NEW AMPHIBOLITE ANALYSES FROM THE CENTRAL APPALACHIAN PIEDMONT OF PENNSYLVANIA: GEOCHEMISTRY AND TECTONIC AFFINITY

The geochemistry and tectonic affinity of interlayered amphibolites is useful in distinguishing different schist units in the Central Appalachian Piedmont of Pennsylvania. Here we report the results of whole and trace element chemical analysis of three amphibolite samples collected along the Rosemont Shear Zone (RSZ). Two samples are from the Wissahickon Fm. in Philadelphia (Ge24-01, Ge19-02); the third (WN10-01) is from West Grove Metamorphic Suite (WGMS) in Chester County.

Ge19-02 plots in volcanic arc fields on tectonic discrimination diagrams; exhibits a flat REE pattern at ~10x chondrite and shows enrichment in incompatible elements and a small negative Nb-anomaly on MORB-normalized trace element plots. This sample fits within the previously described Smedley Park Amphibolite, metamorphosed back-arc basin basalt (BABB) interlayered in the type-section of the Wissahickon Fm (Bosbyshell et al., 2015).

WN10-01 consistently plots as within-plate basalt on various diagrams, exhibits a negative slope on REE plots with enrichment of LREEs of ~80x chondritic values, and incompatible elemental enrichment TE plots of similar magnitude to other samples. This sample is consistent with the previously described White Clay Amphibolite, which represents continental initial rift basalt (Smith and Barnes, 2004).

Ge24-01 plots in a range of fields on discrimination diagrams, included MORB, volcanic arc, and within-plate basalt. REE plots show enrichment in LREEs with a flat pattern in HREEs that parallels that of Ge19-02 at higher values. Incompatible trace elements are enriched; high field strength elements also plot parallel to Ge19-02. This sample resembles the Bridgewater Amphibole (BWA) (Bosbyshell et al., 2015) but is geographically removed from other BWA samples. This suggests that similar amphibolites remain unidentified within the Wissahickon Fm. Alternately, its location within the RSZ could indicate that the sample location is the result of tectonic juxtaposition.