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

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


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.