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.

Typescript. Includes vita and abstract. Includes bibliographical references (leaves 155-177). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.