FIELD RELATIONS AND PETROGENESIS OF AMPHIBOLITES FROM THE WEBSTER- ADDIE/WILLITS REGION, EASTERN NC BLUE RIDGE Rachel SORARUF, Anne SCHERER, Judy HARDEN, Rory MCILMOIL, Jeff RYAN, Ginny PETERSON, Steve YURKOVICH, Jonathan BURR, Sarah KRUSE, Skylar PRIMM, Jonathan NEWBY, David DOUGHTY, Clayton LOEHN, Bryan ALLISON, Christina BRUINSMA, Rachel SHANNON, Susan AKERS 2001 National Science Foundation Summer REU Program at University of South Florida and Western Carolina University Department of Geology, University of South Florida, 4202 East Fowler Ave, Tampa, FL Department of Geosciences, Western Carolina Univ, Cullowhee, NC Abstract Amphibolites in the vicinity of the Webster-Addie (WA) ultramafic body, and in and around Willits, (WI) in the eastern Blue Ridge province of NC, occur as lenticular exposures of varying size (from < 1 m across to road cut and map-scale occurrences), generally enclosed in biotite schists and gneisses of the Tallulah Falls Formation. Textures of these rocks are strongly migmatitic to gneissic. Typical WA/WI amphibolite mineral assemblages are hbld+plag+qz+opaques ±biotite± garnet with accessory chlorite, epidote, sphene, zircon, and apatite. Leucosome samples show myrmekitic textures, which are unusual in metamorphic rocks. Amphibolites often occur in close proximity to the Webster-Addie ultramafic body, but do not show clear contact relationships with ultramafic rocks. Geochemically, WA-WI amphibolites range from 49-63% SiO 2, 1-9% MgO, 14-17% Al 2 O 3, 4-16% Fe and % TiO 2. These rocks plot as a single cluster on most major and trace element variation diagrams, suggesting amphibolites are compositionally similar across the region. Based on their trajectory on a TAS diagram, the volatile-free compositions of WA-WI amphibolites are consistent with basaltic andesite to andesite protoliths, On an AFM diagram, WI-WA amphibolites follow a more "calc-alkaline" trend. The geochemistry of WA-WI amphibolites are unlike amphibolites from either the Buck Creek or Carroll Knob mafic-ultramafic complexes to the SW, as they are inconsistent with mafic cumulate compositions. There appears to be no petrogenetic relationship between WA-WI amphibolites and the spatially associated Webster- Addie or Balsam Gap ultramafic bodies. Our observations point to significant variation in amphibolite protoliths and petrogenesis along strike within the central Blue Ridge of southwestern North Carolina, Assessing amphibolite compositions and origins may prove to be a useful tectonic discriminant in evaluating Blue Ridge terrane associations. Generalized geology of the southwestern North Carolina Blue Ridge showing location of mafic and ultramafic rocks Generalized geology of the Webster-Addie (WA) and Balsam Gap (BG) areas. 1 3 Detailed geologic maps of the Webster-Addie and Balsam areas. (1) Distribution of mafic and ultramafic rocks in the Blanton Branch and Willets areas. (2) – (4) Detailed maps of the Blanton Branch, Balsam Gap, and Chestnut Gap areas, respectively, including structural features. 4 Detailed mapping was conducted in the vicinity of the northeastern Webster-Addie body, the Willets area and the Balsam Gap area by four different mapping groups. The focus of mapping was to determine the spatial distribution of ultramafic and mafic rocks and characterize the contact relations between lithologies. Both map and outcrop-scale mapping helped to determine the character and distribution of amphibolite bodies and their relationship to the ultramafic rocks 2