Metamorphosed stratiform mafic and ultramafic rocks include: greenish-gray, locally layered, coarse-grained metagabbro; dark-greenish-black schistose metabasalt; and, gray to grayish-green talc-chlorite-tremolite schist. Mineralogy: (1) chlorite + epidote + plagioclase + quartz + titanite + ilmenite; (2) chlorite + actinolite + biotite + epidote + titanite + plagioclase + quartz; (3) chlorite + actinolite + talc + dolomite + magnetite-ilmenite; (4) tremolite + chlorite + magnetite-ilmenite; (5) serpentine + talc + chlorite + actinolite ± olivine ± augite. Geophysical signature: strike-elongate positive magnetic anomaly. Metamorphosed mafic and ultramafic complexes are generally sheet-like bodies, concordant near the base of the Alligator Back and Charlottesville Formations. In Nelson County, these rocks are cut by metagabbroic dikes (CZmd) that are likely related to the Catoctin. Hess (1933) reports a stratified association of ultramafic, mafic, and minor silicic lithologies at Schuyler which he attributes to in situ differentiation of a sheet-like concordant intrusion. Glover and others (1989) report a non-tectonized intrusive contact between Charlottesville Formation metasiltstone and ultramafic schist at Schulyer. In contrast, Conley (1985) presents evidence that ultramafic-mafic complexes in Franklin County are tectonicly-emplaced slices of oceanic crust (ophiolites). Tectonic setting and mode of emplacement for these rock assemblages remain enigmatic; correlation of complexes in the southwestern Piedmont with those in the central Blue Ridge may ultimately prove invalid.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.
Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.