Coupling Vertical Transect Zircon (U-Th)/he and Raman Spectroscopy Data to Constrain Colorado Front Range Evolution

Havranek, Rachel E.

Graduate Thesis Or Dissertation

Coupling Vertical Transect Zircon (U-Th)/he and Raman Spectroscopy Data to Constrain Colorado Front Range Evolution Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/4q77fr59r

Abstract

The Front Range is the largest and easternmost Laramide uplift in Colorado and exposes primarily Proterozoic basement. Front Range “fourteeners” with their large vertical relief and abundant zircon provide the opportunity to 1) use the variable He retentivity of a suite of highly damaged zircons to better constrain the Neoproterozoic – Late Cretaceous evolution of the Colorado Front Range and 2) integrate zircon Raman spectroscopy data with zircon (U-Th)/He (ZHe) data to improve the understanding of how radiation damage influences the kinetics of ZHe diffusion and annealing. Pikes Peak samples were collected at elevations from 2084 m to 4297 m. ZHe dates for 6 samples range from 762 ± 87 Ma to 82 ± 6 Ma and display uniform negative date-eU correlations, consistent with reduced He retentivity at high radiation damage doses. There is no correlation between elevation and date. ZHe data are in broad agreement with previously published apatite fission-track (AFT) dates from the same elevation range that vary from 449 ± 57 Ma to 45 ± 4 Ma and were used to interpret the position of the Late Cretaceous 110°C isotherm at a modern elevation 2600 m. In contrast to the Pikes Peak results, existing ZHe and AFT dates from an elevation profile on Longs Peak, a “fourteener” in the northern Front Range are uniformly Laramide (76 ± 21 Ma to 43 ± 5 Ma) in age, implying a higher Late Cretaceous geothermal gradient in the northern Front Range than the southern Front Range. The ZHe data are compatible with the hypothesis that north to south differences in the thickness of the Pierre Shale prior to denudation caused the spatial variability of the geothermal gradient. Raman data indicate that partial annealing of radiation damage did not strongly affect the zircon suite from Pikes Peak. Alpha dose estimates obtained from Raman spectroscopy data are in broad agreement with those suggested the by ZHe results. Further integration of Raman spectroscopy and ZHe data in the future will improve understanding of how damage accumulation, damage annealing, and parent isotope zonation influence complex ZHe data sets.

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