Middle Proterozoic rocks contain hornblende, hypersthene, almandine, red biotite, rod and bleb perthite, blue quartz, and sphene, which indicate upper amphibolite- to granulite-facies metamorphism during the Middle Proterozoic Grenville orogeny. The hornblende gneiss (Yhg, chemical sample 5, table 1, and isotopic sample 6, fig. 3) yields a 40Ar/39Ar hornblende cooling age of 945 Ma (Kunk and others, 1993). During the late Paleozoic Alleghanian orogeny, the gneisses, along with their cover rocks, were metamorphosed to greenschist facies. This retrograde event saussuritized the plagioclase and chloritized the amphibole, pyroxene, garnet, and biotite.
Deformation in the Blue Ridge-South Mountain anticlinorium occurred under greenschist-facies conditions at about 350°C and 3.5 kbar pressure, corresponding to an overburden of 10.6 km (Elliott, 1973). Muscovite in cleavage on the east limb of the anticlinorium in the adjacent Point of Rocks quadrangle has yielded an 40Ar/39Ar cooling age of 340 Ma (Burton and others, 1992b; Kunk and others, 1993).
Late Proterozoic to Lower Cambrian metasedimentary and metavolcanic rocks contain mineral assemblages that are stable at greenschist-facies conditions. Albite, chlorite, and magnetite are found in all of these rocks; actinolite and epidote are abundant in metabasalt, and sericite is abundant in the aluminosilicate-rich rocks. Epidosite is most common in metabasalt flows (Reed and Morgan, 1971) but is also found in metadiabase dikes as a result of circulating fluids (Espenshade, 1986).
White quartz veins are common in all units. The Purcell Knob antiformal syncline exposes isoclinally folded quartz veins in phyllite of the Swift Run Formation. The White Rock thrust fault is parallel to bedding in the Weverton Formation and is marked by brecciated blocks of quartzite that float in a matrix of vein quartz. Abundant residual blocks and boulders of white vein quartz are common in areas underlain by garnet monzogranite, yet they are rarely seen in outcrop.
U.S. Geological Survey, U.S. Department of the Interior
This page is https://pubs.usgs.gov/bulletin/b2123/meta.html
Contact: Scott Southworth
Last modified 08.29.00
This page is https://pubs.usgs.gov/bulletin/b2123/meta.html
Contact: Scott Southworth
Last modified 08.29.00