Petrology of the Ashe Formation, Shortt's Knob area, Floyd County, Virginia

Abstract

A small area within the Ashe Formation has been mapped at the scale 1:6000 and studied petrographically and chemically. The rocks have been metamorphosed into the lowermost amphibolite facies and folded. The gross structure is homoclinal, generally N 80° E, >50°SE. Amphibolites (39%), chlorite schists (8%), and quartzofeldspathic schists and gneisses (53%) are the major lithologies.

Amphibolite assemblages are hornblende + clinozoisite and/or epidote + plagioclase + sphene ± quartz ± garnet. Amphibolites may be divided into four types on the basis of texture, sphene-content, and epidote-composition. Chlorite schists are ultrabasic rocks containing the assemblage Mg-chlorite + actinolite and/or cummingtonite + dolomite + ilmenite ± olivine. Metasediments generally contain biotite. The most distinctive metasediment is black sulfidic schist containing 15-20% amorphous carbon. The assemblage dolomite + quartz + muscovite + plagioclase + biotite occurs in the only carbonate outcrop.

Anorthite content of plagioclase ranges between 18 and 40 mol % but depends on lithology. Amphibolite hornblendes have at least one A1^IV per formula unit. Epidote group minerals have compositions generally <Ps₅ or >Ps₁₀ and the concept of a single miscibility gap is not supported.

Upper P-T limits will be defined by the reaction dolomite + quartz + muscovite -> plagioclase + biotite + calcite + vapor and probably limit the temperature to <550°C. The reaction zoisite + CO₂ -> anorthite + calcite + H₂O limits the CO₂ content of the metamorphic fluid over most of the area to low values, probably <0.1 mol fraction.

The amphibolites probably represent basaltic flows, some amygdaloidal. Chlorite schists are interpreted to have been picritic sills.