DETERMINING PROTOLITH AND METAMORPHIC GRADE FOR BIOTITE-GARNET GNEISS IN THE RUBY RANGE, SW MT

The goal of this research is to determine the protolith or protoliths and conditions of metamorphism of biotite-garnet gneisses (BGG) found in the Precambrian Dillon Gneiss of the Ruby Range (southwest MT). Fifteen samples were collected from a 3 km transect along Cottonwood Creek road. Ten of these fifteen samples were analyzed for whole rock geochemistry (both major elements by XRF and trace elements including REE by ICP-MS), and thirteen out of the fifteen samples were made into thin sections for SEM analysis and studied petrologically. The interpretations of protoliths and tectonic environment that arise from these results will be given during the poster presentation.

The Ruby Range contains uplifted exposures of Precambrian crystalline basement rocks, which are considered part of the Montana Metasedimentary terrane (MMT) of the NW Wyoming province (Mogk et al., 1992). The Dillon Gneiss is structurally between two other Precambrian metamorphic suites in a west-dipping sequence (James, 1990). The Dillon Gneiss is known to include mylonitized leucogranite with an intrusive age of ~2.43 Ga yielded by U-Pb dating, and pelitic gneisses with metamorphic ages of both ~2.43 and ~1.77 Ga yielded by LASS monazite dating (Baldwin et al., 2017; Baker et al., 2017).

Although some sections of the Ruby Range have been closely studied, the Dillon Gneiss has been neglected because it is a complex, undivided suite of rocks that is volumetrically dominated by quartzofeldspathic gneiss. The BGG was targeted for this study because it is possible to determine the temperature of metamorphism where garnet and biotite occur in equilibrium. Ultimately, the goal is to understand what continental dynamics were in motion in Laurentia during the time of gneiss metamorphism. With this information, understanding how continents are created and then grow from the amalgamation of smaller bodies can continue to expand.