Tectonic Implications of Tertiary Intrusion and Shearing within the Bitterroot Dome, Northeastern Idaho Batholith

Abstract

The Bitterroot dome contains plutons of the northeastern Idaho batholith and their regionally metamorphosed country rocks. Several mesozonal to katazonal intrusions in the core of the dome have zircon U-Pb ages (lower intercept on concordia) which range from . Epizonal plutons in the northern Idaho batholith region have Eocene K-Ar ages. An extensive mylonite zone developed on the eastern margin of the dome during the latter stages of uplift. Zircon (lower intercept) ages from three sheared plutons in this zone are , and The fact that significant shallow to deep-seated intrusion and mylonitization occurred during Eocene time requires a careful evaluation of Early Tertiary tectonism in the northern Idaho batholith region. Cretaceous events appear to include multiple deformation, metamorphism, and intrusion of mesozonal to katazonal plutons into the evolving root zone of the northern Rocky Mountain overthrust belt. Early Tertiary events appear to include intrusion of mesozonal to epizonal plutons into the Late Cretaceous igneous-metamorphic complex and diapiric uprise of the Bitterroot dome accompanied by mylonitization on its eastern flank that was superimposed on the already existing thrust system.