CONSTRAINTS ON THE P-T PATH FOR UPLIFT OF THE UCHEE BELT BASED ON MICROTHERMOMETRY OF FLUID INCLUSIONS IN METAMORPHIC ROCK

The Uchee Belt of west central Georgia and eastern Alabama is the southernmost exposure of the Appalachian Piedmont, composed mainly of amphibolite grade felsic and hornblende gneiss, schist, and amphibolite. The Uchee Belt is an east plunging synform consisting of a north and south limb. Chalokwu and Kuehner (1992) report peak metamorphism at 10 kbars and 750 C and retrograde metamorphism at 6 kbars and 524 C. Rocks of the Uchee Belt contain numerous fluid inclusions that were probably trapped during the uplift history of the belt. We determined the composition of and conducted microthermometric experiments on fluid inclusions from samples collected from different areas of the north and south limbs of the Uchee Belt. We calculated isochores for the inclusions that may define constraints on the uplift portion of the P-T path. In particular, we studied quartz in the form of veins and pegmatitic segregation associated with metamorphism. Studies of these samples revealed fluid inclusions of varying composition including 2-phase CO2 rich inclusions, as well as 2- and 3- phase inclusions containing various combinations of H20, CO2, and NaCl. Samples from the north limb collected from Motts gneiss contain an abundance of pure CO2 two-phase inclusions running along fracture planes, indicating that they are secondary or pseudosecondary, while a number of 3-phase inclusions appear to be primary. 2-phase aqueous inclusions are rare in samples from the north. Samples from the south limb contain similar inclusions, although there is a larger number of 3-phase inclusions. Quartz samples from Phenix City gneiss at Eagle and Phenix City Dam and Kendall Creek on the south limb contain 2-phase H2O-NaCl and 3-phase CO2-H2O-NaCl fluid inclusions whose minimum trapping conditions range from 200-360 C at relatively low P. Some of the CO2-H2O-NaCl inclusions had isochores roughly parallel to the proposed P-T path. Samples collected from Motts gneiss in the north limb contain 2-phase pure CO2 inclusions, which homogenize between 10-25 C and produce slightly more shallow isochores. When all of the isochores calculated thus far are superimposed with data for peak and retrograde metamorphism, the P-T path must travel in a downward clockwise curve as pressure and temperature decrease in order to capture all of the inclusions.