The tectonic region that encompasses Southwestern Montana is a petrologically complex area containing several batholiths and thrust faults, underlined by Precambrian basement rock and capped by sedimentary rocks. Intrusive volcanism of Southwest Montana best represented by the Pioneer and Boulder batholiths is a product of the eastward subduction of the Farallon Plate underneath the North American Plate during the Mesozoic time. Geochemical modeling made evident that the Pioneer and Boulder batholiths have a comagmatic relationship. This conclusion is derived from variation, spider and REE diagrams along with petrographic and geochemical models. The intrusion of these batholiths is likely related to the emplacement of a detached portion of the Idaho batholith known as the Sapphire block. Future models that are outside of the scope of this research must consider the evidence proposed in this document to produce an overarching model for the intrusion of the Pioneer and Boulder batholiths in the incredibly dynamic tectonic setting of the Mesozoic.

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.