DECIPHERING THE NEAR-SURFACE EVOLUTION OF TWO COLORADO FRONT RANGE FOURTEENERS USING (U/TH)-HE DATE-ELEVATION TRANSECTS OF RADIATION-DAMAGED ZIRCON

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.