PALMETTO GRANITE-BREVARD ZONE-KATY CREEK FAULT RELATIONSHIPS AS ILLUSTRATED ON THE PALMETTO, CAMPBELLTON, RICO, AND WINSTON, 7.5-MINUTE QUADRANGLES: GEORGIA PIEDMONT/BLUE RIDGE

Named for the town of Palmetto, Fulton County, Georgia, the Palmetto granite is a massive, poorly foliated to unfoliated, light-gray, porphyritic, muscovite-biotite-quartz plagioclase-microcline granite. Commonly contains zoned microcline phenocrysts as long as 5cm. The granite tends to form pavement-style outcrops and large pedestal-boulder outcrops; and weathers to a tan-yellow saprolite with microcline phenocrysts and quartz grains. Where more deeply weathered, the soil is typically light-red. The Plametto Granite is lithologically identical to the Ben Hill Granite.

The age of the Palmetto Granite was first reported by Marvin (1981) as ~291 Ma using K-Ar whole rock methods. The granite intrudes felsic gneisses and schists of the Piedmont and contains very large xenoliths of sillimanite schist. The outcrop pattern of the Palmetto is unique for Permian plutons in the Piedmont with its northern part being generally parallel to the Brevard Zone (BZ) and Katy Creek Fault; the Katy Creek Fault forms the northwestern boundary of the granite near Campbellton, GA. Southwest of Campbellton, the extent of the Palmetto broadens and has an outcrop pattern more typical of Piedmont plutons. On the western margin of the granite near Rico, GA, and on the eastern margin of the granite north of Palmetto, GA, the Palmetto cuts across lithologic units in a typical intrusive relationship.

Preliminary age dates of mica growth in the Brevard Zone of Alabama by Poole (per com) suggest that the last shearing movement occurred ~317 Ma. This, coupled with an age of the Palmetto (~291 Ma), constrains emplacement of the Clairmont thrust sheet along the Katy Creek Fault. Based on these ages and the contact relations along the northern boundary (Katy Creek Fault) and eastern & western boundaries (intrusive), we interpret the granite to represent a syn-tectonic granite that was emplaced during late-stage movement along the Katy Creek Fault. Movement on the Katy Creek Fault appears to be dextral-reverse. This movement appears to be responsible for the overall outcrop pattern of the Palmetto Granite.