Checking billy barr’s snow log…
The Elk and West Elk Mountains of southwest Colorado are dotted with Mid-Cenozoic plutons that can provide important geochronological and thermochronological data about the transitional interval between the end of Colorado Mineral Belt (COMB) magmatism and the onset of San Juan volcanic field activity. 40 Ar/ 39 Ar biotite and U/Pb zircon geochronology are used to determine intrusion age whereas low temperature apatite fission-track (AFT) and apatite (U-Th)/He (AHe) thermochronology constrain southwest Colorado exhumation. Exhumation patterns provide possible links to presentday low velocity zones in the upper mantle. Intrusion ages (relative to Fish Canyon sanidine at 28.02 Ma with 2 sigma errors) decrease from north to south: Mt. Sopris 34.59 0.12; Capitol 33.96 0.06; Snowmass 33.82 0.04; Marcellina 30.76 0.11; E. Beckwith 29.62 0.11; and Mt. Gunnison 29.78 0.17 Ma and can be broadly grouped into ca. 34 Ma and 30 Ma suites. The AFT ages .0 Ma) and AHe ages (Sopris 12.4 -9.9; Capitol 11.2 -8.2; Snowmass 15.0 -5.8; Ragged 11.6 -5.6; Beckwith 29.4 -15.1; and Gunnison 34.6 -32.0 Ma) for all the transects are either concordant with or younger than their coexisting intrusion ages. An exception is Mt Gunnison, which has AHe ages that are older than the coexisting biotite age. In general, the majority of the plutons experienced slow exhumation that started immediately to 10 Ma after emplacement. However, the two southernmost plutons, Beckwith and Gunnison, underwent exhumation earlier than the northern plutons and at a much more rapid rate. Additionally, the low temperature data for these two plutons is equal within error to their Ar/Ar biotite ages and demonstrate emplacement structurally above the ambient 110C isotherm. This data and compilation of regional thermochronological results demonstrate highly variable AFT and AHe ages that appear to indicate a pulsed 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.
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