THE ULTRAMAFIC BODIES OF SOUTHEASTERN PENNSYLVANIA: POSSIBLE DISMEMBERED LAYERED MAFIC INTRUSION

An investigation of ultramafic bodies in the Appalachian Piedmont Provence in Southeastern Pennsylvania (PA) was undertaken to understand the nature of the bodies. The majority of the 75+ “mapped” ultramafic bodies in the region are now inaccessible due to the extensive urban development. The current study has examined 15 accessible ultramafic bodies in the area. Geochemical analyses of major and trace elements and thin sections were obtained from 60 of the 210 samples collected. Characterization of field relationships, mineralogy, hand sample and thin section textures, and geochemical trends were completed to formulate a region-wide summary of ultramafic bodies in the PA Piedmont. Several theories have been proposed to explain the origin of the Piedmont ultramafic bodies including: portions of ophiolites; diapiric rise of mantle through transtension; or a lower arc differentiates associated with collisional arc terranes from the Taconic Orogeny (~470 Ma). Trace elements indicate that the ultramafic protoliths are related to the collided Taconic island arc.

Most PA Piedmont ultramafic bodies are present along regional shear zones and four distinct styles of alteration have been identified: complete serpentinization, blackwall alteration, sheared blackwall alteration, and siliceous alteration. The westernmost bodies of the PA Piedmont exhibit complete serpentinization with textures and relict olivines suggesting an olivine-rich protolith. Bodies present in the eastern PA Piedmont mostly display blackwall alteration: some show orderly concentric zones of alteration, while others show significant shear deformation (unit duplication, strain shadows). When relict cores of the blackwalled bodies are present they reveal unaltered orthopyroxenite and norite lithologies with cumulate textures. Bodies with siliceous alteration show near complete replacement by quartz via contact metamorphism with adjacent granite intrusions. The southeastern Pennsylvania ultramafic bodies may represent a dismembered layered mafic complex from the base of an island arc that was sheared southward along dextral shear zones that widen from a central northern point, sending the dunite based to the west and the less ultramafic (higher stratigraphic units) orthopyroxenite and norite components more easterly.