Field Relations and Petrogenesis of Amphibolites from the Webster-Addie/Willits Region, Eastern NC Blue Ridge

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Poster Session

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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 roadcut and map-scale occurrences), generally enclosed in biotiteschists 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 is 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% SiO2, 1-9% MgO, 14-17% Al2O3, 4-16% Fe203 and 0.6-1.6% TiO2. 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.

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Citation / Publisher Attribution

Presented at the North-Central Section (36th) and Southeastern Section (51st), GSA Joint Annual Meeting on April 4, 2002