BEDROCK GEOLOGIC MAP OF THE MARSHALL 7.5' QUADRANGLE, NORTH CAROLINA: RECENT MAPPING AND SHRIMP GEOCHRONOLOGY OF PROTEROZOIC ROCKS WITHIN THE BLUE RIDGE BASEMENT COMPLEX

The Marshall 7.5' quadrangle exposes Mesoproterozoic to Paleozoic rocks within the Blue Ridge province of western North Carolina that record the tectonic evolution of the southern Appalachians. The dominant rock type (and map unit) within the quadrangle is biotite-rich granitoid gneiss that characteristically includes outcrop-scale to locally mappable lenses of varied lithologies. These lithologies include biotite hornblende gneiss, calc-silicate & marble, pristine to altered mafic & ultramafic bodies, and compositionally distinct leucocratic gneisses. Regionally recognized hornblende-rich, locally migmatitic, mafic gneisses crop out in the SW portion of the quadrangle and do not extend as far north as earlier mapping suggested. Meso- to Neoproterozoic meta-intrusive mafic bodies throughout the map area may be related to the Bakersville intrusive suite. In the NW portion of the quadrangle a SE-dipping belt of mylonitic Max Patch granite structurally overlies younger, Ocoee group metasediments.

The dominant fabric is a SE dipping foliation, except in the SE where structural trends define a broad folded pattern with gentle SSE-plunging axes. Several Paleozoic high-strain zones strike NE, dip moderately to the SE, and are characterized by abundant greenschist-facies minerals. Asymmetric porphyroclasts and down-dip mineral elongation lineations indicate top-to-the-NW, reverse shear sense.

U-Pb SHRIMP geochronology in progress on several samples from the Marshall and surrounding area aims to resolve previously undetermined or poorly constrained age relations. Geochronology samples include 1) the Max Patch granite, and distinctive, coarse-grained, possibly Max Patch-derived protomylonitic rocks, 2) pristine mafic (meta-gabbroic) bodies possibly related to the Bakersville meta-intrusive suite, and 3) isolated granulite bodies. Based on field observations including the diversity of lithologies and occurrence of granulites, rocks in the Marshall area are reminiscent of the Mars Hill Terrane (MHT), a distinctive assemblage of anomalously older basement rocks that appear out of place with surrounding Laurentian derived crust. Our new data will test connections with the MHT and evaluate the tectonic significance of rocks exposed in this portion of the southern Appalachian basement.