OLIGOCENE MAFIC MAGMATISM IN THE GRAVELLY RANGE, SOUTHWESTERN MONTANA (U.S.A.): NEW CONSTRAINTS ON THE DILLON VOLCANICS

The Dillon volcanics are a mapped package of basalt to rhyolite intrusions and lavas that crop out in southwest Montana (U.S.A.), primarily around the town of Dillon. Dillon volcanism has been divided into three episodes, where the first was partially coeval with regional Absaroka, Challis, and Lowland Creek volcanism (e.g., lower Dillon volcanics) ca. ~54-39 Ma. The Middle and Upper Dillon volcanics occurred ca. ~33-16 Ma, coeval with the onset and occurrence of widespread lithospheric extension and sedimentary basin formation in southwest MT and adjacent Idaho. Tectonic models proposed to explain this widespread period of magmatism are debated and center on two primary models: [1] an overall decrease in the subduction angle and/or slab rollback; [2] or slab tearing and slab foundering, coupled with asthenosphere upwelling. To address these models, we have started a regional study of the Middle-Upper Dillon volcanics. Here, we present new age and bulk rock geochemical data from a reconnaissance suite of samples collected in the Gravelly Range and the adjacent Centennial Valley, MT. Five new samples yield groundmass ⁴⁰Ar/³⁹Ar dates of ~32-30 Ma, which overlaps with some locally reported ages, including the ca. ~28.6 ± 0.8 Ma Black Butte stock. Previously published bulk rock major element data for Dillon volcanics show a subalkaline to high-K, calc-alkaline suite and the aforementioned basalt to rhyolite compositions. These rocks also have volcanic arc-like element signatures (e.g., large-ion lithophile element, LILE, enrichments and high field strength element, HFSE, depletions relative to primitive mantle) and strong positive Pb anomalies. The new suite of rocks are mildly alkaline to calc-alkaline basalts and trachybasalts. Relative to primitive mantle, all lack significant HFSE depletions and not nearly as enriched in LILEs; their overall trace element patterns are more similar to ocean island basalts and much younger Snake River plain basalts. Regionally, major and trace element geochemical analyses only show subtle variation between the Absaroka, Challis, and lower Dillon volcanics. Ongoing work will be focused on deciphering the geochemical differences exhibited by our new suite of Middle Dillon rocks and the older, regionally extensive magmatism of the Absaroka-Challis-lower Dillon volcanic event.