Abstract
Intermediate igneous rocks exposed in the Badger Pass area and 3.5 km away in the McDowell Springs area of Beaverhead County, Montana, previously mapped as Cretaceous intrusive (Ki), and Cretaceous undifferentiated volcanics (Kvu) respectively, exhibit little geochemical variation. Trace element, and lead isotope analyses provide strong evidence allowing for a single source. REE patterns, obtained through ID-ICP-MS, are essentially identical. Mineral/melt Eu analyses reveal that Eu behaved predominantly as a divalent cation, refuting an earlier study asserting that trivalent Eu dominated. Data suggest rocks were formed under low oxygen activity conditions, not oxidizing conditions as previously reported. Geochemical data combined with field mapping allow us to establish the temporal relationship between late Cretaceous thrusting, intrusion, and volcanism in this locale. Folding, faulting and thrusting were significantly, if not entirely, completed prior to the commencement of volcanism. Volcanism included contemporaneous thrust plate intrusion, foreland extrusion, and hypabyssal foreland intrusion.
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