CONSTRAINTS ON THE THERMOBAROMETRIC EVOLUTION OF AMPHIBOLITES FROM THE SOUTHERN APPALACHIAN INNER PIEDMONT, WEST-CENTRAL GEORGIA

The development of quantitative pressure-temperature-time (PTt) paths for metamorphic rocks can provide important constraints on the rates of change in conditions during orogenesis and can be used to test orogen-scale tectonic models. This study highlights an ongoing effort to evaluate PT evolution in the Inner Piedmont of west-central Georgia using a suite of mineral, whole-rock, and in-situ trace-element thermobarometric techniques to assess the metamorphic conditions experienced by banded amphibolites exposed in Carroll County, Georgia. As field-based stratigraphic and, therefore, tectonic interpretations of these rocks are contentious, analysis of petrologic constraints and the application of recently-developed thermobarometers may help shed additional light on the evolution of the Southern Appalachian orogen.

The amphibolites in this study are generally coarsely-layered, with alternating centimeter-scale bands of pargasitic amphibole + oligoclase-rich layers and epidote + quartz-rich layers. Peak conditions based on major-element exchange thermometers were previously estimated at 620-670 °C. To further quantify the PT path, in-situ accessory-phase thermobarometers (e.g. Zr-in-rutile, Zr-in-titanite) were then applied. The textural relationships of Ti- and Zr-bearing accessory phases (ilmenite, titanite, rutile, zircon) in these samples are well-characterized: titanites are abundant throughout the samples, contain epidote and amphibole as inclusions, and can be found as rims on rutile; rutile is only present as an inclusion within titanite; zircon, while not abundant, can be found both as inclusions and in the matrix. These relationships suggest that rutile grew during an earlier stage of the metamorphic history, while titanites grew at or near peak conditions. LA-ICP-MS analyses of rutile and titanite, conducted at The University of Texas at Austin, support this interpretation: Zr-in-rutile thermometry gives temperatures of 625 ± 35 °C, while Zr-in-titanite thermometry results cluster near 700 - 720 °C. Combined with thermobarometric models and construction of pseudosections, these data can help to resolve PT paths for the Southern Appalachians and refine tectonic models for orogenesis.