Faculty Scholarly Dissemination Grants

Modeling P-T-Time Paths Within The Chunky Gal Mountain Fault, Central Blue Ridge, North Carolina

Department

Geology Department

College

College of Liberal Arts and Sciences

Date Range

2014-2015

Disciplines

Social and Behavioral Sciences

Abstract

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 Grts 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 Grts and matrix Ky. All samples have accessory Ilm, Rt, and Mnz. Textural observations suggest a stability progression from Sil -> Ky and Ilm -> Rt . Large Grts 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.

Conference Name

Geological Society of America Annual meeting

Conference Location

Vancouver, British Columbia, Canada

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