CONSTRAINING THE TIMING OF ARCHEAN METAMORPHIC EVENTS IN HIGH-GRADE ROCKS FROM THE BEARTOOTH MOUNTAINS OF MONTANA AND WYOMING VIA U-PB MONAZITE GEOCHRONOLOGY

U-Th-Pb monazite geochronology is used to constrain the Archean metamorphic history of crustal rocks from the Beartooth Mountains of Montana and Wyoming. Determining the ages of these events is important for developing a comprehensive understanding of the tectono-metamorphic evolution of the Wyoming Province as a whole. Ages (U-Th-Pb) of individual grains were obtained by laser ablation ICP-MS methods at Washington State University and U, Th, Y, and Ce X-ray maps and back-scatter electron images were collected at Louisiana State University. Compositionally, monazite grains do not exhibit elemental- or age- core-to-rim zonation, nor do they typically display concentric growth faces in BSE imagery. The youngest ages (~2.5 Ga) were obtained for samples on the western side of the range that marks the boundary between the Beartooth-Bighorn Magmatic Zone (BBMZ) and the Montana Metasedimentary Terrane (MMT). Monazites from a highly tectonized quartzofeldspathic gneiss in Yankee Jim Canyon along the Yellowstone River yield a limited age-range near 2.5 Ga that supports previous 2.5 Ga U-Pb zircon dates from this boundary zone. Significantly, western samples do not record any of the 1.7-1.9 Ga events of the Great Falls orogeny that are pervasive in the MMT. In contrast, U-Pb ages of monazites from multiple rock types in the eastern Beartooth Mountains (BBMZ) record three metamorphic events: an M1 granulite facies event ~2.80-2.82 Ga, an M2 amphibolite facies overprint ~2.74-2.79 Ga, and an M3 granulite facies overprint ~2.72-2.74 Ga. The lack of zoning in these monazites suggest partial to complete grain dissolution and precipitation during these events. Although the M2 overprint is recorded most widely, all of the monazite age ranges defined appear to be related to the emplacement of the voluminous Long Lake Magmatic Complex (2.79-2.83 Ga U-Pb zircon). The LLMC intruded a range of 3.1-3.5 Ga supracrustal and tonalite-trondhjemite-granodiorite (TTG) gneisses in the root of a Mesoarchean continental margin arc and fluids associated with crystallization of these magmas permeated the older supracrustal rocks. Monazite age differences in eastern samples likely reflect the response of different host rock bulk compositions to these fluids and/or fluids formed at different stages in the evolution of the arc.