CHEMICAL HOMOGENIZATION DURING RETROGRADE SLATE FORMATION

The origin of the Peach Bottom Slate (PBS) has been long debated and here we propose that it formed from enveloping schists by retrograde processes within an Alleghanian strike-slip shear zone. In addition to its legendary hardness, the PBS is chemically homogeneous across its 1-2 km width and shows enrichment of relatively immobile elements (Al, Fe, REEs, Sc, Ti, Y) and corresponding depletions in alkali elements (Ca, Mg, Na, Sr) in comparison to surrounding units, and shale and slate units elsewhere. Of the major elements, only Si and K have comparable values. Comparison of the geochemistry of the PBS and its likely precursor, the Drumore Tectonite (DT), substantiates field observations related to outcrop scale homogeneities and indicates chemical variations, presumably related to original compositional layers, exist in the DT that are now essentially absent from the PBS. For example, silica content along the PBS sampling transect is 57.43+/-1.9%, that of the DT varies from 61.08+/-11.3%. Similar relationships can be seen in nearly all trace elements (e.g. Thorium - PBS = 14.7+/-0.7; DT = 14.9+/-3.6 ppm). Both LOI and Total Carbon values are higher in the PBS and may be the consequence of introduction of fluids during retrograde metamorphism. Comparison of rare earth element patterns with estimates of the upper crust (shale composites) indicates that the PBS samples have a flat pattern with a slight depletion in the LREEs, but contain 1.5-2x the REE concentrations of upper-crust values. Compared to the PBS, the DT REE patterns are highly similar but, on the average, less enriched and considerably more variable. The REE patterns of the Peter's Creek and Octoraro Schists, the suspected precursors to the PBS and DT, have the same trends. Geochemical trends, taken together with field evidence, physical properties (hardness, density, durability) and textural characteristics, indicate that the PBS is the end product of intense shearing at relatively low-grade conditions within a splay of a major Alleghanian shear zone. The Drumore Tectonite is believed to represent an intermediate stage of this process in which deformation and chemical homogenization was less extreme.