DID FLUIDS CONTROL THE ORIGIN AND DEFORMATION OF THE PEACH BOTTOM SLATE IN SOUTHEASTERN PENNSYLVANIA?

The Peach Bottom Slate (PBS) in the central Appalachian Piedmont Province is one of the most distinctive lithologies in the region; yet, its origin is still debated. The traditional view is that the PBS is a metamorphosed organic-rich mudstone that is part of the stratigraphic sequence of metamorphic rocks in the region. Recent work by Valentino (1994), however, suggests that the texture and unique composition of the PBS are the result of shearing of a protolith caught in a splay of the Pleasant Grove shear zone during the Late Paleozoic Alleghanian orogeny. Given that fluids can localize and control the behavior of shear zones, a study of the fluid history of the Peach Bottom and surrounding units was undertaken to see what role, if any, fluids may have played in the deformation and the origin of the PBS.

Fluids were characterized from abundant pre-, syn- and post-S₁ quartz veins in the PBS and adjacent schists using fluid inclusion microthermometry and CL microscopy. The uniform luminescence of quartz in veins of all ages and in all units indicates they filled by crystal growth into a fluid-filled cavity under static conditions. Data from primary and secondary fluid inclusions indicate a surprisingly uniform fluid history. Quartz in all veins, regardless of age or host unit, was precipitated from a CO₂-H₂O-NaCl+/-CH₄ fluid. The salinity of the aqueous fluid is less than 2 wt. % NaCl equivalent and the density of the carbonic fluid varies between 0.915 and 0.773 g/cm³. The only significant variation in composition is the CH₄ concentration, which is somewhat lower in the main segment of the Pleasant Grove shear zone than in surrounding units, including the PBS. Heterogeneous trapping of immiscible H₂O-rich and CO₂-rich fluids allows determination of trapping temperatures and pressures. For pre- and syn-S₁ veins they were ~250°C and 120-200 MPa, respectively. For the post-S₁ veins, the temperature may have been somewhat higher. The fluids are thought to have been derived from metamorphism of underlying rocks including carbonates overridden by the Martic thrust.

Given the similarities in fluid composition and trapping conditions between the PBS and adjacent units for much of the deformational history, any unique texture and/or composition of the PBS must be related to factors other than the fluid history.