TIMING AND P-T CONDITIONS OF DEFORMATIONAL AND METAMORPHIC EVENTS IN ROCKS ON THE FLOOR OF THE SUSQUEHANNA RIVER, HOLTWOOD, PA

Paleozoic rocks of SE PA preserve multiple deformation fabrics and garnet-grade assemblages. However, the timing of the events and their correlation with regional orogenic events is currently uncertain. Garnet grade pelitic schists of the early Paleozoic Octoraro formation are 80-90% exposed in a 3 km² exposure below the Holtwood Dam on the lower Susquehanna River in Pennsylvania. The stratigraphy progresses from greenstone volcanics (oldest) under the dam apron through garnet and magnetite bearing schists with abundant quartz layers along foliation, to sulfidic schists below the Norman Wood Bridge to quartz and feldspathic schists beyond the bridge. In the central part of the area garnet grains up to 5 mm in diameter locally form crystal clots up to 5 cm, possibly as a result of hydrothermal alteration of aluminum-rich volcanic clasts associated with the greenstone volcanics. Inclusion trails in large plagioclase porphyroblasts preserve at least three distinct deformational fabrics with only the later two preserved in the matrix. Aligned ilmenite and tourmaline inclusions define an early layering (S₁), which is spectacularly crenulated by two later foliations, all within the undeformed albite porphyroblasts. Pervasive quartz stringers, residual from metamorphic reactions, define S₁ in outcrop, and regional relationships suggest correlation of this D₁ event with the Taconian (~450 Ma) orogeny. The dominant schistosity (S₂), possibly of Taconian age as well, is subparallel to S₁ and trends generally east-west with a northwest vergeance. Superimposed is a widespread, steeply southeast-dipping spaced cleavage (S₃) of probable Alleghanian age (~325-265 Ma) with associated local, well-defined F₃ folds. This deformational event reactivated and rotated S₂ and created space for the deposition of large bull quartz veins by hydrothermal fluids, suggesting cooling of the rocks (exhumation?) from ductile to brittle conditions during deformation. Microprobe analyses are currently underway to constrain the P-T conditions and absolute timing of each deformational event. It is hoped that these superbly exposed rocks can provide a high-resolution P-T-D-t path that can be correlated with other exposures around the region.