GEOCHEMISTRY: A TOOL FOR MAPPING METASEDIMENTARY ROCKS?

The Pennsylvania Piedmont, first mapped early in the 20th century, is currently being systematically remapped by the PA Geologic Survey. Here we report on mapping conducted in schist bounded by the Embreeville thrust on the south and the Martic Line on the north.

The original mapping identified two units: the albite-chlorite schist member (informally, Octoraro schist, Os) of the Wissahickon Formation and the Peters Creek schist (PCs). Mapping conducted for this study identifies five lithologies. PCs is subdivided into three members from south to north: schist, tectonite, and variably-tectonized schist. Two members, black phyllonite (Obp) and Os, comprise the albite-chlorite schist. These units resemble mapping reported by Valentino et al. (1994). Because the map units are identified on the basis of structural fabrics, it is implied that the area consists of two metasedimentary lithologies that have undergone significant strain partitioning.

To test this hypothesis, the WCU Spring 2003 petrology class collected representative samples from each map unit and analyzed them using XRF spectrometry at Franklin & Marshall College. All samples are similar with a fairly narrow range of concentration for Rb, K, V, Cr, La, Ce, Th, Pb, Ti, and Nb. The elements Ba, Sr, Zr, Y, Sc, Co, and Ni also have a fairly narrow range of concentration, but may show systematic differences between less and more tectonized samples. Ca has the largest variation – from about 13,000 ppm in PCs to 570-790 ppm in the most tectonized units. When normalized to the North American Shale Composite (NASC), the trace element patterns for all units are markedly similar. If these data reflect a primary feature of the sedimentary protolith, it would be reasonable to propose a single sedimentary formation.

Minor variations in some elements may suggest 3 sub-groups reflecting variation in accumulated strain. Microscopic analysis indicates that the dominant foliation in the tectonized units formed to a significant degree via pressure solution. This mechanism could account for differences in Ca and possibly Ba, Sr, Zr, Y, Sc, Co, and Ni. Observed mineralogic differences may reflect primary characteristics of the sediments, metamorphic grade, and deformation history.

Petrology students: M Arles, J Dippy, T Ellis, A Geyser, M Gyves, L Hackett, K Issa, N Lynde, and J Timbers.