Collaborative Research: Identifying the Limitations and Expanding the Utility of Detrital Mineral U-Pb Geochronology to Tectonic Studies

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Provenance studies utilizing radiometric ages of detrital minerals (particularly zircon) have expanded greatly over the past decade, driven largely by advances in microgeochronometric methods. The very same properties that make zircon such a powerful geochronometer (extremely high closure temperature and very low solubility) render it virtually inert during collisional orogenesis, except at upper amphibolite to granulite facies and anatexis. Thus, an entire cycle of collision, crustal thickening, heating and loading, and exhumation could pass in some terranes and there would be no record of that cycle in the detrital record using conventional approaches for detrital zircon analysis. This research project is testing the utility of determining the Uranium-Thorium-Lead ages of detrital monazite, which holds the potential for providing a far richer history of tectonometamorphic events compared to analyzing only detrital zircon cores. Additionally, the hypothesis that very small detrital zircon crystals, and thin metamorphic rims on detrital zircon, provide a substantially different chronological view of an orogen compared to that obtained by analyzing the central portions of larger grains is being tested. Clastic sedimentary rocks and alluvium from the southern Appalachians are being sampled for their heavy mineral suites. The Appalachian orogen is chosen because this belt has experienced the full spectrum of tectonic processes, rather than being constructed largely by a single process such as arc magmatism. In addition, to fully test these hypotheses it is necessary to collect the detrital data from a region where the details of the magmatic, metamorphic, and deformational history are well established. This will allow establishment of the most robust protocols to minimize the effects of age bias, both those naturally inherent to detrital minerals, and potential laboratory induced bias during grain selection. Only once the results of detrital age information are available from a well-studied, polydeformed orogen can they be best applied to tectonic studies of other more poorly understood orogenic belts. The results of this study will have major implications for the interpretation of the worldwide detrital mineral age record. The procedures and tools developed here that are most successful for resolving the problems that currently exist with the interpretation of the conventional detrital zircon database will then be available for application by other researchers working in any orogenic belt, for any division of geologic time, to best use detrital mineral dating as a provenance indicator, as an aid in plate reconstructions, and for much more accurate elucidation of major terrane tectonothermal events.