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

Provenance studies utilizing radiometric ages of detrital minerals (particularly zircon) have expanded greatly over the past decade, driven largely by advances in microgeochronometric methods (ion microprobe and laser ablation). 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 (analyzing cores of large [> 100 ~m] grains). New approaches are clearly needed if detrital mineral analysis can recover the critical tectonic record from regions where much of the original orogenic belt exists only in the form of clastic sediments. The research proposed here will test the utility of determining the U-Th-Pb ages of detrital monazite, which we speculate holds the potential for providing a far richer history of tectonometamorphic events compared to analyzing only detrital zircon cores. We will also test the hypothesis that very small detrital zircon crystals, and thin metanlOrphic 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. Clastic sedimentary rocks and alluvium from the southern Appalachians will be 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 our 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 us to establish 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. Broader Impacts This proposal allows for an intellectual exchange between students at the University of Kentucky (UK) and at Syracuse University (SU) as all of the students will interact with peers and mentors at both institutions during field work and during analytical sessions. Funding of this proposal also will permit us to pursue supplemental funding for undergraduate students from underrepresented groups as part of the EPSCoR programs (UK). Particular efforts are aimed at non-traditional female students returning to school after raising families who now wish to pursue an advanced degree, and Appalachian students from rural eastern KentuckJ' counties. UK is involved in publicly-funded initiatives for enhancing science education and research for minority students and those from the Appalachian region (e.g., AMSTEMM: Appalachian & Minority Science, Technology, Engineering, & Mathematics Majors). One such undergraduate is already being mentored by David Moecher. Scott Samson will work with a middle school teacher from the Syracuse inner city, which has a very high percentage of minority students. During the summer the teacher willieam the art of heavy mineral separation and SEM imaging, as part of a research for teachers program. He will utilize his research by establishing special topics projects for his ninth grade students. The students will present their work as part of a special "field trip" day to So. Samson will work with geology sophomore Emily Feinberg on the SEM analysis of the detrital minerals. New PhD student Jack Hietpas will also join the SU group. An additional educational/scientific benefit from this research will include the establishment of a web-based detrital zircon and monazite age database with an emphasis on SEM images, and chemical and isotopic details. Researchers can access the database for their own research, and will be encouraged to add their own detrital mineral data thus increasing the database utility.