Southeastern Section - 66th Annual Meeting - 2017

Paper No. 20-2
Presentation Time: 8:00 AM-12:00 PM


HARSTAD, Rylleigh P.1, WHITMORE, John P.1, STELTENPOHL, Mark G.1, KATH, Randy L.2, TEFEND, Karen S.2 and SCHWARTZ, Joshua J.3, (1)Department of Geosciences, Auburn University, 2050 Beard-Eaves Coliseum, Auburn, AL 36849, (2)Department of Geosciences, University of West Georgia, 1601 Maple St, Carrollton, GA 30118, (3)Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330,

The relationship between the lithologies of the Jacksons Gap Group (JGG) defining the Brevard shear zone (BSZ) in Alabama to those of the structurally lower Emuckfaw Group (EG) of the Eastern Blue Ridge (EBR) has been a topic of interest over the past several years. In this study, we report field and petrographic observations from the Long Island Creek Gneiss (LICG), the most continuously exposed granitic body intruding the BSZ and EBR of Alabama and Georgia. The LICG is typically a medium- to coarse-grained and moderately- to well-foliated and lineated titanite-epidote-biotite-quartz-feldspar granite gneiss. Due to the very high feldspar content of the LICG, weathering yields a characteristic light yellow-colored soil, leading to the term Yellow Dirt Gneiss as it is known for exposures in Yellow Dirt, Georgia. The LICG has been mapped for nearly 150 km along strike and notably, the LICG can be traced across the zone of most intense shearing within the BSZ and there is no apparent offset where it is crossed by the BSZ, although crystal-plastic mylonitic textures are prevalent (e.g., deformation twins, strain-related myrmekite, and asymmetric mica fish). Near metro-Atlanta the LICG occurs southeast of the BSZ; to the southwest, in Heard County, Georgia, it occurs completely within the BSZ and is intensely sheared and silicified (flinty crush rock); further southwest, in Randolph County, Alabama, the LICG occurs northwest of the BSZ. The only age reported for the LICG is a Rb-Sr whole-rock isochron-age of ~460 Ma. Comparing characteristics of the LICG with other intrusive bodies found within the EG (i.e., Zana Gneiss, Kowaliga Gneiss) may provide new insights into the relationship between the JGG and the EBR. Samples of the LICG were collected from exposures within the 1:24K Roanoke East, Alabama quadrangle for geochemical analysis using ICP-MS and ICP-OES methods and geochronologic analysis using LA-SF-ICP-MS methods. Geochemical and geochronological data will be available at the time of presentation.