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

Presenter: Erik Richardson

Faculty Sponsor: Michael L. Williams

School: UMass Amherst

Research Area: Geology and Earth Sciences

Session: Poster Session 4, 2:15 PM - 3:00 PM, 163, C5

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, layers of garnet-rich pelitic schist near the Rowe-Moretown boundary 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 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. 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. Ongoing 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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