Zircon (U-Th)/He Age-eU Correlations Reveal Long-Term Thermal History of Laurentian Basement
Devon A. Orme, Stanford University
William R. Guenthner, University of Illinois Urbana-Champaign
Andrew K. Laskowski, University of Arizona
Peter W. Reiners, University of Arizona
The broad temperature sensitivity of the zircon (U-Th)/He thermochronologic system, as a function of accumulated radiation damage, provides a unique tool to constrain long-term (> 1 Ga) thermal histories of cratons. We use zircon (U-Th)/He thermochronology to understand the Proterozoic-Phanerozoic thermal history of Laurentian basement in the northern Rocky Mountain region of the western United States. Zircon (U-Th)/He ages of Wyoming craton crystalline rocks from the hanging wall of major Laramide thrust faults in the Wind River Range (54 ages) show distinct negative correlations with effective uranium (eU) concentrations. Zircons show a range of ages from 582 Ma at low eU to 33 Ma across a wide range of higher eU from about 1000-7000 ppm. We use a radiation damage and annealing model to fit these age-eU correlations, documenting a multistage thermal history of the Precambrian basement. The best fit to the Wind Rivers data involves two phases of cooling: 1800-1600 Ma and 900-700 Ma followed by slower cooling until 525 Ma. During the Phanerozoic, these samples were heated to maximum temperatures between 160-125°C, prior to Laramide cooling to 50°C between 60-40 Ma. The most recent parts of this thermal history are consistent with published apatite (U-Th)/He and fission-track constraints on the timing of Laramide exhumation in the Wind Rivers (~60 Ma), but our model also constrains the pre-Laramide history of these rocks. These data are consistent with cooling associated with Yavapai-Mazatazal tectonism followed by two phases of Meso- and Neoproterozoic intracratonic extension.