A GEO AND THERMOCHRONOLOGICAL STUDY OF THE ELK AND WEST ELK MOUNTAIN RANGE, SOUTHWEST COLORADO

The Elk Mountains and West Elk Mountains of southwest Colorado are dotted with Mid-Tertiary plutons that can provide important data about the transitional interval between the end of Colorado Mineral Belt (COMB) magmatism and the onset of San Juan volcanic field activity. Additionally low temperature thermochronology will help constrain southwest Colorado exhumation and any possible link to a present-day low velocity zone in the upper mantle known as the Aspen Anomaly. These large laccoliths and stocks intrude thick exposures of Pennsylvanian/Permian and Cretaceous strata, as well as some smaller exposures of Early Tertiary strata. Seven elevation transects were collected from five laccoliths and one stock. Samples were typically collected at a 500-foot elevation increment, providing an average of five to six samples for each pluton. Results to date for several samples of the Snowmass stock taken over a large elevation range yield concordant LA-ICPMS U/Pb zircon and ⁴⁰Ar/³⁹Ar biotite ages. The zircon ages are 33.22 +1.9/-0.86 and 34.57 +0.48/-0.96 Ma for Capitol Peak and 33.83+0.76/-0.82 Ma for Snowmass Peak. Biotite total gas ages cluster between 33.8 and 34.0 Ma (relative to Fish Canyon sanidine at 28.02 Ma). K-feldspar age spectra are nearly flat and are slightly younger (0.2 to 0.5 Ma) than the intrusion age, indicating cooling below at least 250°C soon after emplacement. A K-feldspar for the lowest elevation sampled at Capitol Peak has a spectrum with an age gradient between about 28 and 34 Ma that indicates cooling from 250°C to about 150°C during this time. Combined these results indicate that plutonic rocks that make up Capitol Peak and Snowmass Peak are well approximated by a single large intrusion. This contrasts with older COMB plutons that are often comprised of multiple and highly age discordant intrusions.