DETERMINING THE ORIGIN OF GRANODIORITES IN THE PENNSYLVANIAN PIEDMONT USING GEOCHEMICAL AND PETROGRAPHIC METHODS

Petrographic and geochemical analyses were used to characterize two granodioritic bodies located in the Piedmont Province of Southeastern Pennsylvania. These small (~300 by 150 m) granodioritic bodies are within 2.25 km of each other, located 15 km north-northwest of downtown Philadelphia, PA. One is referred to here as the Girl Scout Granodiorite, due to its proximity to a girl scout camp near Manor Road in Lafayette Hill, the other we refer to as the Bells Mill Granodiorite, due to its location near Bells Mill Road in Philadelphia. At both of these locations the bodies are surrounded by the Wissahickon Schist. The granodioritic bodies are thought to be contemporaneous with the Taconic orogeny, an island arc collision with the eastern margin of Laurentia approximately 450 million years ago. Theories that may explain the origin of the granodioritic bodies include partial melt accumulation during orogenic crustal thickening or magmatism generated within the colliding arc. To the best of our knowledge the origin of these rocks have not previously been characterized.

Petrographic and geochemical analyses were completed. Whole rock geochemical data plotted on discrimination diagrams suggests that the granodiorites have an arc origin. The granodioritic bodies are peraluminous and subalkaline with geochemistry similar to the Springfield Granodiorite located ~7 km south. Outcrop and petrographic thin section analyses reveal deformation in the Bells Mill Granodiorite with foliations parallel to the adjacent Wissahickon Schist while the Girl Scout Granodiorite exhibits a more cumulate texture. The presence of deformation in the Bells Mill Granodiorite can be interpreted to suggest it solidified pre- or syn-tectonically with the regional metamorphosis accompanying the Taconic Orogeny while the Girl Scout Granodiorite may not fully have solidified until after the peak in local stresses and failed to record similar strain.