GEOCHEMICAL AND PETROGRAPHIC CHARACTERIZATION OF PIEDMONT GRANITES, LIMA PA

Using geochemical and petrographic methods, small granitic bodies (~1 x 2 km) ~20 km west of Philadelphia (in Lima, PA) were examined to better understand the orogenic setting during emplacement. To the best of our knowledge, the Lima granites have not been previously characterized, but may be related to the nearby (~10 km to the east) Springfield granodiorite. Several hypotheses have been proposed for the emplacement of granitic rocks in the Appalachian Piedmont. Emplacement may have occurred before the Taconic orogeny (formed in volcanic island arcs), during the orogeny (as syntectonic anorogenic melt products), or after the orogeny (from within plate regional extension). To address these gaps in knowledge, fieldwork was conducted to map and collect samples which were analyzed by petrographic and geochemical methods. Using the results of bulk whole rock analyses, CIPW normalizations were calculated to model average abundances with most samples exhibiting a monzogranite-type composition. Trace element compositions plotted on petrographic discrimination diagrams suggests the Lima samples may have originated as within plate granites or post-collision granites with a minority of samples plotting as volcanic arc granites. Rare earth element and multi-element variation diagrams show a negative slope consistent with arc magmatism, with pegmatitic samples exhibiting a positive europium anomaly. Examination showed that the Lima body shares characteristics with the Springfield granodiorite. Geochemical data supports a within plate or post-collisional origin, possibly during post-Taconic Silurian extension in the region, which is consistent with hypotheses for the origin of the Springfield granodiorite.