LARGE-VOLUME PYROCLASTIC DEPOSITS ALONG THE EASTERN EDGE OF THE BOULDER BATHOLITH, SOUTHWESTERN MONTANA

The Boulder Batholith and Elkhorn Mountains Volcanics (EMV) formed concurrently during shortening between ~85-76 Ma near the end of Mesozoic Cordillera arc magmatism. Together they represent an exceptionally well-preserved and voluminous record of continental arc magmatism. We report field and analytical results from studies of the EMV and Boulder Batholith over a distance of 50 km along their contact, from I-90 near Pipestone, north to Boulder, Montana. At Pipestone the EMV are metamorphosed to biotite hornfels and have a sheared contact with the Boulder Batholith. To the north, ~25 km, batholith rocks intrude and cut the foliation of EMV. Outward from their contact with the Boulder Batholith, the EMV consist mainly of dacitic tuffs that correlate to the middle member of the formation. The EMV section is ~700 m thick and consists of three large-volume ignimbrite sheets capped by two thin and local ignimbrite units. Rheomorphic flow lineations show NNE-SSW and E-W bidirectional flow that both parallels and is oblique to their contact with the Boulder Batholith. The age of the section is constrained by a weighted mean U/Pb zircon age of 84.9 +/- 2.6 (2 standard error of mean) obtained from the basal ignimbrite. Diorite sills intrude the section and yield ages of 79 and 81 Ma (⁴⁰Ar/³⁹Ar from hornblende). These data suggest that the section formed between ~80-85 Ma. The EMV middle member ignimbrite sheets appear to correlate over the distance of our study. If the ignimbrite sheets correlate with units exposed farther to the north and west, then individual eruption volumes may rival the largest observed on Earth (>1,000 km³).