2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 210-68
Presentation Time: 9:00 AM-6:30 PM


ASHWORTH, Alan W., BERG, Christopher A. and ASHLEY, Aaron Wolfgang, Department of Geosciences, University of West Georgia, 1601 Maple Street, Carrollton, GA 30118, aashwor4@my.westga.edu

Two rock specimens collected from the Garnet Hill locality in Paulding County, Georgia--part of the Dahlonega Gold Belt (DGB) terrane of the Southern Appalachians--are the focus of this study. Rocks at Garnet Hill are hydrothermally-altered and metamorphosed seafloor basaltic rocks associated with sulfide-rich zones and banded iron formations within the DGB. Both samples (GH-1 and GH-2) are composed of large (~1 centimeter in diameter) porphyroblasts of garnet in a fine-grained phyllitic chlorite- and quartz-rich matrix, with lesser proportions of magnetite and ilmenite. Multiple thin-sections were prepared orthogonal to the mica-defined foliation for detailed petrographic, mineralogic, and geochemical analyses of each sample.

Comparison of the samples reveals several textural differences. The porphyroblastic garnets in GH-1 are much more idiomorphic and contain fewer inclusions than the garnets in GH-2. The matrix is coarser-grained in sample GH-2, and distinct quartz-rich and chlorite-rich layers in the matrix are much more common than they are in GH-1. The overgrowth of either chlorite-rich or quartz-rich layers appears to control the density of inclusions within GH-2 garnets, reflecting nutrient supply. Garnets in GH-2 contain obvious core and rim domains that are characterized by different populations and proportions of aligned mineral inclusions. The internal fabric defined by aligned ilmenite and quartz inclusions in the garnet cores is distinct from the matrix foliation; garnet rims are very inclusion-rich, the internal fabric wraps the garnet cores and is continuous with the matrix. Garnets in GH-1 do contain identifiable core and rim domains noted by patterns of aligned ilmenite inclusions. In both samples, there are complex garnet clusters in which individual garnet nuclei coalesced during growth of garnet rim domains.

Data from mineral and bulk chemical analyses will be used to constrain models for the P-T conditions during metamorphism of these samples. Insights gained from analyses of petrographic and geochemical processes active at the micron-scale can help refine our understanding of the kilometer-scale orogenic processes that formed the Southern Appalachians.