GEO AND THERMOCHRONOLOGY OF ELK AND WEST ELK MOUNTAIN RANGE PLUTONS, SOUTHWEST COLORADO: TEMPORAL AND SPATIAL INTRUSION AND EXHUMATION VARIATIONS

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. ⁴⁰Ar/³⁹Ar biotite and U/Pb zircon geochronology are used to determine intrusion age where as low temperature apatite fission-track (AFT) and apatite (U-Th)/He (AHe) thermochronology will help constrain southwest Colorado exhumation and any possible link to a present-day low velocity zone in the upper mantle. Seven elevation transects (150 m increments) were collected from six plutons. Intrusion ages (relative to Fish Canyon sanidine at 28.02 Ma with 2 sigma errors) decrease from north to south and are: Mt. Sopris 34.93 ± 0.14; Capitol 33.96 ± 0.06; Snowmass 33.82 ± 0.04; Marcellina 30.76 ± 0.06; E. Beckwith 29.68 ± 0.04; and Mt. Gunnison 29.66 ± 0.03 Ma and can be broadly grouped into ca. 35 Ma and 30 Ma suites. AFT ages for Snowmass Mountain and Capitol Peak are substantially younger than emplacement ages. Snowmass AFT ages vary systematically with elevation and range from 28.8 to 21.7 Ma and at Capitol Peak they have less variation between 22.3 and 23.8 Ma. It appears that Capitol cooled later than Snowmass, which may indicate a south tilting exhumation surface across the pluton. In contrast to Snowmass and Capitol, the Beckwith laccolith has AFT cooling ages of 30.8 to 31.0 Ma that are equal within error to Ar/Ar biotite ages and demonstrate emplacement structurally above the ambient 110°C isotherm. Additional AFT and (U-Th)/He results for these plutons will refine local and regional cooling patterns. This data and compilation of regional thermochronological results demonstrate highly variable AFT and AHe ages that appear to indicate a pulsed (in time) and discrete (in space) Cenozoic exhumation history. Short wavelength upper mantle velocity variations may correlate to thermal history differences of exposed rock at the 100’s of meter scale with the overall low velocity mantle driving dynamic rock uplift and high surface elevation that is not correlated to crustal thickness variation.