29-6 Reevaluating Progressive Metamorphism in Dutchess County, New York, Using Novel Thermodynamic Modeling Methods and Tools

Session: Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories (Posters)

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Abstract:

Metamorphism in Dutchess County, NY, represents a classic example of a Barrovian sequence (Balk 1936 & Barth 1936), ranging from unmetamorphosed shales to granulite facies. Previous studies of this sequence have constrained temperature (T) and pressure (P) of the prograde sequence from ~475ºC, c. 3-4 kbar in the garnet zone to ~720C, c. 5-6 kbar across the second-sillimanite isograd (Whitney et al., 1996). Over the past decades, advances in the development of thermodynamic models for metapelites in expanded chemical systems (Holland & Powell, 2011; White et al., 2014a, 2014b; Forshaw & Pattison 2023) as well as the development of methods and software for the collection and processing of thin section-scale geochemical maps using electron probe microanalysis (EPMA) (e.g., Lanari et al., 2014; Duesterhoeft & Lanari, 2020) have grown the toolbox to investigate metamorphic mineral assemblages and the P,T information they record. These advances warrant a re-examination of the metamorphic conditions recorded in this classic Barrovian sequence, nearly thirty years after the first quantitative P,T estimates of Whitney et al. (1996), and close to a century after the first descriptions of this sequence. For this study, Wavelength-Dispersive X-Ray Spectroscopy (WDS) was used to collect EPMA maps of samples from the garnet, lower-staurolite, upper-staurolite, and kyanite zones. Map data were processed using XMapTools 4.4 (Lanari et al. 2014; 2019) to quantify mineral compositions and local or effective bulk rock compositions. Bingo-Antidote (Lanari & Hermann 2020) and Theriak-Domino (de Capitani & Petrakakis 2010) were used to evaluate relative P,T conditions of metamorphism. The results provide an updated insight into the metamorphic history of these rocks and the P-T path(s) experienced by these rocks. The garnet zone sample records initial burial and garnet growth conditions at 490 ± 10°C and c. 3.4 kbar determined from pseudosection garnet isopleth modeling in the MnNCKFMASHTO system. Excluding the garnet core from the bulk composition and using garnet mantle compositions yields peak P-T conditions at c. 570°C and c. 6.1 kbar. Finally, pseudosection modeling indicates isothermal retrogression to roughly 540-560°C and 2-3.5 kbar. This analysis will be expanded to staurolite and kyanite zone samples and compared with P,T results from Whitney et al. (1996) to provide updated constraints on the conditions of metamorphism recorded by garnet growth in the Dutchess County Barrovian sequence.

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