PETROLOGICAL, GEOCHEMICAL, AND STRUCTURAL INVESTIGATION OF ALTERATION ZONES IN THE YOUNGSFORD ROAD ULTRAMAFIC BODY, SOUTHEASTERN PENNSYLVANIA

An ultramafic body in the Piedmont Province of southeastern Pennsylvania was examined through geochemical, petrological, and structural analyses to gain understanding about its zoned alteration and geologic history. The Piedmont Province of the Central Appalachian Mountains contains numerous ultramafic bodies within Grenville (1100 Ma) gneisses and Taconic (485 Ma) schists. While much is known about the geologic history of the Appalachian Piedmont, the source of the ultramafic bodies remains unresolved. The Youngsford Road ultramafic body, located in NW Philadelphia, is an elongate body (~1.5 by 0.2 miles) trending SW to NE conformably occurring within the Wissahickon Fm. The conformable relationship suggests that the ultramafic body was emplaced in the Wissahickon Fm prior to peak metamorphism of amphibolite facies (415 Ma). The NW side of the body is fault bound by the Rosemont Shear Zone separating it from the Baltimore Gneiss. On the SE side of the ultramafic body is an elongate amphibolite body (0.3 miles long) between the Wissahickon Fm and the ultramafic body. The Youngsford Road ultramafic body contains the following three major concentric alteration zones: (1) a relatively unaltered pyroxenite core grading to (2) an anthophyllite rock containing serpentine pods grading to (3) an outer zone of talc schist before contact with the country rock.

This study included the production of a geologic map incorporating structural, petrological, and geochemical analyses of the Youngsford Road ultramafic body and its zones. Thin-section analysis showed the pyroxenite core to contain fractured orthopyroxenes with small amounts of talc. Whole rock geochemistry revealed the core to be mostly anhydrous. The textures and geochemistry of the core suggest only minor alteration while other zones are considerably more hydrous and completely recrystallized. Composition profiles from the core to the country rock show a decrease in SiO₂ and MgO and an increase in H₂O. The serpentine pods have a gradational chemistry with the adjacent anthophyllite rock and exhibit alteration mesh textures surrounding relic olivine grains. Based on petrologic and geochemical evidence, it appears that the protolith of the Youngsford Road ultramafic body may have been a layered ultramafic unit of olivine-rich lenses within an orthopyroxenite.