Abstract

Fission-track (FT) thermochronology of detrital apatite and zircon from synorogenic sedimentary rocks in the Kiowa Cored Well and the Castle Pines drillhole in the Denver Basin can be used to document a generally predictable unroofing sequence for the southern part of the Colorado Front Range. First, Phanerozoic sedimentary rocks that covered the Front Range in late Mesozoic time contributed recycled sedimentary and far-traveled volcanic zircon and apatite to the oldest sediments as Laramide deformation began. Next, two significant pulses of volcanic grains are recorded in both the detrital zircon and apatite FT age populations in the Kiowa Cored Well as volcanoes along the Colorado Mineral Belt dominated the landscape. The volcanic contribution is not as significant in the Castle Pines well, which is more proximal to the mountain front compared to the Kiowa Cored Well. Higher in the stratigraphic sequence, a mix of volcanic grains, recycled grains from the Phanerozoic cover, and Proterozoic basement grains is present, with the percentage of basement grains in the mixture increasing up section. Finally, in the youngest part of the sequence, metamict zircon grains from the basement and apatite grains, derived from the Proterozoic basement with 50 to 70 Ma apatite fission-track cooling ages derived from below the base of the apatite partial annealing zone in the Front Range, dominate the age populations. Complexities in the form of pulses of volcanic or basement components are superimposed on the simple pattern. This study demonstrates the power of analyzing both apatite and zircon when examining detrital grains derived from a basement-cored uplift where basement resided at shallow levels of the crust (<4 km) prior to deformation.

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