THE WISSAHICKON SCHIST AND RELATED ROCKS: STUDIES OF METAMORPHISM IN SE PENNSYLVANIA

For over 100 years metamorphic studies have played a significant role in geological interpretations in SE Pennsylvania. Early workers recognized lithological differences due to metamorphism that increases in intensity south and east across the region. Bascom, who named the metamorphosed argillaceous rocks of the Piedmont Wissahickon Formation, attributed the higher metamorphism at the type locality in the Philadelphia area relative to rocks to the west and north to intrusion of igneous rocks. In 1929 her students, Knopf and Jonas, mapped metamorphic intensity in Pennsylvania and Maryland and reported evidence for superimposed metamorphic events including retrograde metamorphism. They related deformation and metamorphism and recognized that superimposed fabrics may reflect superimposed metamorphic events. Using the ideas of metamorphic zones and isograds developed in Scotland, McKinstry (1949) and Weiss (1949) published isograd maps. Wyckoff (1952) applied the concept of metamorphic facies to the schist to stress the role of rock chemistry on mineral occurrence. She suggested an early contact metamorphic event based on occurrences of andalusite and sillimanite, followed by kyanite-bearing regional metamorphism related to introduction of granitic material. Subsequent papers on details of metamorphic mineral properties and chemistry contributed little to unraveling the details of the metamorphism; that awaited the next major advance, experimental work that defines P-T conditions for metamorphic assemblages. Using these data and careful petrographic work, Wagner (1975) identified two prograde metamorphic events in the Precambrian gneiss and Mark (1977) recognized low P overprinted by high P metamorphism in the Wissahickon N of the Wilmington Complex, N Delaware. Srogi, Plank and Bosbyshell further documented a thermal aureole around the Wilmington Complex locally overprinted by high P metamorphism. These studies led to our present identification of at least 3 metamorphic events: basement gneiss granulite facies metamorphism; high T, low P metamorphism around the Wilmington complex; and a superimposed regional metamorphism grading from greenschist to amphibolite facies from NW to SE across the region. Most of the evidence was seen by the early workers; we have but worked out the details.