TEXTURAL, ACCESSORY-PHASE, AND MODELLING CONSTRAINTS ON THE METAMORPHIC EVOLUTION OF AMPHIBOLITE GNEISS, SOUTHERN APPALACHIAN INNER PIEDMONT, WEST-CENTRAL GEORGIA

Constraints on pressure-temperature-time (PTt) paths for metamorphic rocks provide a means to test orogen-scale models for tectonic assembly of the Southern Appalachians. This study focuses on a continuing effort to quantify the evolution of metamorphic conditions in the Inner Piedmont of west-central Georgia through the examination of textural fabrics, analyses of mineral chemistry, and construction of equilibrium models. The fine-scale, in-situ petrologic and thermobarometric constraints from these approaches are especially useful for increasing the spatial and temporal resolution of tectonic models for assembly of this portion of the Appalachian orogen. Results of these analyses will be presented for samples of amphibolite gneiss exposed in Carroll County, Georgia, for which structural and stratigraphic interpretations are contentious; recent mapping and whole-rock geochemistry suggests this area was part of an Ordovician back-arc system.

These amphibolites are composed of alternating centimeter-scale bands of pargasitic amphibole + oligoclase interlayered with epidote + quartz-rich bands. Amphiboles and plagioclase lack pronounced chemical zoning. Epidotes show Ti and Fe zoning, which links growth of the epidotes and changes in Ti-bearing accessory phase stability. Some layers also contain symplectitic intergrowths of epidote + quartz. Ilmenite and titanite are common accessory minerals in these rocks. Some ilmenites are rimmed by titanite, which also contain inclusions of amphibole and epidote. Rutiles are only present as inclusions. These textural relationships suggest that rutile growth occurred during an earlier stage of the metamorphism, while titanites grew at or near peak PT conditions. Zircon is rare but present within both mineralogic layers and as inclusions in Ti-bearing phases. Peak conditions based on major-element hornblende-plagioclase exchange thermometry were previously estimated at 620-670 °C. In-situ trace-element thermometry data give temperatures of 625 ± 35 °C for Zr-in-rutile and results that cluster near 700 °C for a much larger dataset of Zr-in-titanite analyses. These results, together with equilibrium models, will provide better constraints on the PT evolution of this portion of the Southern Appalachians.