AMPHIBOLITE GEOCHEMISTRY IN THE WISSAHICKON FORMATION, PHILADELPHIA, PA: NEW RESULTS AND IMPLICATIONS FOR WILMINGTON COMPLEX – LAURENTIA COLLISION

Two distinct types of amphibolite are present in the Wissahickon Formation east of the Rosemont shear zone: Type 1, which are similar to highly-evolved Fe-Ti basalts and which more closely resemble within-plate basalts than MORB or arc tholeiites; and Type 2, which resemble modern arc/backarc tholeites. These types have previously been labeled the Bridgewater and Smedley Park amphibolites, respectively. New data demonstrate the occurrence of both amphibolites as far east as Philadelphia. It can now be shown that the amphibolites occur in geographically distinct belts, interlayered within somewhat different metasedimentary rock. Type 2 tholeiitic amphibolites occur in quartz-rich metasedimentary rocks while the Type 1, Fe-Ti type, occurs in less-mature or volcanogenic, relatively plagioclase-rich rocks. The Fe-Ti amphibolites (Type 1) are structurally and stratigraphically lower than the arc tholeiites (Type 2). The amphibolites have not been dated directly, however, based on the volcanic arc geochemistry of the Type 2 amphibolites, and the presence of cross-cutting Silurian plutonic rock, are interpreted to be similar in age to the Ordovician (~480 Ma) Wilmington Complex. These results suggest that the Wissahickon Formation east of the Rosemont shear zone comprises sediments originally deposited in a back-arc basin during the Ordovician.

The Wilmington Complex and Wissahickon Formation occupy a crustal block that lies outboard of the younger, peri-Laurentian James Run arc (~ 460 Ma, Horton et al., 1998). The most pervasive period of metamorphism in the Wissahickon Formation and right-lateral transpressive deformation in the Rosemont shear zone are Devonian in age. We suggest that the Wilmington Complex/Wissahickon block originated elsewhere in the orogen, possibly as part of a peri-Gondwanan arc, and achieved its present location in the Central Appalachians during Devonian tectonism.