Paper No. 6
Presentation Time: 1:30 PM-5:00 PM
DETAILED PETROGRAPHIC AND MICROSTRUCTURAL ANALYSIS OF SHEARED BIOTITE GNEISS WITHIN THE KINNARD CREEK FAULT, JASPER COUNTY, GEORGIA
Detailed geologic mapping and field observations have identified the Kinnard Creek fault within the Inner Piedmont Cat Square terrane of Central Georgia. Previous work interpreted the structure as a southwest-directed, dextral-oblique thrust fault (Davis, 2010). Motion along the fault is correlated with Acadian or Neoacadian deformational pulses (Heubner et al., 2011). Unusual porphyroclastic hornblende, garnet, and feldspar crystals are present in a narrow band within and immediately adjacent to the fault zone. Orthogonal sections, cut either parallel to or perpendicular to the mineral stretching lineation, were prepared in order to examine the structural, textural, spatial, and chemical characteristics of the rocks exposed in the Kinnard Creek Fault in more detail using petrographic and SEM-EDS analytical techniques. Garnet and hornblende porphyroclasts contained abundant inclusions of ilmenite and plagioclase feldspar. While inclusion density within garnet generally increases toward the rim of the crystal; the inclusions are not concordant with the matrix fabrics, indicating that garnet growth was pre-kinematic. Garnet does not appear to be strongly resorbed. Hornblende porphyroclasts also appear to be pre-kinematic; some contain tension cracks that have been filled with biotite, and σ-type recrystallized tails are visible in hand sample and thin-section. Plagioclase crystals commonly exhibit σ-type tails. Biotite, alkali feldspar, muscovite, and quartz are common phases in the matrix; accessory apatite, zircon, and epidote-group minerals are present in the matrix and as inclusions within the hornblende and garnet porphyroclasts. Preliminary chemical characterization found slight increases in Mn content in garnet rims, with elevated Ca and Mg contents in the near-rim. These zoning patterns may correspond to the changes in inclusion density. Combined with microstructural characterization, compositional data can better constrain the evolution and timing of metamorphic conditions before and during shearing within the Kinnard Creek Fault.