TIMING OF BRITTLE DEFORMATION IN THE CAÑON CITY EMBAYMENT, CO, BY LA-ICP-MS U-PB DATING OF CARBONATE VEINS: A FEASIBILITY STUDY

The Rocky Mountain Front Range has an average elevation of 2-3 km through multiple uplift events over the past ~100 million years. However, little is known about the exact timings of the brittle deformation caused by these uplift events. The use of U-Pb carbonate dating via laser ablation- inductively coupled plasma – mass spectrometry (LA-ICP-MS) offers a solution. Carbonate minerals have the potential to precipitate along faults and cracks to form slickenfibers and crack-seal type veins. This complex system may result in the capturing of uranium, particularly if the mineral forming fluid travels through strata containing elevated uranium levels. This study tests the feasibility of using U-Pb LA-ICP-MS dating on fault-hosted carbonates to determine the absolute timing of brittle deformation in the Front Range region.

Fault-hosted carbonates from a variety of locations and strata within the Cañon City embayment region were processed into polished thick sections (ca. 100 µm thick) and dated using LA-ICP-MS U-Pb geochronology. Samples from carbonate veins crosscutting the Jurassic Morrison Fm. at the Marsh Felch Dinosaur Quarry produce a high precision, pre-Laramide, Cretaceous age coinciding with the timing of the Sevier orogeny and basin formation. A texturally younger vein from the same outcrop yields a less precise Laramide date, indicating a second phase of deformation and associated fluid flow. Samples from fault surfaces in the Ordovician Harding Fm. along Shelf Road show data interpreted to represent open-system behavior from Mid-Cretaceous to Neogene. They likely reflect the result of multiple fluid flow events triggered by uplift causing calcite growth/recrystallization. Future work will use laser ablation U-Pb analysis of carbonates in combination with detailed fieldwork and petrographic analysis to produce improved interpretations of the absolute timings of brittle deformation in the southern Rocky Mountains.