ALTERATION OF THE NEWTOWN SQUARE AND LIMA ULTRAMAFIC BODIES IN DELAWARE COUNTY, PENNSYLVANIA

Utilizing field work, geochemistry, and petrography, two ultramafic bodies (UMB’s) in the Pennsylvania Piedmont were examined revealing varying degrees and types of alteration. The two bodies examined, Newtown Square UMB and Lima UMB, are located in Delaware County, PA, and are separated by 1 km. The bodies are oblong (~6 km x 3 km, trending NNE and SSW) and surrounded by the Wissahickon schist apart from the western edge where they border the Rosemont shear zone. The Newtown Square and Lima bodies are the two largest UMB’s in a series of early Paleozoic UMB’s in the Pennsylvania Piedmont which were emplaced during the Taconic Orogeny (470 Ma) and subsequently deformed. Trace elements plotted on discrimination diagrams indicate an island arc setting suggesting they were part of the collided arc system.

The Newtown Square UMB exhibits limited alteration (orthopyroxenite and norite lithologies) on the western side and blackwall alteration (zones of increasing hydration towards the country rock) on the eastern side of the body. The lack of alteration on the western side may be due to the presumed source of metamorphic fluids (the anhydrous, granulite-facies, mafic Baltimore gneiss) or this area’s juxtaposition to the Rosemont shear zone. In contrast, the blackwall alteration of the eastern side provides a gradient from a nearly unaltered core to zones of increasing hydration (anthophyllite, talc, serpentine, respectively) to the silica-rich country rock, the Wissahickon schist. The Lima UMB contains similar rocks to the Newtown Square UMB (pyroxenite and serpentinite), however, the most abundant rock in the Lima UMB is a severely-altered ultramafic rock characterized by extensive honey-comb veining and up to 90% silica content. The occurrence of this “reaction rock” is likely due to contact metamorphism with multiple mapped granitic intrusions adjacent to the Lima body. The widespread presence of the reaction rock suggests that the granitic bodies may be continuous at depth representing one large granitic body. Geochemical and petrographic examination of these UMB’s reveal an island arc affinity for their origin and subsequent deformation was significantly influenced by adjacent Baltimore Gneiss, Wissahickon schist, and granitic intrusions each imprinting a distinct style of alteration on the adjacent UMB’s.