Doctor of Philosophy (PhD)


Geology and Geophysics

Document Type



Archean-aged rocks represent the only record of Earth development during a critical period of its early development. The Archean igneous and metamorphic rocks that comprise the eastern Beartooth Mountains of Montana preserve a complex geologic history characterized by multiple overprinting metamorphic events that is challenging to reconstruct. Constraining parameters associated with development of this important Archean setting such as the regional metamorphic pressure-temperature path as well as the number and timing of tectonometamorphic events is critical for reconstructing early Earth evolution in the Archean. This study reconstructs the igneous and regional metamorphic history that resulted from the main stages of tectonism in the eastern Beartooth Mountains at ~2.79-2.83 Ga using data from xenolithic garnet migmatites, a cordierite-orthopyroxene rock (COR), and the Quad Creek metanorite (QCMN). Detailed petrography and noted field relationships are paired with mineral- and whole-rock-based P-T modeling and U-Pb zircon geochronology to interpret the petrogenesis of these rocks.

P-T modeling demonstrates that migmatites and the COR preserve a polymetamorphic history with a general clockwise metamorphic P-T path. Once peak metamorphic pressures were reached during prograde metamorphism, xenolithic rocks experienced approximately isobaric heating to peak granulite-facies conditions of ~0.50-0.70 GPa and 750-800 ºC (M1) and later overprinting by two successively lower-grade hydration events: an amphibolite-facies event (M2) and a greenschist-facies event (M3). Zircon U-Pb geochronology of the QCMN indicates that it was emplaced (and subsequently metamorphosed) after the main stages of xenolith metamorphism ~2.79 Ga, synchronous with the waning stages of plutonism producing the Long Lake Magmatic Complex (LLMC) that hosts studied xenolithic lithologies. The modeled P-T path of xenolithic rocks and the interpreted petrogenetic history of the QCMN demonstrate that the rocks in the study area were produced in a compressional environment associated with a continental arc subduction zone. This research provides definitive evidence for modern-style plate tectonic processes being operative in the eastern Beartooth Mountains during the Archean eon at ~2.79-2.83 Ga.



Committee Chair

Henry, Darrell J.



Available for download on Thursday, March 18, 2027