Year of Award
Master of Science (MS)
Department or School/College
Department of Geosciences
Julia A. Baldwin
Rebecca Bendick, Michael D. DeGrandpre
Ruby Range, Big Sky orogeny, monazite, petrochonology, phase equilibria
University of Montana
Tectonics and Structure
Phase equilibria modeling and monazite petrochronology reveal a complex history for the Ruby Range, SW Montana, which provides insights into the reactivation and growth of the northwestern Wyoming craton. Metapelites and orthoamphibolites from three areas were investigated. Stone Creek, the structurally highest area along the western flank, records a clockwise P–T path with peak conditions of ~6.5 kbar and ~760 °C and post-peak conditions of ~6.0 kbar and ~680 °C. Monazite growth in this area occurred at 1790–1723 Ma, with monazite inclusions in garnet at 1757 ± 8 Ma. Structurally deeper rocks at Elk Gulch in the south-central part of the range record a clockwise P–T path with peak conditions of ~9 kbar and ~780 °C and post-peak conditions of ~5–7 kbar and ~660 °C. Monazite growth in this area occurred at 1784–1733 Ma, with monazite inclusions in garnet at 1769 ± 11 and 1757 ± 8 Ma for two samples. Monazite inclusions in cordierite record growth at 1752 ± 8 Ma. Five monazite grains contain older detrital cores. The structurally deepest rocks at Sweetwater Creek on the eastern flank record a clockwise P–T path with peak conditions of ~8–9 kbar and 750–800°C and post-peak conditions of ~5 kbar and 650–700 °C. Monazite records variable mixing between ca. 2.5 and 1.75 Ga components, with an early population of 2520–2452 Ma relict cores and a younger population at 1783–1713 Ma. Monazite inclusions in garnet record growth at 1754 ± 6 Ma. The lack of older monazite in the Stone Creek area and shallower burial depths suggest that these sediments were deposited after the ca. 2.5 Ga event, either accumulating on a passive margin or juxtaposed as an allochthonous block post-2450 Ma. P–T–t paths from all three areas support a collisional setting for the Ruby Range during the Paleoproterozoic. Prograde metamorphism occurred from 1790–1760 Ma, followed by near isothermal decompression and re-equilibration following post-orogenic extension at 1760–1710 Ma. Collectively, these data indicate that rocks of the Ruby Range record a transition from reworked Archean craton to younger metasedimentary sequences representing Paleoproterozoic growth at the craton margin.
Cramer, Martin, "Proterozoic tectonometamorphic evolution of the Ruby Range, SW Montana, USA: Insights from phase equilibria modeling and in situ monazite petrochronology" (2015). Graduate Student Theses, Dissertations, & Professional Papers. 4385.
© Copyright 2015 Martin Cramer