STRUCTURE AND PETROLOGY OF COMPLEXLY DEFORMED GNEISSES IN THE MAULDIN 7.5-MINUTE QUADRANGLE, WESTERN INNER PIEDMONT, GREENVILLE, SOUTH CAROLINA

Outcrops on the Greenville Technical College Barton Campus, Greenville, SC (Mauldin 7.5' quadrangle) expose lithologic and geologic features (rock contacts, ductile and brittle structures) in multiple rock units which are useful for understanding the local Six Mile crystalline thrust sheet relationships in the Inner Piedmont. A systematic investigation involving field observations, meter-scale detailed geologic mapping of individual exposures compiled into GIS, and structural and petrographic analyses indicate a complex metamorphic, intrusive, polyphase folding, and ductile and brittle deformation history. Major rock units are migmatitic fine- to medium-crystalline biotite-hornblende-quartzo-feldspathic gneiss with accessory garnet (grading into amphibolite), psammitic biotite gneiss, schistose garnet-sillimanite-biotite gneiss, migmatitic quartzo-feldspathic gneiss, and garnet-muscovite-biotite-microcline augen gneiss (BAG). Augen in BAG have myrmekite rims. Concordant pegmatites with mafic selvedges are localized adjacent to psammitic gneiss, separating it from BAG; discordant pegmatites are common in hornblende-bearing rocks, although present in all lithologies. Interlayered metamorphic units have southwest-dipping foliations and compositional layering. Polyphase fold generations include locally transposed isoclinal folds, west-vergent overturned flexure folds, and chevron folds (plunging gently ~N80W). The latter deform bundles of aligned sillimanite needles in gneiss. Ductile faults are present in BAG. Fractures mimic the trend of regional N60E faults. Sill-like bodies of BAG intrude all units, with sharp scalloped contacts. Foliation in BAG produced by the alignment of microcline augen may be a relict flow foliation, as evidence of ductile shearing is not shown. Retrograde metamorphism is indicated by the formation of muscovite and chlorite at the expense of prograde garnet and sillimanite. The retrograde effect could represent contact metamorphism by BAG.