MULTI-MINERAL (U-TH)/HE THERMOCHRONOMETRY OF THE STANDARD BOULDER AND OUTCROPS OF THE MCCLURE MOUNTAIN SYENITE, WET MOUNTAINS, COLORADO

The McClure Mountain syenite of Colorado’s Wet Mountains is an important U-Pb and ⁴⁰Ar/³⁹Ar standard. The best known occurrence of this standard is a boulder in a parking lot thought to be sourced from a nearby gravel quarry. This boulder yields zircon, titanite, and apatite U-Pb dates, as well as hornblende ⁴⁰Ar/³⁹Ar dates, that are indistinguishable at ~524 Ma. Our study had two goals: 1) to use the diversity of U-Th rich minerals in this rock, including baddeleyite, titanite, zircon, and apatite, to characterize the relative temperature sensitivities of different (U-Th)/He thermochronometers, and 2) to identify new sampling localities within the McClure Mountain Complex that are comparable to the standard boulder.

(U-Th)/He analysis of minerals from the standard boulder yielded reproducible dates of 543 ± 5 Ma, 498 ± 9 Ma, and 459 ± 20 Ma for baddeleyite, titanite, and zircon, respectively. This pattern of He dates is compatible with the estimated relative closure temperatures of these phases. Apatite He dates are positively correlated with eU concentration and range from 70 to 245 Ma. Three samples were collected from the McClure Mountain Complex and separated for accessory minerals. Zircon and apatite were extracted from all three samples, but titanite and baddeleyite were obtained from only one of them. The one outcrop sample containing titanite, zircon, and baddeleyite yielded He data that overlap with the results from the standard boulder. Apatite (U-Th)/He data were acquired for all three outcrop samples. Minor variations in the apatite dates among these three samples and the standard boulder suggest that different parts of the complex underwent different cooling histories. Based on the apatite He data and mineral assortment we suspect that the standard boulder’s provenance is the quarry.