TIMING AND RATE OF EXHUMATION OF THE BOULDER AND PIONEER BATHOLITHS IN SOUTHWESTERN MONTANA

The Boulder and Pioneer Batholiths are Late Cretaceous intrusive complexes exposed in southwestern Montana. Both batholiths intrude a sequence of Precambrian through Mesozoic sedimentary rocks and the Boulder batholith also intrudes its own volcanic cover, the Elkhorn Mountain volcanics. Investigations in these easternmost batholiths of the North American Cordillera have suggested that their construction was intimately linked to deformation taking place in the foreland fold and thrust belt of the Cordillera in latest Cretaceous through Eocene time. The depth and timing of emplacement as well as cooling history of the batholiths are poorly understood yet critical to understand spatial and temporal relationships between magmatism, volcanism, deformation and exhumation in the fold and thrust belt.

(U-Th)/He in apatite thermochronology for 3 phases of the Boulder batholith were collected at high and low elevations in each phase and yield cooling ages that range from latest Cretaceous to Eocene in age. The Butte Granite ages are 66.8 ± 7.5 Ma and 51 ± 4.3 Ma, Rader Creek pluton ages are 55.3 ± 4.0 Ma and 57 ± 4.8 Ma and Camp Thunderbird/Colorado Gulch plutons 60 ± 6.6 Ma and 56 ± 4.6 Ma. Only the Rader Creek ages are not internally consistent but are within error. These data yield vertical uplift rates for the Boulder batholith that range from 0.05 to 0.5 mm/yr. Using Al-in-hbl barometry from Houston and Dilles, 2013 for the Butte Granite, which put emplacement depths in the 5.5-9.5 km range, and the new (U-Th)/He ages, uplift rates range from 0.1 to 0.7 mm/yr. Together these rates of uplift suggest a consistent exhumation of the batholith from emplacement to shallow levels in the crust. The timing of uplift of the Boulder batholith lies right at the transition between Sevier style folding and thrusting and Laramide style block uplifts and understanding the timing and rates of uplift can help constrain the shirt in tectonic regime as well as how sedimentary basin evolution during this timeframe. (U-Th)/He thermobarometry on apatites from the Pioneer batholith are in progress and will help elucidate the exhumation of the batholith and Late Cretaceous-Early Eocene uplift history and basin evolution in southwestern Montana.