FORMATION OF ULTRA-PHYLLONITE WITHIN THE MID-ATLANTIC PIEDMONT TRANSCURRENT DUPLEX

While slaty fabrics have been shown to form within highly sheared rocks, there are no examples in the scientific literature where volumetrically significant, let alone extensively quarried, tectonic slate has formed in this manner. Such a rock outcrops in strike-parallel quarries on Slate Ridge in Pennsylvania and Maryland, and was sold as the Peach Bottom Slate through the 19^th and early 20^thcenturies. In contrast to the rocks in classical slate belts, the Peach Bottom Slate originated by dynamic recrystallization and strain hardening of higher-grade protoliths within a shear zone that was accompanied by retrograde metamorphism. Here we refer to slate formed in this manner as ultra-phyllonite.

The near east-west structural trend of the rocks of the Appalachian Orogen between the NY Promontory and the PA Reentrant provided a continent-scale restraining bend that resulted in the development of a major Alleghanian transcurrent duplex. Various splays within the duplex cross pelitic units, resulting in the development of thick zones of mid-crustal phyllonite and in this case, ultra-phyllonite. The ultra-phyllonite resembles slate formed by prograde metamorphism of mudrocks. But it differs in several fundamental ways including: 1) development along a splay of a regional transpressional shear system; 2) envelopment by, and gradation into, higher grade phyllonite across and along strike; 3) abundant, kinematically-significant, ultramicroscopic porphyroclasts that display dextral shear sense; 4) remarkable hardness, density, and durability; 5) lack of the vestiges of original sedimentary and/or compositional layering; 6) homogeneous chemical composition; and 7) Nd isotopic systematics that are nearly identical to adjacent schist and phyllonite units. Field, microstructural, geophysical, petrographic, density, and geochemical data will be presented that document a previously unrecognized mechanism for the origin of volumetrically significant tectonic slate (ultra-phyllonite) developed from higher grade schist units through intense and localized high-strain. The example discussed here is likely not unique and this study may lead to the recognition and reevaluation of slate belts that occur in similar tectonic and structural settings.