PETROLOGY AND GEOCHEMISTRY OF THE BELLS MILL ROAD ULTRAMAFIC BODY, PHILADELPHIA PA Kerrigan, Ryan J. 1 , Mengason, Michael J. 2 , and Simboli, Lorin N. 1 , (1) Department of Energy and Earth Resources, University of Pittsburgh at Johnstown, 450 Schoolhouse Road, Johnstown, PA 15904 (2) National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899 Abstract A petrographic and geochemical assessment of the Bells Mill Road ultramafic body was conducted to examine its alteration history and to test hypotheses on the source of the ultramafic protolith. The Bells Mill Road ultramafic body is located in the Piedmont region of southeastern Pennsylvania within Wissahickon Valley Park at the intersection of Bells Mill Road and Wissahickon Creek. The Bells Mill Road ultramafic body is one of several altered ultramafic bodies scattered throughout the Pennsylvanian Piedmont province. The local area is host to rocks that are metamorphosed from greenschist to amphibolite facies during accretion onto the continental margin during the Taconic orogeny (450-470 Ma) that overlay a basement composed of higher grade metamorphic rocks of Grenville age (1.0-1.2 Ga). The geologic history of the Pennsylvanian Piedmont has been thoroughly studied, but the origin of the ultramafic bodies remains a source of contention. There are several competing hypotheses to explain the protolith of these ultramafics, namely: oceanic crust (ophiolite), diapiric mantle, and arc-related magmatic differentiates. Geologic mapping of the body has shown the following lithologic zones: serpentine-talc rock, talc-tremolite schist, anthophyllite- chlorite schist, chlorite schist, and talc-serpentine schist. Spinel chemistry was obtained using EDS and whole-rock chemistry was obtained using ICP-MS and XRF. Elemental concentrations of spinel minerals reveal significant metamorphism (greenschist to amphibolite facies), altering the inherited protolith signals. However, plotting whole-rock trace element concentrations on multiple petrogenetic discrimination diagrams confirms an island arc origin. A minority of samples exhibit a mid-ocean ridge basalt affinity on petrogenetic diagrams, however, these rocks are rich in Al-phases (garnet, kyanite, and corundum), which may suggest a high level of recrystallization/metasomatism altering inherited signals. The ultramafic body is most likely the ultramafic differentiate of an arc-related magma system. Introduction The Bells Mill Road ultramafic body is part of a set of exotic rocks located in the Pennsylvanian Piedmont that have evaded proper characterization and scientists remain uncertain about their exact origin. Published geologic maps that contain coverage of the ultramafic bodies (Bascom et al., 1909; Weiss, 1949; Amenta, 1974; Amenta et al., 1974; Berg and Dodge, 1980; Bosbyshell, 2006; among others), contain contradictory information pertaining to the precise locations and contact relationships between the ultramafic bodies and adjacent rocks. Furthermore, all of the geologic maps for this region lump the ultramafic bodies as single units and do not differentiate between the various lithologies within the ultramafic bodies. Field work 16 GANJ XXXIV Annual Meeting Proceedings and Field Guide - 2017