2012 Geological Society of America (GSA) Annual Meeting and Exposition
The terrane-bounding Chunky Gal Mountain Fault (CGMF) in the Southern Appalachian Central Blue Ridge lies adjacent to the Buck Creek-Chunky Gal mafic-ultramafic complex (BCC), an ocean crustal fragment that experienced peak (Taconian) conditions of ~ 825 oC, 1.2 GPa. Mylonitic amphibolites, garnet-biotite, and sillimanite-garnet-biotite gneisses from the fault zone preserve complex physical and chemical textures that may help constrain the emplacement history of the BCC.
In the type exposure, the CGMF separates BCC amphibolite from biotite gneisses and includes several focused shear zones. A compositionally layered amphibolite mylonite (11J1A) from the main fault zone shows apparent normal shear and includes a hornblende-rich domain with biotite-plagioclase-sphene, a scapolite-rich domain with epidote-sphene, and a biotite-rich ultramylonite with garnet, epidote, and plagioclase porphyroclasts. Garnet shows complex zoning.
A nearby biotite gneiss ultramylonite (11L3) with sinistral asymmetry includes asymmetric garnet, K-feldspar, plagioclase, and muscovite porphyroclasts. Garnet zoning suggests preservation of outward prograde zoning (increasing Mg/decreasing Ca). Increased Ca and Mn with decreasing Mg toward the rims is distinctive.
Large garnets from biotite mylonites with a shallow sillimanite lineation in the nearby Jake Ridge outcrop (JR22) preserve 3 distinct garnet growth periods. Inclusion rich, high Ca/high Mg cores are surrounded by inclusion-free zones with distinctly lower Ca, and slightly higher Mg. Symmetrical triangular tails include fibrous sillimanite and preserve a distinct increase in Ca with decreasing Mg.
Preliminary use of TWQ, Theriak-Domino, and Hb-Pl thermometry suggests minimum peak temperatures of ~ 750 oC, and matrix deformation conditions of ~ 600 oC. Pressure estimates are more complex. Garnet growth zoning in both JR22 and 11L3 are similar to zoning reported for large garnets at Winding Stair gap, where peak granulite facies are documented. However, in these two samples, we see an additional sharp increase in Ca toward the rims that has not been previously reported. This may indicate a relatively late increase in pressure at reasonably high (sillimanite-stable) temperature conditions within the Chunky Gal Mountain Fault.