THE ROSETTA STONE OF THE CENTRAL APPALACHIAN PIEDMONT IN PENNSYLVANIA: CHESTER CREEK TRAIL EXPOSURES

The contact between upper amphibolite facies metasedimentary rock of the Wissahickon Fm and metaigneous rock of the Wilmington Complex magmatic arc is exposed in an outcrop along the Chester Creek Trail near Glen Riddle, Pa. Prior to recognition of the significance of the exposure some 20+ years ago, this contact was modeled as a thrust fault. In an early study of metamorphic overprinting, Crawford and Mark (1982) proposed an overthrust model with the Wilm Cmplx occupying the structurally highest block. Wagner and Srogi (1987) subsequently postulated the presence of an inverted metamorphic gradient in the metasedimentary rock beneath the Wilm Cmplx. However, the Chester Creek Trail exposures, and data obtained from the rock visible there, demonstrate the original contact to be intrusive and not a thrust fault.

Portions of the Wilm Cmplx, including the Chester Creek Trail exposure, contain mafic layers with distinct boninitic geochemistry. Thin (<1 m) boninitic amphibolite layers are also present in the Wiss Fm at the contact. The layers are concordant and attenuated within the dominant foliation and folded by younger deformation. Their presence in both units demonstrates that the contact developed very early in their history and suggests an intrusive relationship.

The complex sequence of deformation and metamorphism in the Wissahickon is clearly delineated in this exposure. Two episodes of metamorphism are evident: early muscovite-absent, sillimanite-bearing assemblages are overprinted by kyanite and muscovite. Sil is aligned in the dominant S₂ foliation which is axial planar to isoclinal folds (F₂); the oldest foliation S₁ is present in the hinges of these folds. F₂ isoclines are re-folded (F₃/S₃) and S₁ through S₃ are deformed by an outcrop-scale shear zone, S₄. S₄ foliation is defined by aligned Ky + Ms. A younger crenulation, S₅, affects all these elements.

EPMA U-Th-total Pb monazite geochronology dates Sil-grade metamorphism at ~440 Ma. ~476 Ma monazite, the same age as igneous zircon in the Wilm Cmplx here, is interpreted as evidence of contact metamorphism, and supports an intrusive contact. If the metasedimentary rock here is indeed arc-basement, then 630-640 Ma detrital zircon ages from the Wissahickon Fm demonstrate that the Wilmington Complex originated as a peri-Gondwanan arc.