Master of Science (MS)
Geology and Geophysics
Compositionally-diverse Archean metamorphic rocks occur as xenoliths within the well-dated 2.8 Ga granitic gneisses that volumetrically-dominate the eastern Beartooth Mountains of the northern Wyoming Province. Common mafic lithologies in the Quad Creek-High Line Trails-Christmas Lake-Wyoming Creek-Line Creek Plateau area include granitic gneiss, amphibolites, mafic granulites and biotite and garnet-bearing peraluminous gneisses. The region also has sillimanite-biotite ± cordierite ± garnet gneisses and migmatites, as well as a single occurrence of a noteworthy orthopyroxene-cordierite gneiss with late-stage anthophyllite. A combination of detailed petrographic analysis and thermobarometric modeling were used to constrain the metamorphic conditions of the xenoliths prior to inclusion in the granitic rocks as well as during intrusion of the granitic rocks. P-T conditions of the mafic rocks, sillimanite-biotite gneisses and the orthopyroxene-cordierite gneiss were determined using “classical” geothermobarometry, a petrogenetic grid and “pseudosection” modeling of bulk rock compositions using different amounts of H2O, up to H2O saturation. Temperatures are estimated by the hornblende-plagioclase, garnet-biotite and Ti-in-biotite geothermometers. Pressures are estimated in garnet-bearing assemblages by the garnet-biotite-plagioclase-quartz (GBPQ) and garnet-amphibole-plagioclase-quartz (GAPQ) geobarometers. Pseuodosections and a petrogenetic grid identified areas in P-T space where assemblages of interest are stable and where specific reactions occur. Combined results from thermobarometry, petrogenetic grid, pseuodosections and petrographic observations indicate four metamorphic events M1-M4. Results from melt thermobarometry for mafic xenoliths suggest initial derivation of the protolith from a mid-ocean ridge environment or from subduction-zone volcanism. The earliest event, M1, is preserved in all three rock types and represents pre-intrusion conditions at granulite facies (~775-800°C, 6.0-7.0 kbar) prior to tectonic mixing of lithologies. A subsequent upper-amphibolite facies and a mid-amphibole facies event with rehydration related to the series of 2.8 Ga granitic intrusions are seen at ~750°C and 650°C. The associated pressures are not well-constrained. A monazite inclusion in a garnet suggests the date of amphibolite facies metamorphism is 2.9 Ga and the amphibolite facies events may be temporally equivalent. The most recent event M4 is a greenschist facies overprint at ~450°C, possibly related to regional open folding.
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Will, Celina N., "Temperature and pressure conditions of Archean amphibolite-granulite facies metamorphic xenoliths from the eastern Beartooth Mountains, Montana and Wyoming, USA" (2013). LSU Master's Theses. 3997.