Pressure-Temperature Path Followed During Exhumation of the Southern Appalachians - Constraints from Microthermometry of Fluid Inclusions in Metamorphic Rocks from the Uchee Belt, Western Georgia and Eastern Alabama

The Uchee belt, the southernmost exposure of the Appalachian Piedmont is a complexly deformed metamorphic complex exposed in west central Georgia and east central Alabama extending parallel to strike from southwest to northeast. It consists of mainly mafic, pelitic and felsic units. We have studied the composition and homogenization temperature of fluid inclusions in samples collected from an amphibolite grade, biotite gneiss unit known as the Phenix City Gneiss. We collected samples in two locations, one below the Eagle and Phenix Dam on the Chattahochee River Phenix City, Alabama and the other at Kendall Creek about 30 kilometers to the east of Phenix City, Alabama. The sample from the Eagle and Phenix Dam was collected from a pegmatititic segregation that lies within the foliation of the gneiss and appears to be partial melt generated at peak metamorphic conditions. Fluid inclusion chips were prepared from a quartz grain within the pegmatite. The samples from Kendall Creek area consist of a folded pegmatite, also occurs in the Phenix City Gneiss, formed at peak metamorphic conditions as well as a late stage quartz vein that cuts the pegmatite. Fluid inclusions from the Eagle and Phenix Dam location included three phase, CO₂-H₂O-NaCl inclusions as well as 2 phase H₂O-NaCl inclusions. Fluid inclusions from the quartz vein at Kendall Creek consisted of arrays of 2 phase H₂O-NaCl inclusions and CO₂. Both the three phase CO₂ rich inclusions and two phase aqueous inclusions were studied. CO₂ rich inclusions occur both as isolated inclusions and as planar arrays of secondary inclusions. Although two isochores from the isolated CO₂ bearing inclusions pass through estimates of peak and retrograde metamorphic conditions (Chalokwu and Kuehner, Am. Min., 1992), homogenization temperatures for this inclusion assemblage vary widely and most inclusions have lower densities. These inclusions are interpreted as having been formed early and subsequently been reequilibrated during uplift due to over pressurization. Isochores for the aqueous fluid inclusions have lower densities and presumably record entrapment during exhumation. Homogenization temperatures and isochores for the lowest density fluid inclusion assemblages cluster tightly and do not appear to have been reequilibrated. By combining estimates of retrograde metamorphic conditions (Chalokwu and Kuehner, Am. Min., 1992) with the homogenization temperatures from the fluid inclusion assemblage with the lowest densities, we can establish constraints on the P-T path these high grade metamorphic rocks followed during exhumation. We find that the exhumation was characterized by near isothermal decompression from about 6 kb and 500 C to 0.2 kb and a minimum of 380 C. Similar P-T paths have been observed for other high grade metamorphic terrains including portions of the Northern Appalachians (Hames et al., Geology, 1989; Winslow et al., J. Metamorphic Geology, 1994).