1 Undergraduate Research (Posters)

Saturday, 31 March 2012
7:00 PM - 9:00 PM
Grand Ballroom

DEFORMATION AND METAMORPHIC CONSTRAINTS ON ALUMINOUS ROCKS FROM THE CHUNKY GAL MOUNTAIN FAULT IN THE JAKE RIDGE EXPOSURE, SOUTHWESTERN NORTH CAROLINA

Paper number: 1-42
Program: Technical Sessions
Time: 7:00 PM – 9:00 PM

Authors

Abstract

The large Jake Ridge road cut along Hwy 64 in southwestern North Carolina exposes aluminous sillimanite-garnet rich gneisses, apparently deformed by the Chunky Gal Mountain Fault. The rocks appear compositionally similar to the granulite-facies aluminous rocks at Winding Stair Gap (~10km to the northeast) and preserve textural evidence of a complex deformation and metamorphic history.Observations using the petrographic microscope, SEM/EDS, and preliminary microprobe data indicate a dominant mineral assemblage that consists of garnet, biotite, sillimanite, plagioclase, and quartz. The relatively larger (2-5mm) garnet grains in the slide preserve a complex textural and compositional zoning. The cores of the garnets (up to 0.5-1.0 mm) are rich with inclusions that define trails nearly perpendicular to the external foliation. Inclusion minerals are mostly quartz with minor biotite, plagioclase, ilmenite, apatite, and monazite. A sharp boundary separates the cores from an inclusion-free zone that extends up to 1 mm to the boundaries of the grains. The garnets are unusual in that they appear to have continued to grow into the tail regions of the grains creating garnets that look like eyes. The tail portions of the garnet are mostly symmetrical and are loaded with fibrous sillimanite grains that are oriented semi-parallel to the prismatic sillimanite grains of the matrix.

Preliminary zoning profiles in garnet show a decrease in Ca from core to rim and relatively flat profiles for other elements, apart from evidence of retrograde zoning near the rims. Work on thermobarometry is in progress. A final stage of deformation is suggested by fractures in garnet, approximately perpendicular to the matrix fabric. Myrmekite-rimmed K-feldspar and a pale-green mica appear to have grown in these fractured domains, but not elsewhere within the rock. Evaluation of P-T conditions in the context of complex textures in the Jake Ridge samples and comparison with mylonites from the type outcrop of the Chunky Gal Mountain Fault and published P-T-t conditions for granulite facies rocks at Winding Stair Gap may help to better understand the P-T-deformation history of the fault with respect to surrounding rocks.