Academic Affiliation: Missouri State University
After serving his country in the United States Navy for eight years, Wesley Weisberg decided to return to school to pursue his passion for the geosciences and is now a junior at Missouri State University working towards a B.S. in Geology. Inspired by Earth sciences at a young age by his father, Wesley’s focuses are in petrology, mineralogy, geo/thermochronology, structural geology, and tectonics and he plans on developing these fields further in graduate school. His research at RESESS involved the study of an intrusive syenite in Colorado’s Wet Mountains by use of (U-Th)/He thermochronology to help constrain thermal histories and to develop new ways to analyze thermal histories specific to (U-Th)/He thermochronology. Study of this syenite and units like it will contribute to a more complete geologic picture of orogenic processes of Colorado’s Ancestral Rockies and the Laramide uplift.
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.