Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard
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New zircon, titanite, and apatite (U-Th)/He (ZHe, THe, AHe) data for four samples of the 524 Ma McClure Mountain syenite (MMS) in the Wet Mountains of Colorado constrain the <200 °C thermal history of this 40Ar/39Ar geochronology standard. ZHe dates vary from 420 to 567 Ma, do not correlate with their limited eU range (66–388 ppm), have dispersion attributable to eU zonation, and show no systematic variation between samples. THe dates are 488 to 515 Ma. AHe dates range from 70 to 211 Ma, with some inter-sample variability that could be due to modest post-70 Ma displacements along faults. Our dataset is consistent with recently published (U-Th)/He data for the MMS, although the published ZHe results extend to higher eU and define a negative date-eU correlation not captured by our lower eU zircons. Existing U-Pb and 40Ar/39Ar thermochronologic data, together with the THe and ZHe results, document rapid post-emplacement cooling of the MMS to ≤200 °C by ~500 Ma. A recent (U-Th)/He study inferred that the MMS subsequently underwent simple slow monotonic cooling until present-day. However, geologic relationships are incompatible with such a history, instead requiring that rocks in the vicinity of the MMS were at the surface at ~480 Ma, ~180 Ma, and ~60 Ma, with possible phases of burial and erosion of varying magnitude between these times. Inverse thermal history modeling can simultaneously satisfy the new and previously published ZHe and AHe data while honoring these geologic constraints. The outcomes of this work highlight that date-eU patterns can be generated by protracted cooling as well as by multiphase heating and cooling histories, and underscore the importance of integrating geologic data to help identify the most probable explanation for a (U-Th)/He dataset.
Weisberg, Wesley R.; Metcalf, James R.; and Flowers, Rebecca M., "Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard" (2018). All Faculty Scholarship for the College of the Sciences. 173.
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