U-PB AND LU-HF GEOCHRONOLOGIC CONSTRAINTS ON THE PROTEROZOIC EVOLUTION OF THE RUBY RANGE, MONTANA

The northern Wyoming province preserves a long history of tectonic activity during the Proterozoic. The details of these geological events, particularly at the NW margin, remain elusive. This study utilizes U-Pb monazite/zircon and Lu-Hf garnet geochronology in samples from the Ruby Range, SW Montana to investigate the timing and duration of garnet and monazite/zircon growth, and to place constraints on the metamorphic and melting events that have affected these rocks. Geochronologic 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. The western side of the range contains Grt-Sil-Bt paragneisses and Grt-St-Crd orthoamphibolites that record protracted monazite growth between 1.79–1.72 Ga. Inclusions in garnet and matrix grains record similar ages. In the south-central part of the range, Grt-Sil-Bt-Ksp migmatitic paragneisses and Grt-Crd orthoamphibolites record growth at 1.78–1.73 Ga. Along the southeastern flank of the range, monazite in Grt-Sil-Crd paragneisses records complex mixed ages between 2.45 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. Rim analyses within inclusions record ages ranging from 1.79–1.72 Ga. Collectively, these data indicate that the Ruby Range records the transition from rocks that are dominated by Paleoproterozoic metamorphism to rocks that experienced an older event at ~2.45 Ga. This history has also been elucidated by new garnet Lu-Hf dating of 2 samples: a mylonitic garnet leucogneiss that intrudes the base of the metasedimentary sequence on the western flank of the range and a pelitic gneiss that is 2 km up section from the leucogneiss. The garnet Lu-Hf analyses on the leucogneiss yield a well-defined date of 2428.1± 6.8 Ma based on 3 garnet fractions and an associated whole rock. These results confirm that this earlier Proterozoic event records a period of crustal melting preserved in the Ruby Range that predates metamorphism attributed to the Big Sky orogeny. The pelitic gneiss yields a date of 1798.0 ± 9.9 Ma based on four garnet and two whole-rock fraction. This younger date is interpreted to reflect regional metamorphism during the Big Sky orogeny.