Graduation Year

2015

Graduation Month

May

Document Type

Thesis - Campus Access Only

Degree Name

Bachelor of Science

School or Department

Geosciences

Major

Geosciences

Faculty Mentor

Julie Baldwin

Faculty Mentor Department

Geosciences

Subject Categories

Geochemistry | Geology | Tectonics and Structure

Abstract

The Ruby Range in southwest Montana preserves a record of the tectonic events that occurred >2.5 to 1.7 billion years ago and records the growth and evolution of the North American continent at that time. In particular, metamorphic rocks and deformational structures found in the range preserve evidence for a regional continental collision 1.79–1.72 billion years ago, which has been termed the Big Sky orogeny. Garnet amphibolites found in the three distinct compositional suites hosted within the Ruby Range, the Christensen Ranch Metasedimentary Suite, the Dillon Gneiss Suite, and the Pre-Cherry Creek Suite, contain appropriate mineral assemblages to constrain the peak pressures and temperatures at which the rocks were metamorphosed. By determining the peak pressure and temperature conditions experienced by the amphibolites, the grade of metamorphism that occurred during orogenesis was determined. Samples of garnet amphibolites were collected from the Ruby Range for whole rock geochemistry, petrography, and SEM (scanning electron microscope) mineral chemistry analysis. Major and trace element compositional data was obtained for one of the three suites to determine possible protolith rock types and petrogenesis. Mineral chemistry data was also collected with the SEM-EDS (energy dispersive spectroscopy) to use in the calculation of the pressure and temperature conditions for each of the three compositional suites. This information determined that the three suites in the Ruby Range experienced different pressures and temperatures and showed no trend in peak metamorphic conditions across the range. The peak conditions for the Christensen Ranch Metasedimentary Suite and Pre-Cherry Creek Suite of 670-780 °C and 7.4-9.9 kbar, suggest overall higher pressures during the Big Sky orogeny then given by previous studies. The peak conditions for the Dillon Gneiss Suite also suggest greater peak conditions with a result of 830-870 °C and 9-10 kbar that puts it in to lower granulite facies. The P-T estimates from this study, along with previous studies in the Ruby Range are consistent with estimates of peak metamorphism in the Tobacco Root Mountains that have been correlated to the Big Sky orogeny. The information obtained from the Ruby Range amphibolites can be integrated with data from other lithologies to gain a better understanding of the origins and characterize the metamorphic event(s) that affected this margin of North America during the Big Sky orogeny.

Honors College Research Project

Yes

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© Copyright 2015 Brianna Berg