AGS Monthly Meeting

Title: Structure, Geochemistry, Geochronology, and 2- and 3-dimensional strain analysis of the Long Island Creek Gneiss with proximity to the zone of most intense shearing in the Brevard Zone in western Georgia and eastern Alabama

Abstract:  Two- and three-dimensional strain analysis of the Long Island Creek Gneiss (LICG) was conducted from natural exposures in western Georgia and eastern Alabama.  The LICG is typically a sphene (titanite)-epidote-biotite-quartz-feldspar gneissic granite that is medium- to coarse-grained and moderately- to well- foliated; locally the unit contains muscovite and microcline.  The outcrop belt of the LICG is one of the longest and most continuous exposures of gneissic granite in the region, and is the most continuous lithology found within the Brevard Zone (BZ).  The LICG is a recognizable marker unit that has been mapped for ~140km along strike, from Sandy Springs, GA, to southwest of Roanoke, AL, and is crossed by the zone of most intense shearing (ZIS) within the BZ.  The LICG and BZ are subparallel with a divergence/convergence angle of ~7 degrees.

A total of 13 hand samples of the LICG were collected along the strike belt from Sandy Springs, GA to Roanoke, AL.  Strain analysis was conducted on the hand samples and the final ellipticity (R_f) and final orientation (ɸ) were calculated. The measured strain in the LICG generally decreases from R_f=3.3 near the ZIS to R_f=1.8 near Sandy Springs, GA, north of the ZIS.

Samples of the LICG were collected from regions north, south, and within the ZIS, and major, minor, trace elements, and REEs were measured by ICP-OES and ICP-MS.  The geochemistry of these samples reveals a probable igneous protolith with a granite to granodiorite composition.  Discrimination diagrams show that half of these samples are A-type granites and the other half have an island arc signature

Three samples of the LICG were ground and sieved for separation of zircons for U-Pb geochronology. Sample LIGC-1, which is located south of the ZIS near Roanoke, Alabama, yielded an age of 449.41 ±0.83 Ma; this sample is very close to the Long Island Creek Fault, south of the ZIS, and just north of the Katy Creek Fault.  Sample LIGC-9, which is located north of the ZIS near Campbellton, Georgia, yielded an age of 449.86 ±0.85 Ma, and Sample LIGC-6, which is located north of the ZIS near Sandy Springs, Georgia, yielded an age of 450.24 ±1.14 Ma.

 Speaker bio:  Randy Kath is professor of geology at West Georgia University. He has a Doctor of Philosophy in Geology from the Institute for the Study of Mineral Deposits at the South Dakota School of Mines and Technology, and over 32 years’ experience in academic, consulting and industry practice.  In addition to teaching engineering geology, structural geology, economic geology, field geology and geologic mapping, Dr. Kath is currently performing geologic mapping and structural interpretation at the 7.5-minute quadrangle scale in the Piedmont/Blue Ridge & Valley and Ridge provinces of northwest Georgia.  These geologic quadrangles are currently being used to identify mineral, aggregate, and groundwater resources by a variety of end users.