PETROGENESIS OF ULTRAMAFIC BODIES IN THE PENNSYLVANIAN PIEDMONT

The structural, petrologic, and geochemical significance of a group of enigmatic ultramafic bodies in the Piedmont Province in southeastern Pennsylvania have been examined to determine their origin and petrological significance. The Piedmont Province of the Appalachian Mountain Belt is composed of several suites of metamorphic and igneous rocks that owe their origin to the collision of multiple terranes on the east coast of North America approximately 1100 to 450 million years ago (Mya). Significant geologic investigations have been completed within this area, however, a series of rock bodies have eluded proper characterization. The ultramafic bodies are elongate pods (1.5 to 0.5 miles long and 0.5 to 0.1 miles thick) and altered to varying degrees. Several theories have been proposed to explain the origin of the bodies: pieces of ocean floor caught within a tectonic collision; igneous rocks that diapirically rose from the mantle below; or part of a magma chamber associated with the terranes that collided with North America ~450 Mya.

Over a dozen ultramafic bodies were examined in the field and samples were collected for petrological and geochemical analyses for major and trace elements. Most bodies are completely serpentinized, however, some exhibit a complex “onion skin” morphology creating zones of alteration from the country rock to the core of the body. The alteration zones allow for the modeling of fluid alteration and infiltration during metamorphism. Areas of intense alteration indicate significant metasomatic reactions with the aluminum-rich country rock as shown by the presence of garnet, kyanite, and corundum in localized zones. Internal shear is present in some of the bodies resulting in duplication of alteration zones. Plotting trace element geochemical data on petrologic discrimination diagrams yield an arc signature supporting the hypothesis that these bodies originated from an ultramafic differentiate at the base of an arc system.