Microstructures and mineral chemistry of feldspars and micas within protomylonitic to mylonitic granite and pegmatite of the Bitterroot dome of the Idaho batholith in western Montana have been studied to determine the metamorphic conditions of deformation. More fully developed mylonitic microstructures in the pegmatites suggest that they experienced stronger ductile deformation than the granite, in contrast to evidence for more competent behavior of pegmatite observed in most mylonitic terrane. This, coupled with occurrence of the pegmatite as streaks and layers parallel to the mylonitic fabric in the granite, suggests that the fabric existed in the granite prior to crystallization of the pegmatite. It also indicates that the pegmatite was deformed not during a later metamorphic event but during the final stages of its crystallization, and thus; during the final stages of crystallization of the batholith itself at this locality. In the granite, myrmekite appears to have formed continuously during deformation at near-magmatic conditions, as a product of a reaction that also produced muscovite from K-feldspar. Two-feldspar geo-thermometry yields a minimum deformation temperature of ∼500 °C; however, the microstructures and field relations stated above suggest that some mylonitization occurred at higher temperatures.

If the intrusive age of the batholith at this locality is Cretaceous, these results are consistent with the tectonic model that mylonitization occurred its the 15- to 17-km-thick Sapphire block slid eastward off an early dome, causing Cretaceous thrusting in the eastern Sapphire block and facilitating rise of the present Bitterroot dome. If the intrusive age is younger, these findings support post-Cretaceous mylonitization that is presumably unrelated to thrusting in the Sapphire block.

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