2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 224-7
Presentation Time: 10:15 AM

MODELING P-T-TIME PATHS WITHIN THE CHUNKY GAL MOUNTAIN FAULT, CENTRAL BLUE RIDGE, NORTH CAROLINA


ARMSTRONG, Eric1, PETERSON, Virginia L.1 and GILEWSKI, Carlene A.2, (1)Geology Department, Grand Valley State University, Allendale, MI 49401, (2)Grand Valley State University, Geology Department, Allendale, MI 49401, armseric@mail.gvsu.edu

The terrane-bounding Chunky Gal Mountain Fault (CGMF) in the Southern Appalachian Central Blue Ridge forms part of the boundary of the Buck Creek-Chunky Gal mafic-ultramafic complex. Fault movement during peak Taconian metamorphism (~460 Ma) may have emplaced this ocean crustal fragment. Four fault zone samples represent variations in protolith, assemblage, and strain. Complex garnet zoning profiles help constrain the fault zone P-T path.

Sample L3 is a Bt-rich ultramylonite with Grt, Pl, Kfs, and Ms porphyroclasts. JR22, 13-3b and 13-5a are from a more aluminous exposure; all contain Bt, Pl porphyroclasts, and abundant rounded to eye-shaped Grt’s with fibrous Sil inclusions near the rim. JR22 and 13-5a are protomylonites; the matrix of JR22 is rich in coarse Sil whereas 13-5a has Ky and Ms. 13-3b is migmatitic with the largest Grt’s and matrix Ky. All samples have accessory Ilm, Rt, and Mnz. Textural observations suggest a stability progression from Sil -> Ky and Ilm -> Rt .

Large Grt’s are mostly inclusion free with Mg-rich, Ca-poor, and inclusion-rich cores and fibrolite inclusions in some rims. Core to rim profiles display a distinct drop in Ca (a Ca-poor ‘moat’) and a modest Mg increase. Outward from the ‘moat', this trend reverses with increasing Ca and Mn and decreasing Mg toward the rim.

T-P conditions estimated using TWQ suggest minimum peak conditions of 750-800oC, 0.6-0.7 GPa within the Sil stability field. Grt rims equilibrated within the Ky stability field at ~550-600oC, 0.7-0.8 GPa. Equilibrium assemblage diagrams created with Theriak-Domino (TD) using whole-rock geochemistry suggest similar T-P conditions.

TD models of Grt isopleth thermobarometry yield a counterclockwise P-T path. The initial (core-‘moat’) stage generates decreasing pressure (~0.1GPa) with a slight increase (~10oC) in temperature. ‘Moat’ to rim preserves a nearly isothermal spike in pressure followed by a distinct, near-isobaric temperature decrease. The pressure spike may indicate loading across the CGMF during peak metamorphism.

Work is in progress to better constrain P-T path models by accounting for Grt fractionation with approaches that model Grt growth and diffusion. These efforts must account for early diffusional re-equilibration of Grt profiles. Some preliminary results are consistent with initial P-T path estimates.