THERMAL HISTORY DETERMINATION OF THE BOULDER BATHOLITH, MONTANA THROUGH 40AR/39AR THERMOCHRONOLOGY

Incremental heating ⁴⁰Ar/³⁹Ar measurements have been performed with seven samples from the Boulder Batholith in southwestern Montana to assess how the thermal history experienced by the batholith relate to “Butte” mineralization and/or crustal extension related to the nearby Anaconda detachment fault. Previously published zircon U-Pb ages of the sampled portion of the Boulder Batholith are 78 ± 1 Ma. In this study, biotite and K-feldspar were analyzed. Biotites yield relatively flat ⁴⁰Ar/³⁹Ar age spectra and total gas ages between 73-78 Ma. K-feldspar on the other hand yield more pronounced age gradients and total gas ages between 65-72 Ma. For comparison, previously acquired apatite (U-Th)/He ages range from 51-67 Ma. Collectively, the new ⁴⁰Ar/³⁹Ar thermal history results from the Boulder batholith predate the timing of extension along the Anaconda detachment that is reported to have occurred between ca. 53-39 Ma based upon results from the exhumed footwall. While this relationship is expected for the shallower hanging wall rocks of the Boulder Batholith, we are also evaluating potential transient heating effects related to the post-extension mineralization.