EOCENE IGNIMBRITE FLARE-UP IN THE NORTHEASTERN U.S. CORDILLERA: TIMING AND STYLE OF VOLCANISM IN THE LOWLAND CREEK VOLCANIC FIELD OF SOUTHWESTERN MONTANA

The Lowland Creek Volcanic field (LCV) formed from 53 Ma to 49 Ma during the “challis volcanic episode”, and the waning stages of thick-skinned deformation in SW Montana. The LCV is unusual with respect to most challis-aged volcanic rocks in Montana because it consists mainly of evolved composition caldera deposits that erupted into hanging wall block faults of an active detachment fault, the Anaconda Metamorphic Core Complex. The volcanic pile is exposed in a 65 km long, NE-trending, belt within the Late Cretaceous Boulder Batholith. The LCV averages 10 km wide, is up to 2,000 m thick, and has an original estimated volume of 310 km3. Most of the preserved deposits are intra caldera facies tuffs and lava domes that formed in three general stages. Stage 1 caldera-forming eruptions produced primarily gas and pumice-rich rhyolite (75.1-70.1 wt. % SiO₂) tuffs between 52.9 Ma and 51.8 Ma. The rhyolite tuffs were subsequently tilted 15 - 25°. Stage 2 eruptions are characterized by intra caldera rhyodacite (72.0-65.0 wt. % SiO₂) lavas, vitrophyre, fumaroles, and breccia that formed between 51.8 Ma and 51.1 Ma. Rhyodacite lavas are banked into NE-striking normal faults that exhibit 200 -300 m of local paleotopography. Stage 3 activity produced isolated dacite-andesite lava domes, rhyolite dikes, breccia pipes, and diatremes between 51.1 Ma and 49.0 Ma. Breccia pipes and diatremes have Au and Ag ores that likely formed during caldera resurgence. Stage 3 rhyolite dikes contain inherited Archean-Cretaceous zircons, which suggests LCV magmas utilized crustal-scale weaknesses during their formation. Results of this study favor a model in which the LCV formed in a transtensional pull-apart basin that reflects the onset of slab roll-back of the Farallon Plate. Slab roll-back is potentially the trigger for “ignimbrite flare-up” in the northeastern U.S. Cordillera.