Abstract
The Blue Ridge is one of over a dozen provinces located in the much larger Appalachian Mountain Range which spans across the eastern United States. The Appalachian Mountains began to form close to 500 million years ago and rocks within the Blue Ridge Mountains have been dated to be over a billion years in age. While this mountain belt is well known, the specifics of how the rocks that are visible today ended up in their present location is still the subject of current studies. The rocks within the Blue Ridge Province can provide a good deal of information regarding the early stages of the Appalachian Mountain chain. The mineral content as well as the deformational features in the rocks here can be examined to identify the conditions under which they formed. The Central Blue Ridge can be particularly beneficial to study because the rocks here have undergone extreme deformation. The large Jake Ridge road cut is found in this province along Hwy 64 in southwestern North Carolina. This road cut exposes aluminous sillimanitegarnet 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 semiparallel 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 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.