INVESTIGATION OF CHEMICAL METASOMATISM OF A BLUE RIDGE CHARNOCKITE

A Grenville-age megacrystic granitoid, formerly known as the Vesuvius Megaporphyry, contains an assemblage of large plagioclase lathes of up to 5 cm in a finer matrix of plagioclase, quartz and orthopyroxene. It is typically gray throughout, the megacrysts being highly noticeable only on weathered faces. It is one of four rock types that make up the Pedlar River Charnockite Suite (formerly Pedlar River Pluton) which intrudes the Nellysford Granulite gneiss, and is found in the central portion of the pluton. The outcrop shows progressive metasomatism towards unakite, first along cracks and fissures, then throughout larger sections of the parental material. It is hypothesized that the rock transitions into the large unakite outcrops found in Turkey Hollow, approximately 0.5 miles to the east. Although it is known that unakite is a product of granitic metasomatism, it is thought that plagioclase alters to epidote minerals + other minerals (saussuritization) giving the green color, while retaining the pink orthoclase and quartz. In this location, the plagioclase megacrysts alter to pink orthoclase, while the matrix transitions to the green colors of epidotes. Blue quartz occurs in the unakite, and this is probably primary and unaltered. Detailed petrographic, SEM, EDS, and whole rock analyses will show the processes and chemical changes taking place from the unaltered megacrystic granite to unakite. Understanding the metasomatic alteration taking place here can provide clues to the evolution and stability of the Proterozoic lithospheric mantle.