GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 345-4
Presentation Time: 9:00 AM-6:30 PM


HAVRANEK, Rachel Elizabeth1, FLOWERS, Rebecca M.2, JOHNSON, Joshua1 and KELLY, Nigel M.3, (1)Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309, (2)Department of Geological Sciences, University of Colorado Boulder, UCB 399, Boulder, CO 80309, (3)Department of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309-0399,

The Colorado Front Range is the largest and easternmost uplift in Colorado and exposes primarily Proterozoic basement. The Late Cretaceous – Early Tertiary Laramide cooling history of the Front Range is relatively well-constrained by apatite fission-track (AFT) thermochronologic data and other geologic constraints. However, neither the Proterozoic through Ancestral Rockies (~300 Ma) nor the post-Laramide history of the Rockies are well understood. We acquired elevation transects of (U-Th)/He dates from Longs Peak and Pikes Peak, two Colorado “fourteeners”, to address two questions. First, we aim to use the results to better inform the pre- and post-Laramide evolution of the Rockies. Second, we hope to better constrain the temperature sensitivity of radiation damaged zircons in the (U-Th)/He system through comparison with AFT results for vertical transects on the same peaks, because a recent study (Guenthner et al., 2013) demonstrated that zircon’s He retentivity is significantly reduced at high radiation damage levels. Pikes Peak samples were collected at elevations of ~2100 m to ~4300 m with ZHe dates ranging from 100 – 770 Ma that demonstrate strong negative date-eU correlations, consistent with reduced He retentivity at high damage levels. These data are in broad agreement with AFT data from the same elevation range that fall between 50-500 Ma. In contrast, the Longs Peak ZHe data from elevations of ~2800 to ~4300 m range from 40 - 150 Ma, with the majority of dates falling between 40 – 80 Ma. These dates show no correlation with eU, and agree with AFT dates of 40 – 80 Ma from the same samples. Together, these data support previous AFT work that documents a systematic north to south variability in the Laramide cooling history. Additionally, we obtained a Raman spectroscopy data set for the Pikes Peak sample suite to evaluate the degree and variability of radiation damage in the suite. The Raman data show a range of damage levels in zircons from each sample, although internal zonation within each grain is limited.