THERMOBAROMETRY AND METAMORPHIC MINERAL GROWTH SEQUENCES IN BANDED AMPHIBOLITES FROM CARROLL COUNTY, GEORGIA, SOUTHERN APPALACHIANS

Quantitative pressure-temperature-time (PTt) paths constrain changes in metamorphic conditions during orogenesis and test models of tectonic evolution. This study highlights the results of continuing work that seeks to provide better constraints on PT evolution in the Southern Appalachians of western Georgia. Fresh samples of banded amphibolite gneisses have been collected from two localities in the vicinity of Carrollton, west-central Georgia. These amphibolites are part of the Ropes Creek Metabasalt, part of the Inner Piedmont province of the Southern Appalachians. The foliation within these rocks consists of alternating centimeter-scale bands of pargasitic amphibole + oligoclase-rich layers and epidote + quartz-rich layers. Zircon, rutile, ilmenite, and titanite are present as accessory phases; detailed characterization of the spatial distribution of these minerals was used to determine their growth history. Titanite is present in both amphibole- and epidote-bearing layers, and commonly contains inclusions of amphibole or epidote, indicating that it grew at or near peak metamorphic conditions. Zircon is found as inclusions within titanite and within the matrix of each layer, indicating that zircon maintained equilibrium throughout metamorphism. Rutile is overgrown by titanite and absent from the matrix, indicating that rutile is not part of the equilibrium assemblage representing peak metamorphic conditions. Previously completed hornblende-plagioclase major-element thermometry PT estimates of 620-660 °C and 6-8 kbar are supplemented by more sophisticated thermobarometric models and trace-element thermobarometric techniques. Systematics of Ti and Zr distribution among metamorphic minerals phases have been the basis of several recently-developed thermobarometers (e.g. TitaniQ [Wark and Watson, 2006, CMP 152: 743-754], Zr-in-Titanite [Hayden et al., 2008, CMP 155: 529-540]); when used in conjunction with compositional zoning patterns in major rock-forming phases, these can significantly improve the resolution of PTt paths calculated for this portion of the Southern Appalachians.