamorphism. These data will constrain exhumation rates associated with their eclogite- to granulite- and amphibolite-facies decompression. Rates will be calculated based on the age difference (determined by U-Pb geochronology of accessory zircon and other phases) between superposed mineral growth zones corresponding to different pressures (depths) of formation (as inferred from mineral inclusions, REE geochemistry, and thermobarometry of garnet-bearing mineral assemblages). This study is likely to significantly advance our knowledge of the geologic history of the N. Qaidam terrane, and provide new perspectives on UHP exhumation processes in general. Broader impacts. Central to this research is collaboration with Chinese colleagues, thereby enhancing international scientific exchange and cooperation. Collaborative research during our previous research has resulted in six journal publications, and we expect to continue this productive collaboration. The trace element analysis routine developed during our previous and continuing work expands capabilities of the state-of-the-art, multi-user Stanford-USGS SHRIMP-RG facility, thereby enhancing the utility of this facility for research and education. This project will also provide training and research experience for Stanford University graduate and undergraduate students.