Sm-Nd Thermochronology of Garnets in Metamorphic Rocks: A New Method and Tectonic Applications

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Ducea/GangulyEAR-0087424We will quantitatively investigate cooling and exhumation rates of deep crustal rocks from orogenic environments using a new thermochronological method. The basic scientific goal of the application is to asses the relative contribution of normal faulting, ductile thinning and erosion to rock exhumation. This understanding is a fundamental concept to add to our knowledge of continental tectonics. This method permits determination of cooling rates from the age profile or the difference between the core and bulk ages of a mineral by a single radiogenic decay system. We will determine the Sm-Nd ages of central and rim segments of garnet crystals by isotope dilution and thermal ionization mass spectrometry. The ages will be determined from small quantities of samples drilled from the cores of defined dimensions. These data will be combined with conventional thermobarometry based on ion exchange and mineral reactions to constrain the exhumation history of the rocks above 500 oC. Integration of the data derived from the new and conventional thermochronology with petrological observations would permit construction of a more comprehensive picture of the exhumation history of high-grade metamorphic rocks than has been possible so far. The suites of samples are from a collisional orogen, the Himalaya Mountains (Nepal), arc root assemblages from the Sierra Nevada (California), and a subduction complex, the Catalina Island (California).