PORPHYROBLAST-MATRIX AND STRUCTURAL RELATIONS ALONG THE METAMORPHIC FIELD GRADIENT IN THE WISSAHICKON SCHIST TYPE SECTION, SE PENNSYLVANIA

The Wissahickon schist type locality along Wissahickon Creek in the SE Pennsylvania piedmont (Philadelphia), is composed of pelitic schist interlayered with psammitic gneiss and quartzite (0.1-1m thick). The type locality is known for its complete record of Barrovian metamorphic zones. The metamorphic field gradient increases progressively SSE in the creek valley for 5km from chlorite to second sillimanite zone, at a high angle to the ENE-WSW strike of the structures. The chlorite zone in the N is within and adjacent to the 070-striking, dextral-transcurrent Huntingdon Valley shear zone system (Carboniferous). A transect along the creek is across both the metamorphic zones and the structural grain, permitting documentation of the relations between the metamorphic textures and strain fabrics. We mapped along the east valley slope starting in the chlorite zone (Bells Mill) to the SSE for 2.8km, into the kyanite zone (Valley Green). With increasing grade there is both general coarsening in the pelitic layers and shallowing of foliation dip, but relatively consistent ENE-WSW strike. Foliation is steeply-to-moderately dipping in the lower grade zones (N), and shallowly-to-moderately dipping in the higher grade zones (S). Fold hinge lines plunge shallowly, and are subparallel to both mineral lineations and foliation strikes. Interlimb angles are consistently tight in the lower grade rocks, and variable in the higher grade rocks. Folds are of both layer contacts and foliation in the higher grade rocks, but only of the contacts in the chlorite zone. Crenulation is common in all zones. Oriented specimens of pelitic schist were collected from 50 stations, and thin sections were made according to fabrics in order to document the progression of porphyroblast-matrix fabrics along the field gradient. Microstructures show that chlorite and biotite are fabric-forming in the chlorite and biotite zones, with garnet porphyroclasts indicating retrograde metamorphism. That relation persists into garnet zone, and is not seen in our samples from the staurolite and kyanite zones. These data illustrate a direct relation between the metamorphic field gradient and the strain associated with the shear zone system in the N, consistent with previous studies that cite regional deformation the cause of the metamorphic zone pattern in this part of the Appalachian orogen.