Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 18-4
Presentation Time: 9:00 AM


KAEMPFER, Jenna, Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green St., Urbana, IL 61801, GUENTHNER, William, Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 and PEARSON, David, Geosciences, Idaho State University, 921 S. 8th Ave, STOP 8072, Pocatello, ID 83209

Archean and Paleoproterozoic basement rocks of southwestern Montana have been subjected to multiple tectono-thermal events since ~3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt-Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier-Laramide orogeny. The emerging application of the zircon (U-Th)/He thermochronometer to understand deep-time (Precambrian and older) thermal histories provides new opportunities to interpret the long-term tectonic evolution of this region. We investigate this prolonged tectono-thermal history by integrating multiple low temperature thermochronometers (apatite (U-Th)/He, zircon (U-Th)/He and apatite fission track) and inverse modeling the results with the Monte Carlo Markov-chain program HeFTy. Our data were collected from nine basement localities (n=55 zircon and n=26 apatite aliquots) in the northern and southern Madison ranges, the Ruby-Gravelly ranges, and the Tobacco Root Mountains. Negative relationships between single aliquot date and effective uranium (a proxy for radiation damage) in our zircon (U-Th)/He data are interpreted with a thermal history model that considers the damage-He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures in excess of 400 ˚C to near surface conditions between 800 and 510 Ma, with some variation between ranges. Our long-term, low-temperature thermal record for these southwestern Montana basement ranges shows that: (1) these basement blocks experienced multiple episodes of upper crustal exhumation and burial since Archean time and (2) Neoproterozoic cooling below 200 ˚C was coincident with early rifting of Rodinia and Snowball Earth glaciation. Our low temperature thermochronological results demonstrate the power of integrating multiple low-temperature thermochronometers to understand previously undetermined Neoproterozoic thermal events in basement rocks.