PROTEROZOIC TECTONISM IN SOUTHWESTERN MONTANA: DEEP-TIME LOW-TEMPERATURE THERMOCHRONOLOGY OF BASEMENT ROCKS

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.