Ultraphyllonite origin for slate, mid-Atlantic Piedmont, USA

Structural, petrographic, and geochemical criteria are applied in combination to determine the petrologic origins of slaty rocks in polydeformed metasedimentary terranes. Multiple lines of evidence suggest that the Peach Bottom slate (PBs), mined over several centuries, is an ultraphyllonite. It occurs within an Alleghanian dextral retraining bend within the Appalachian Piedmont, as a segment of the Pleasant Grove-Huntington Valley shear zone system (>200 km long). The PBs differs markedly from other slates, in that, it contains abundant, micro-scale remnant porphyroclasts of higher-grade minerals. The PBs has major, trace and rare earth element geochemistry and 147Sm/143Nd ratios that are similar to the surrounding pelitic rocks. When compared to average slates, the geochemistry of the PBs exhibits depletion in alkali earth and alkali elements.It is concluded that the PBs belt formed by ductile shear within a strike–slip duplex and is the low-grade metamorphic product derived by shearing higher-grade upper greenschist facies schists. This origin explains its extreme hardest, high density, and chemical homogeneity. With across strike thickness of ultraphyllonite approaching a kilometer and a strike length >30 km, its volume is similar to mylonite zones developed in granitoid rocks of higher grade terranes. Compared to these rocks, the PBs differs in composition, compositional homogeneity, fabric, and strain across the entire belt. The PBs cannot be a unique occurrence of a slate-belt that formed through deformation processes, therefore raising question about the origin of some pelitic slate-belts that occur within a similar tectonic setting in other orogens.