PETROLOGY AND GEOCHEMISTRY OF CLASTS IN COARSE-GRAINED METASEDIMENTS OF THE OCOEE SUPERGROUP, COHUTTA AREA, GEORGIA: AN EROSIONAL RECORD OF THE GRANITIC BASEMENT

The basal coarse-grained metasediments of the Late Proterozoic Ocoee Supergroup provide a unique opportunity to determine the nature of the poorly exposed underlying Precambrian basement rock. In contrast to the interbedded and overlying fine-grained metasediments that have been substantially changed by metamorphism, the coarse-grained sedimentary rocks and their clasts are characterized by a more stable and structurally competent anhydrous mineral assemblage. Original textures and mineral compositions are commonly preserved, therefore sedimentary and igneous names are used to describe these rocks. To determine the nature of the source area, clasts were collected from a variety of metasediments including diamictite, cobble-conglomerate, and pebbly sandstone. The granitic composition of the source rock for these basal units of the Ocoee Supergroup is evident from the highly arkosic composition of the sandstones. Petrographic, x-ray and geochemical data from approximately twenty samples further indicate a texturally diverse but compositionally similar source area. Most of the clasts are granites; however, a few samples of gneiss were also found. The igneous clasts are predominantly coarse-to fine-grained two-feldspar (subsolvus) granites. Zircon is a common and sometimes very abundant accessory phase. These granitic clasts display similar mineral and whole-rock major-and trace-element compositions, but are characterized by highly variable textures. The presence of blue quartz in several of the granites commonly results in a darker color more typical of diorite than granite. Several clasts of gneiss were also collected that are similar in composition to the granitic clasts, possibly representing a source rock for the granitic magmas.