PRESSURE-TEMPERATURE AND TEMPORAL CONSTRAINTS ON REGIONAL METAMORPHISM NEAR BIG THOMPSON CANYON, COLORADO, USA

Paleoproterozoic supracrustal basement exposures in Colorado commonly reached migmatite-grade, limiting the potential for rocks to record later, lower-grade tectonometamorphic events. Preservation of a regional metamorphic gradient from garnet- to migmatite-grade in metasedimentary rocks of the Colorado Rockies near Big Thompson Canyon (BTC) is enigmatic, and offers a unique research opportunity to investigate the Pressure (P) - Temperature (T) - time (t) history during the Proterozoic. The broader tectonic significance of this metamorphic pattern, and associated deformation fabrics, requires a thorough understanding of the reaction history, P-T conditions, and timing. One topic of ongoing debate centers on whether these rocks experienced a single (c. 1.7 Ga) or multiple (c. 1.7, 1.6 and 1.4 Ga) cycles of metamorphism.

We aim to constrain PTt-paths of BTC rocks using thermodynamic techniques and monazite U-Th-Pb_total geochronology. P-T estimates from mica-rich metapelites range from ~0.3 GPa and 520°C for the garnet-zone (Grt, Bt, Ms, Pl, Qz), 0.45 GPa and 560°C for the staurolite-zone (Grt, St, Bt, Ms, Pl, Qz), and 0.45 GPa and 690°C for the migmatite zone (Grt, Crd, Pl, Kfs, Crd, Bt, Sil, Qz). Monazite dates from the staurolite-zone range from ~1.72-1.67 Ga, the youngest age from monazite included in fresh staurolite, suggesting that mineral assemblages and P-T conditions for that sample represent equilibration after 1.67 Ga. Additional published data in similar St-bearing samples from BTC suggest staurolite growth may have occurred during the 1.4 Ga event. These data and the identification of multiple generations of index minerals, including new staurolite growth within retrograde pseudomorphs, two chemically distinct garnet generations, and texturally distinct generations of andalusite and sillimanite, support the idea of multiple cycles of metamorphism. Additional monazite dates in all these samples coupled with petrologic modeling may help to decipher the garnet growth history and constrain PT-conditions for each garnet growth event. Ultimately, this study will help unravel the tectonometamorphic history of the northern Colorado Proterozoic basement, and will address the question of why this metamorphic gradient is only preserved in BTC area.