BLACKWALL ALTERATION AND SERPENTINIZATION OF ULTRAMAFIC BODIES IN SOUTHEASTERN PENNSYLVANIA

An examination of ultramafic bodies in southeastern Pennsylvania using field mapping, petrography, and geochemistry has revealed four distinct styles of alteration. The ultramafic bodies are lenticular in shape ranging from 6 km to 0.5 km along their long axis. The bodies are generally adjacent to major shear zones in the region and are considered to have been emplaced during the Taconic Orogeny (470 Ma). Trace elements indicate that the ultramafic protoliths are likely related to the collided island arc rather than ophiolitic or mantle upwelling settings.

Four styles of alteration can be identified: complete serpentinization, blackwall alteration (i.e., a nominally anhydrous cores surrounded by “onion-skin” alteration zones of increasing hydration), sheared blackwall alteration, and siliceous alteration. The westernmost bodies trending west-southwest are encompassed in the Grenvillian (1.0 Ga) mafic Baltimore Gneiss and exhibit complete serpentinization. The serpentine is mainly mesh texture lizardite with relict olivines indicating an olivine-rich protolith. However, the close association with the mafic Baltimore Gneiss may indicate Si-poor fluids reducing the possibility of the anthophyllite-talc alteration seen in the blackwall altered bodies. The remaining three alteration styles are found on the eastern side of the Piedmont within the Taconic (470 Ma) Wissahickon schist trending south-southwest. Two sets of parallel trending bodies closest to northern Philadelphia and adjacent to the Rosemont Shear Zone exhibit blackwall alteration. Counterintuitively, the set closest the Rosemont Shear Zone show little shear deformation and retain their “onion-skin” alteration zones typically with cores of orthopyroxenite. The set slightly further (~1 km) from the Rosemont Shear Zone show significant shear deformation (strain shadows, unit duplication, etc.). The two largest ultramafic bodies show a mix of blackwall alteration, serpentinization, and siliceous alteration. Siliceous alteration is most significant in close proximity with intruding granitic intrusions. The Philadelphia-area ultramafic bodies may represent a dismembered layered mafic complex below an island arc with the olivine-rich stratigraphic bottom to the west and the upper orthopyroxenites to the east.