LATE ARCHEAN TO PALEOPROTEROZOIC EVOLUTION OF THE RUBY RANGE, MONTANA, AND ITS IMPLICATIONS FOR THE GROWTH AND MODIFICATION OF THE NORTHWESTERN WYOMING PROVINCE

An integrated approach to constraining P-T-t histories is required for rocks that have experienced multiple metamorphic events. In this study, monazite in samples from the Ruby Range in southwestern Montana was analyzed by in situ U-Th-Pb/REE laser ablation split stream analysis to investigate the linkages between garnet and monazite growth, the duration of monazite growth, and the P-T histories of the rocks. U-Pb results were integrated with phase equilibria modeling in order to place constraints on the P-T-t paths. Samples from three areas across the range were investigated. Stone Creek on the western side of the range contains Grt-Sil-Bt paragneisses and Grt-St-Crd orthoamphibolites that record protracted monazite growth between 1790-1723 Ma. Inclusions in garnet and matrix grains record similar ages. In the Elk Gulch area in the south-central part of the range, Grt-Sil-Bt-Ksp migmatitic paragneisses and Grt-Crd orthoamphibolites record growth at 1784-1733 Ma. Two monazite inclusions in garnet record spot core ages of 2.4 Ga. In the Sweetwater Creek area on the southeastern flank of the range, monazite in Grt-Sil-Crd paragneisses records complex mixed ages between 2.5 Ga and ca. 1.75 Ga. The oldest monazite ages recorded in this area are 2.52-2.45 Ga cores that occur both as inclusions in garnet and matrix grains. Most grains, regardless of petrographic setting, record mixed ages. Rim analyses record ages ranging from 1791-1722 Ma. Compared to the surrounding Tobacco Root and Highland Mountains, the Ruby Range records lower P-T conditions and less extensive partial melting. Sillimanite is the dominant aluminosilicate and retrograde cordierite rimming garnet is abundant and preserves widespread evidence for equilibration following decompression at ~5 kbar and ~680°C. Collectively these data indicate that the Ruby Range records the transition from rocks that are dominated by Paleoproterozoic metamorphism to rocks that experienced a more cryptic metamorphic event at ~2.5 Ga, which has been correlated to the Tendoy orogeny further south. The central part of the Ruby Range likely records post-orogenic extension and rapid exhumation of this belt, which resulted in high-grade metamorphism and partial melting at 1.75 Ga. The 1.75 Ga event in the Sweetwater Creek area may be reactivation of fabric developed at 2.5 Ga.