Transcript Document

FIELD RELATIONS AND PETROGENESIS OF
AMPHIBOLITES FROM THE WEBSTERADDIE/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 33620
Department of Geosciences, Western Carolina Univ, Cullowhee, NC 28723
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% 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 WebsterAddie 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
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Generalized geology of the Webster-Addie
(WA) and Balsam Gap (BG) areas.
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.
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