GEOCHEMICAL INVESTIGATION OF THE LONG ISLAND CREEK GNEISS: IMPLICATIONS FOR THE PALEOZOIC TECTONICS OF THE SOUTHERN APPALACHIANS

The Long Island Creek Gneiss (LICG) is a long, linear unit oriented NE-SW in western Georgia and eastern Alabama, and is mappable for a distance of ~130km along strike. A geochemical investigation the LICG was conducted to support field investigations conducted by Crawford and Kath (2015), who show that the LICG is cross-cut by the Brevard Zone, a major shear zone regarded by some to be a terrane boundary. Samples of the LICG were collected from regions north, south, and within the zone of intense shearing, and major, minor, trace elements, and REEs were measured by ICP-OES and ICP-MS.

The geochemistry of these samples reveal 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. Samples of LICG from within the Brevard Zone are silicified (75.4 – 92.5 wt% SiO₂), and all LICG samples south of the Brevard Zone have >70 wt% SiO₂. In contrast, LICG samples obtained north of the Brevard Zone have a larger range in SiO2 (66.5 – 77.4 wt% SiO₂), with the lower silica samples <70 wt% SiO₂ having elevated Al₂O₃, Fe₂O₃, MgO and CaO. The lower silica samples also have higher concentrations of Ba, Sr, Co and Ni, consistent with less evolved compositions that may result from crystal accumulation within the magma chamber. Based on the aluminum-saturation index (ASI), and the modified alkali-lime index, (MALI), by Frost and Frost (2008), the LICG samples from within and south of the Brevard Zone are all periluminous, and calc-alkalic, alkali-calcic, or calcic; all of these samples are also ferroan. Samples from north of the Brevard Zone have the same ASI and MALI classifications, but a few of the samples are magnesian.

Based on detailed geologic mapping by Crawford and Kath (2015), mineralogy and shear stress indicators of LICG samples (Hanson et al., 2019), and geochemical analysis, the Long Island Creek Gneiss pre-dates the latest motion of the BZ (~317 Ma), confirming that the Brevard Zone is not a major terrane boundary.