Paper No. 19
Presentation Time: 8:00 AM-12:00 PM
GEOCHEMICAL AND PETROGRAPHIC COMPARISON OF AMPHIBOLITE AND ASSOCIATED GNEISS FROM THE PHENIX CITY GNEISS IN THE UCHEE BELT OF WESTERN GEORGIA
The Phenix City Gneiss (PCG) is the main unit comprising the southern and northern limbs of the Lake Oliver Synform (LOS) within the Uchee Belt (UB) of west-central Georgia and eastern Alabama. Amphibolite and associated gneiss of the PCG are compared within the UB on either side of the LOS using a combination of field relations, petrography, and geochemistry. This work expands the data set of Chalokwu and Hanley (1990), especially for the southern limb rocks. Both northern and southern amphibolite appear as layers, lenses, or xenoliths within the PCG. Chalokwu and Hanley (1990) record amphibolite containing plagioclase, hornblende, and clinopyroxene with garnet being abundant in some rocks. Northern amphibolite from within the Goat Rock Fault are fine grained protomylonitic with plagioclase porphyroclasts and matrix of finer hornblende. Southern amphibolite typically contain hornblende and plagioclase with variable amounts of biotite and contain a variety of accessory minerals, including clinopyroxene, epidote, sphene, zircon, and opaques, and tend to be coarser grained than their northern counterparts. When plotted on total alkali-silica diagram, northern amphibolite have a narrower range in SiO2 content (45-53 wt%), whereas the southern limb shows a wider range (41-57wt%). Amphibolite from the southern limb are primarily alkalic, whereas those from the northern limb tend to be sub-alkalic to tholeiitic with some overlap. Harker diagram plots show an increasing trend in alkali and a decreasing trend in CaO, MgO, and Fe2O3, with increasing SiO2 content. Trace element plots, e.g. Zr vs Nb and Ba vs Sr, reveal that northern limb samples are more clustered in comparison to samples from the south. A plot of Zr/Nb vs Ba/Nb distinguishes northern and southern limb amphibolite. Southern limb amphibolite show modest LREE enrichment relative to HREE. REE slopes vary between locations, as well as within a site, e.g. La/Yb in the southern limb range from 1.2 to 8.5, with La/Yb solely from Lindsay Creek ranging from 1.4 to 7.9. La/Yb in the northern limb range from 1.5 to 2.7, showing more homogeneity than is found in the south. This work in progress will discuss the protolith and tectonic significance of amphibolite and associated gneiss of the UB.