PRELIMINARY INVESTIGATION OF DEFORMATION AND METAMORPHISM IN THE AVONDALE MASSIF, SE PENNSYLVANIA PIEDMONT

The Avondale Massif is one of several bodies of basement gneiss which underlie the central Appalachian Piedmont of Pennsylvania and is among the least well characterized rock in the region. A variety of lithologies are present in the massif including both ortho- and paragneiss. This investigation focuses on a transect of the Avondale Massif along Chester Creek in Delaware County, where quartzo-feldspathic biotite garnet gneiss is the most common lithology. Preliminary field work reveals that the rocks are complexly deformed and strain is highly variable. In spite of this variability, foliation generally dips moderately to the southeast, approximately parallel to the Street Road Fault, a ductile thrust-sense shear zone which separates the Avondale from the underlying West Chester massif. Strain in some portions of the transect is characterized by boudinage which ranges from m-scale boudinage of amphibolite layers to meso-scale foliation boudinage of gneiss, where individual boudins are as large as 10 x 15 m in cross section. Some outcrops are characterized by highly planar foliation while others show evidence for multiple folding episodes.

Petrographic analysis shows that the gneiss consists of monocrystalline quartz ribbons separated by bands of fine grained recrystallized feldspar, similar to high grade “striped gneiss” and indicative of high temperature deformation. However, quartz does exhibit intracrystaline strain, which suggests that deformation continued as the rock cooled. Asymmetric microstructures, including S-C fabrics and sigma tails on garnet indicate top to the northwest sense of shear. Garnet is generally anhedral and surround by plagioclase coronae, a texture that is commonly attributed to high temperature isothermal decompression. Preliminary geothermometry, using Ti in biotite and garnet-biotite Fe-Mg exchange, gives temperatures in the upper amphibolite facies.