Petrogenesis of the Springfield Granodiorite, southeast Pennsylvania Piedmont

Abstract

The Springfield pluton intrudes the Wissahickon schist in the southeast Pennsylvania Piedmont. The mineralogy of the pluton. in order of decreasing abundance is plagioclase, quartz, microcline, biotite, epidote (often with allanite cores) and trace amounts of hornblende. Accessory minerals include sphene, magnetite, apatite and zircon. The magmatic assemblage of quartz+two feldspars+biotite+sphene+hornblende+magnetite permits its utilization of AI-in-hornblende as a geobarometer (Schmidt, 1992) and yields a pressure estimate of 9 kbars. The initial 87Sr/86Sr ratio of 0.708, suggests an evolved source for the magma with respect to mantle compositions, while a Rb-Sr whole rock age of 457 ± 9 Ma suggests a link between peak metamorphic conditions (Crawford and Crawford, 1980; Crawford and Mark, 1987, Sinha, 1988) and magmatism. The pluton is metaluminous to weakly peraluminous, with the aluminum saturation index (ASI) < 1.15. Si02 contents range from 64.1 to 75.1 wt. percent, with all other elements showing a correlation (either positive or negative) with the Si02. The pluton is correlated with two similar bodies to the south, Ellicott City, Maryland and Ellisville, Virginia. The spatial and temporal relationships of the plutons, including the two mica Gunpowder granite, Maryland record a magmatic axis associated with high pressures (>5.0 kb) of emplacement of magmas.