THREE PROTEROZOIC METAMORPHIC EPISODES AND 1.70-1.65 GA AGE OF THE UNCOMPAHGRE FORMATION CONSTRAINED BY MONAZITE AND XENOTIME PETROCHRONOLOGY, NEEDLE MOUNTAINS, COLORADO

The Needle Mountains of southwestern Colorado represent one of the most complete exposures of Proterozoic basement rocks in the western US. Despite this, there are few constraints on the timing, metamorphic conditions, and number of tectonic events and the depositional age of the 2.5 km thick Uncompahgre Formation. We integrate multi-scale compositional mapping, petrologic modeling, and in-situ monazite and xenotime petrochronology to constrain pressure-temperature-time (P-T-t) paths from basement gneisses and cover quartzites in the Needle Mountains. Garnet-staurolite-bearing Twilight Gneiss records peak metamorphic conditions of 600°C at 0.7 GPa at 1764±9 Ma with cooling through ~575°C at 1741±10 Ma. Garnet-K-feldspar-bearing metagraywacke sampled from drill core near Pagosa Springs, CO records M1 amphibolite-facies conditions of 700°C at 1748±9 Ma, followed by M2 granulite-facies conditions of 800°C at 1.1 GPa, ca. 1680-1650 Ga, and an M3 event, ~425°C at 1424±12 Ma. The Uncompahgre Formation unconformably overlies the basement gneisses and contains rounded detrital monazite and xenotime cores and compositionally distinct overgrowths. The depositional age is constrained by detrital monazite (1707±8 Ma) and xenotime (1704±15 Ma) populations and 1670-1600 Ma (weighted mean=1644±5 Ma) metamorphic xenotime. M3 metamorphic temperatures at 1470-1440 Ma ranged from ~300°C in the Uncompahgre Gorge to ~575°C in the Eolus Granite contact aureole. These results, combined with published geochronology, indicate (1) 1.76-1.74 Ga amphibolite facies M1 metamorphism, (2) intrusion of 1.73-1.69 Ga granites, 3) 1.70-1.65 Ga deposition of the Uncompahgre Formation, 4) M2 metamorphism (1.68-1.60 Ga) that ranged from granulite facies near Pagosa Springs to greenschist facies in the cover quartzites of the Needle Mountains, and (5) M3 1.47-1.42 Ga greenschist to amphibolite facies metamorphism. Our results emphasize the importance of multi-scale compositional mapping and -situ acquisition of monazite and xenotime compositional and geochronologic data. This highlights the need for integrated studies to unravel the varying intensities of ca. 1.75, 1.70, 1.65, and 1.45 Ga tectonism during construction of southwestern Laurentia.