INTEGRATING U-PB GEOCHRONOLOGY WITH TRACE ELEMENT GEOCHEMISTRY OF ZIRCON TO CONSTRAIN THE PROVENANCE, METAMORPHIC, AND MAGMATIC HISTORIES OF ROCKS IN THE BITTERROOT CORE COMPLEX, WESTERN MONTANA

U-Pb geochronology and trace element analysis of zircon utilizing a quadrupole LA-ICP-MS have been used to investigate the geologic histories of rocks within the high grade metamorphic footwall of the Bitterroot core complex. Multiple periods of igneous intrusion and related metamorphic events are recorded in these rocks. For this study, four metamorphic rocks from the high grade footwall were analyzed from the Bass Creek Canyon area near Florence, Montana. Meta-anorthosite bodies within the core complex have been proposed to correlate to the 1.78 Ga Boehl’s Butte anorthosite in northern Idaho. Our new data have identified a population of zircons that correlate to this age, indicating these rocks are basement to the overlying Mesoproterozoic Belt Supergroup. Investigation into the trace element composition of these zircon indicates a low concentration of REE, flat REE patterns, and relatively high Th/U and Nb/Ta. Two additional populations of zircon that are distinguishable in age and geochemistry are 59 and 46 Ma, which correlate to main stage plutonism and core complex exhumation. Detrital zircon analysis of the rock that encloses the anorthosite, a sillimanite gneiss, indicates that it was derived from the lowermost unit of the Mesoproterozoic Belt Supergroup, which leads us to infer that the contact between the anorthosite and sillimanite gneiss may represent the basal Belt Supergroup unconformity. This rock shows evidence for partial melting between 72-62 Ma. An amphibolite dike within the sillimanite gneiss yields a protolith crystallization age of 1458 Ma, with overprinting metamorphism at 72 Ma and 62 Ma. The 72 Ma ages are interpreted to reflect crustal thickening during Sevier orogenesis, whereas the Paleocene-Eocene ages reflect anatexis and metamorphism during magmatism and core complex formation. The in situ geochronological analysis approach integrated with trace element geochemistry from the same spots yield important information such as the REE geochemistry and Ti-in zircon temperatures that can aid in the interpretation of the geological significance of the dates obtained. This is especially important in complex area such as this that reflect a variety of physical processes that promote crystallization and new growth of zircon during magmatism and metamorphism.