PETROGRAPHIC AND GEOCHEMICAL EVIDENCE FOR THE TECTONIC ORIGIN OF BELLS MILL ROAD ULTRAMAFIC BODY OF SOUTHEASTERN, PA

Field work, petrography, and whole-rock geochemistry has been completed to understand the nature of the alteration zones, deformation history, and to test origin hypotheses for the Bells Mill Road ultramafic body. The Bells Mill Road ultramafic body is one of several of unknown origin within the Piedmont Province of the Central Appalachian Mountain belt. The Piedmont Province is composed of several suites of metamorphic and igneous rocks that owe their origin to the collision of multiple terranes on the east coast of North America approximately 1100 to 450 million years ago. The Bells Mill Road ultramafic body is an elongate unit (~0.25 by 1 mile in dimensions) emplaced within the Wissahickon Schist of southeastern PA and is oriented parallel to regional foliations (NE-SW trending).

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 source of these tectonically emplaced ultramafics, namely: oceanic crust (ophiolite), diapiric mantle, and arc-related magmatic differentiates.

Work in this area is complicated by contradictory data related to contact locations and lithologic relations. A revised geologic map is presented to show the lithologic zones of alteration within the body. The zones are as follows: serpentine-talc; serpentine-talc-carbonate; talc-anthophyllite-tremolite; chlorite; and anthophyllite-chlorite. Whole-rock chemistry was obtained using ICP-MS and XRF; the chemistry of individual phases was obtained using EDS. The use of geochemical discrimination diagrams support an island arc origin. A small minority of samples show a mid-ocean ridge basalt affinity, however, these rocks are rich in Al-phases (Grt, Ky) 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.