Semester of Graduation
Master of Science (MS)
Geology & Geophysics
Prograde metamorphic pressures and temperatures of Archean high-grade garnet-bearing lithologies from the eastern Beartooth Mountains of Montana provide an important constraint on the tectonometamorphic history of this terrane and the early Earth in general. A particularly useful means to approximate prograde metamorphic conditions is examining entrapment conditions of garnet mineral inclusions during garnet growth. Lithologies of the eastern Beartooth Mountains are well-suited to this approach because of the presence of abundant mineral inclusions within garnet porphyroblasts. Consequently, prograde metamorphic pressures and temperatures in the Beartooth Mountains, conditions that have only been broadly constrained previously, can be more accurately determined and used to constrain the tectonic environment responsible.
Four high-grade garnet-bearing lithologies from the eastern Beartooth Mountains were examined to constrain prograde metamorphic paths: peraluminous migmatites, garnet-biotite gneisses, iron formations, and mafic granulites. Optical petrography and cathodoluminescence (CL) imagery were used to target areas for subsequent Raman and chemical analysis. Quartz inclusion entrapment pressures were calculated by measuring Raman spectroscopic peak position changes of quartz as a result of relative changes in strain. Mineral inclusions and matrix grains were chemically analyzed on an electron microprobe (EMP) and the data were used to calculate prograde entrapment temperatures and peak metamorphic conditions, respectively, using various geothermobarometers.
Inclusion thermometry coupled with Raman barometry predicts prograde entrapment conditions of 675-775°C and 9-11 kbar. Predicted conditions of inclusion entrapment agree with the upper limits of calculated peak metamorphic pressures but are below calculated peak metamorphic temperatures. These data are interpreted to represent inclusion entrapment during isobaric growth of garnet hosts at peak metamorphic pressures as temperatures increased in a clockwise pressure-temperature path. Such prograde paths likely resulted from burial due to collisional tectonics followed by isobaric heating from emplacement of local plutonic bodies. These findings place new constraints on the tectonometamorphic evolution of Beartooth rocks. Additionally, these findings demonstrate that garnet mineral inclusion studies can be used to quantitatively constrain prograde conditions of Beartooth metamorphism.
Cliff-Tuttle, Larry Fisher Jr, "Constraining Prograde Metamorphic Paths in Archean High-grade Garnet-bearing Lithologies from the Eastern Beartooth Mountains, Montana, USA" (2020). LSU Master's Theses. 5082.