Zircon (U-Th)/He data for the Colorado Front Range “fourteeners” and testing Cryogenian exhumation of sub-Great Unconformity basement

Abstract

We present zircon (U-Th)/He(ZHe) data for 17 Proterozoic basement samples from elevation profiles on the three peaks in the Front Range of Colorado that are at elevations over 14,000 ft. (4267 m, locally known as “fourteeners”), with complementary Raman data for a sample subset. ZHe dates from Longs Peak in the northern Front Range are uniformly Late Cretaceous to Eocene (37 ± 1 Ma to 137 ± 2 Ma), while those from Mt. Evans (34 ± 1 to 606 ± 18 Ma) and Pikes Peak (111 ± 5 Ma to 773 ± 17 Ma) in the central and southern Front Range are as old as Cryogenian. The results suggest hotter Late Cretaceous to Eocene burial temperatures to the north, attributable to northward thickening Cretaceous burial as proposed previously. Neoproterozoic sandstone injectites in the central range and kimberlite-hosted Cambrian sedimentary xenoliths in the northern range document that Precambrian basement was at the surface in the Cryogenian and/or in the Cambrian, as inferred previously for basement below the Great Unconformity in the southern range. Thermal history modeling demonstrates that ZHe data from the three fourteeners are compatible with the hypothesis of initial basement exhumation throughout the range by the Cryogenian. The study outcomes highlight important considerations for resolving deep-time thermal histories, including younger burial magnitude, detailed sampling position in structural blocks, availability of paleosurface markers, and construction of inverse thermal history models that integrate geologic knowledge and are designed to test geologically realistic thermal history scenarios.