16-42 Garnet as an Archive of Appalachian Metamorphism: Microstructural and Geochronologic Evidence from Rowe-Moretown Schists in Chester, MA

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The geology of Western Massachusetts involves a complex sequence of tectonostratigraphic units associated with Ordovician island-arc/microcontinent accretions along the Laurentian margin. In the town of Chester, Massachusetts, there are layers of garnet-rich pelitic schist near the Rowe-Moretown boundary. These provide an opportunity to examine the mechanisms, conditions, and timing of the events related to the assembly, and later deformation, of the Northern Appalachian Orogen. In this study, we perform analysis of polished thin sections taken from three sample locations containing abundant garnet. We performed structural analysis of the outcrops in the field, followed by hand-sample description. Billets were cut parallel to both strike and lineation directions, from which the best samples were chosen for thin section preparation. Remnants of an earlier foliation perpendicular to the current foliation, as well as layered and sigmoidal oxide and quartz inclusion trails in garnet were identified using a petrographic microscope. Inclusion trails commonly disappear before reaching the edge of the garnet, suggesting a second stage of growth. Compositional maps at multiple scales were made on the electron microprobe to locate datable phases and to identify locations for quantitative traverses. Xenotime is present, but no monazite was found in these samples. On-going work includes geothermobarometry and forward modeling to constrain the changing metamorphic conditions in the area. Then, xenotime U-Pb geochronology will be used to place temporal constraints on the metamorphism of these rock units. We aim to provide clarification of the tectonic history and deformation involved in the emplacement of the Rowe-Moretown units. This will contribute to a broader understanding of the Taconic orogeny in Western Massachusetts and Appalachian orogenesis of New England.

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