Geology of the Ashe Formation between Fries and Galax, Virginia

Abstract

Field mapping, structural analysis, and mineral equilibria of rocks between Fries and Galax, Virginia provide information on the tectonic and metamorphic history of the Ashe Formation, a series of late Precambrian gneisses, pelitic schists and amphibolites, and the underlying Cranberry Gneiss. The dominant penetrative foliation of the rocks is composed of an early S₁ foliation that is crenulated and transposed by a later S₂ foliation which strikes N 50° E and dips SE. Metamorphic porphyroblast growth of biotite, garnet, staurolite, and kyanite followed penetrative foliation development, with metamorphic grade increasing stratigraphically upward to form inverted isograds. Thrust faulting along the Fries Fault caused extensive mylonitization of the Cranberry Gneiss and offset of the metamorphic isograds. Textural evidence indicates that mylonitization is the greatest in rocks at the Fries Fault trace, and decreases upward through the Cranberry-Ashe contact. No textural evidence of an additional episode of faulting at the AsheCranberry contact was observed, based on lack of a second episode of mylonite, relatively good preservation of sedimentary clasts in the Ashe, and similarity of metamorphic grade.

Mineral equilibria show systematic trends from chlorite-biotite grade to staurolite-kyanite grade. Muscovite compositions show an increase in paragonite component and decrease in celadonite component. Chlorite, biotite, garnet, and staurolite show a systematic increase in Mg/(Mg+ Fe) toward higher grade. AFM topologies indicate that these compositional trends may be explained a series of continuous reactions ( chl + mu=bi + q + V followed by chl + mu + q=gt + bi + V) and discontinuous reactions (gt + chl + mu= st + bi + V followed by st + chl + mu= ky + bi + V). Temperatures inferred from garnet-biotite equilibria are approximately 550 to 600° C in the staurolite zone, with pressures estimated from various equilibria to be between 4.5-7 kb. The inversion of the isograds is interpreted as resulting from overthrusting of the Ashe metasediments during the Paleozoic.

The above information can also be applied to the deformation of the Gossan Lead massive sulfide. Structural data from the Bumbarger mine pit indicates that the massive sulfide underwent the same sequence of structural events as the surrounding rocks. During D₂, shearing of gneisses and schists in a matrix of ductile pyrrhotite caused isoclinal folding, fracture, and rotation of the metasediments while preserving the gross sedimentary layering. This information confirms synsedimentary models of the formation of the Gossan Lead.