INVESTIGATING THE P-T CONDITIONS AND TEMPORAL CONSTRAINTS ON REGIONAL METAMORPHISM NEAR BIG THOMPSON CANYON, COLORADO, USA

Paleoproterozoic metasedimentary rocks near Big Thompson Canyon (BTC) in the northern Colorado Front Range are well known for their regionally zoned metamorphism from garnet- to migmatite-grades. An ongoing debate is whether the area experienced a single (~1.7 Ga) or multiple cycles of metamorphism (~1.7, 1.6 and 1.4 Ga). Reports of multiple generations of key metamorphic minerals appear supportive of the latter. This includes multiple generations of staurolite suggested by Ms + Bt ± Chl pseudomorphs and later fresh staurolite porphyroblasts, as well as two successive garnet generations revealed by compositional zoning. The equilibrium pressure (P) and temperature (T) conditions of these rocks are unclear; several researchers suggest pressures below 0.4 to 0.8 GPa, while others have suggested pressures as high as 1.0 GPa.

This study focuses on constraining the possible P-T-time paths of BTC rocks using thermodynamical techniques (TWQ 2.34 and other independent thermobarometers) and monazite U-Th-total Pb geochronology. Mica-rich metapelites were selected from the garnet (Grt, Bt, Ms, Pl, Qz), staurolite (Grt, St, Bt, Ms, Pl, Qz) and migmatite zones (Grt, Crd, Pl, Kfs, Crd, Bt, Sil, Qz). Preliminary P-T results vary from ~0.3 GPa and 520°C for the garnet-zone and up to 0.45 GPa and 690°C for the migmatite rocks. The staurolite-zone sample yields 0.45 GPa and 560°C. In addition, the occurrence and textural relationships between monazite and other major phases in the staurolite-zone sample are being investigated with the intent of placing timing constraints on staurolite growth and related deformation fabrics. Monazite dates in this sample range from ~1.72-1.67 Ga, the youngest age being found included in fresh staurolite. Therefore, we conclude that the determined P-T conditions for that sample represents equilibration after 1.67 Ga. While the timing of P-T conditions in all metamorphic zones remains unclear, these preliminary results support a low-pressure metamorphic history for the latest event. Future work will include continued monazite dating of samples from intermediate to high grade rocks and additional petrologic modeling to decipher the garnet growth history, notably the difference in P-T conditions between the two garnet generations.