THERMOCHRONOLOGY OF THE MCCLURE MOUNTAIN SYENITE: CHARACTERIZATION OF NEW HE THERMOCHRONOMETERS AND CONSTRAINTS ON THE COOLING HISTORY OF THE WET MOUNTAINS, COLORADO

The McClure Mountain syenite, located in Colorado’s Wet Mountains, is an important U-Pb and ⁴⁰Ar/³⁹Ar standard. Zircon, titanite and apatite U-Pb dates, as well as hornblende ⁴⁰Ar/³⁹Ar dates, are indistinguishable at 524 Ma, recording syenite emplacement followed by rapid cooling to < 500 °C. The diversity of U-Th rich minerals in this rock – including baddelyite, titanite, zircon, and apatite - also make the McClure Mountain syenite well-suited for characterizing the relative temperature sensitivities of (U-Th)/He thermochronometers, which is important for developing new ways to study thermal histories. We acquired (U-Th)/He data for this mineral suite both for this purpose and to better constrain the <500 °C thermal evolution of the Wet Mountains. (U-Th)/He analysis yields reproducible dates of 543 ± 5 Ma, 498 ± 9 Ma, and 459 ± 20 Ma for baddeleyite, titanite, and zircon respectively. The slightly older than expected baddeleyite date is likely due to the use of zircon stopping distances for alpha-ejection that leads to an overestimate of the corrected He date. The younging pattern of He dates is compatible with the estimated relative closure temperatures of these minerals, and extends the history of rapid cooling to temperatures <180 °C within ~80 m.y. of syenite emplacement. Apatite He dates range from 70 to 150 Ma and are positively correlated with eU concentration. Thermal models of these data reveal protracted cooling or reheating during re-burial. This history may be related to Ancestral Rockies tectonism, with final cooling below ~70°C associated with the Laramide orogeny. Together these data improve our quantitative understanding of the region’s tectonic and thermal history and enable better characterization of He thermochronometers.