EOCENE MAGMATISM AND EXTENSION IN SOUTHWEST MONTANA: PRELIMINARY AGE AND GEOCHEMICAL CONSTRAINTS ON THE DILLON VOLCANIC GROUP

Eocene volcanic centers span over ten degrees of longitude in the northern intermountain west and exhibit an equally wide array of magmatic assemblages. Volcanic centers in Montana exemplify magmatic diversity during Eocene time, ranging from silicic granitoids formed in metamorphic core complexes in western Montana, to highly alkaline intrusions emplaced in Late Cretaceous sediments of the Cordilleran foreland to the east. These complex spatio-magmatic trends have proven difficult to reconcile in published tectonic models. Here, we examine volcanogenic sequences of the Eocene Dillon Volcanic Group (DVG) through detailed field mapping, U-Pb zircon geochronology (n = 31), and bulk-rock geochemistry (n = 54) that help to elucidate tectonomagmatic processes in southwest Montana.

The DVG rests on an erosional unconformity truncating Cretaceous and older rocks that are strongly deformed by contractional structures of the Cordilleran fold-thrust belt. Chemically uniform rhyolite lava domes (~49.9–49.2 Ma; 72.1–70.1 wt. % SiO₂) located ~2 km northwest of Dillon, Montana comprise the oldest silicic flows mapped. Approximately 15 km southwest of Dillon, a complex, intertonguing sequence of dacite-andesite flows (50.0–48.6 Ma, 68.2–56.8 wt. % SiO₂) and rhyolite-dacite tuff (49.9–48.1 Ma; 73.6–61.2 wt. % SiO₂) compose the bulk of the DVG and appear to form a volcanic neck locally. These volcanic sequences are overlain by distinctive quartz-eye rhyolite lavas and related ash-flow deposits (48.1–46.6 Ma, 81.9–67.6 wt. % SiO₂). Basalt and basaltic andesite flows (55.4 –46.7 wt. % SiO₂) rest on an angular uniformity that caps the DVG sequence south of Dillon. Andesite-basalt intrusions (60.3–48.6 wt. % SiO₂) cut the Eocene volcanic rocks and subjacent units throughout the map area. All the volcanogenic units are enriched in incompatible elements and contain abundant Precambrian zircon xenocrysts that together reflect substantial fractionation, mixing, and assimilation of melts generated by an active plate margin. The DVG is locally tilted and deformed by numerous faults indicative of E-W to SW-NE extension. We speculate that Eocene magmatism was contemporaneous with the incipient Muddy-Grasshopper normal fault that daylights ~18 km to the west.