PETROLOGIC ANALYSIS OF ULTRAMAFIC ROCK FROM FAULT ZONES IN THE CENTRAL APPALACHIAN PIEDMONT OF SOUTHEASTERN PENNSYLVANIA AND NORTHERN DELAWARE

Ultramafic rocks may serve as indicators of tectonic environment. As such, petrographic and geochemical analysis provide insight into the tectonic processes that shaped the region in which the rocks occur. In the central Appalachian piedmont of southeastern Pennsylvania and northern Delaware, ultramafic rock is found along the trace of three major faults/shear zones which bound internal massifs. From NW to SE these are the Embreeville Thrust, Street Road Fault and Rosemont Shear Zone. The granulite facies West Chester massif and metasedimentary cover are emplaced upon greenschist facies rocks on the Embreeville Thrust; the Street Road fault places amphibolite facies gneiss of the Avondale massif above the West Chester massif and the steeply dipping Rosemont Shear Zone separates the Avondale from the Wilmington Complex and Wissahickon Formation.

We will present the results of petrographic and geochemical analysis of ultramafic rocks from the three fault/shear zones to test whether or not consistent differences are present. The Mesoproterozoic West Chester massif is most likely Laurentian basement, while the Ordovician-aged Wilmington Complex and Wissahickon Formation are the remains of a volcanic/magmatic arc and related basin(s). The origin of the Avondale gneiss is somewhat uncertain, but is likely of Laurentian affinity. Thus, ultramafic rock along the Embreeville Fault may be related to post-Grenville rifting of Rodinia, while ultramafic bodies along the Rosemont will likely exhibit an arc-affinity. The nature of the ultramafic rock along the Street Road Fault may provide insight into the origins of the Avondale gneiss.