16-39 Evaluating Anomalous Pressure-Temperature Conditions in the Ashe Metamorphic Suite, Northwestern North Carolina

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

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Abstract:

A belt of ultramafic rocks in the eastern Blue Ridge of northwestern North Carolina has been suggested to mark a tectonic discontinuity. The ultramafic rocks consist of talc-tremolite and chlorite-amphibole schists, and are hosted within metagraywacke and pelitic and graphitic schists of the Ashe Metamorphic Suite (AMS). Recent peak pressure-temperature calculations from rocks adjacent to the ultramafic unit identified a possible barometric gradient across the proposed discontinuity. Calculated pressures from the southeastern side were anomalously high and may have reached ~1.1 GPa, whereas pressures on the northwestern side were as low as ~0.6 GPa. In a sample of pelitic schist collected along strike from the same unit, atoll garnet porphyroblasts were observed in thin section. The atoll-garnet texture has been interpreted to form under a wide range of conditions, with some studies suggesting a high-pressure origin. The goal of this study is to investigate the metamorphic history, particularly the peak pressure, of the purported high-pressure rocks in the AMS. We will also investigate the metamorphic conditions which led to the formation of the atoll-garnet texture. The peak mineral assemblage within the sample consists of garnet, biotite, plagioclase, and quartz. Textures and microstructures observed in the sample include euhedral garnet with asymmetric tails containing biotite, muscovite, and quartz, graphitic zoning in garnet matching the crystal shape, and atoll-garnet texture with biotite and quartz in the core of the garnet. In some instances, biotite was also observed around the rim of the atoll garnet. Detailed petrographic analysis, high-resolution compositional mapping and quantitative analysis on the electron microprobe, and in-situ U-Th-Pb monazite dating will be completed for key samples. Peak conditions will be calculated using the garnet-biotite geothermometer and the garnet-aluminosilicate-plagioclase (GASP) geobarometer. The timing of metamorphism will be determined by dating monazite grains within the fabric and included within garnet crystals. With this data, we will be able to estimate the crustal depth in which these units were metamorphosed and provide a constraint on the conditions of formation of the atoll-garnet texture. If successful, this study will help confirm the presence of a tectonic boundary in the eastern Blue Ridge and establish the timing of a key chapter in the formation of the southern Appalachian orogen.

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