INVESTIGATION OF AN INVERTED METAMORPHIC GRADIENT BELOW THE EMBREEVILLE THRUST IN THE CENTRAL APPALACHIAN PIEDMONT OF SE PENNSYLVANIA

The Embreeville Thrust is the lowest in a series of ductile, thrust-sense shear zones in high grade metamorphic rock of the central Appalachian Piedmont. Thrust stacking resulted in the highest grade metamorphic rocks occurring in the structurally highest sheet. The Embreeville Thrust places the amphibolite facies Doe Run Schist above the greenschist facies Peters Creek Schist. Outcrops along the East Branch of Brandywine Creek near Downingtown, Pa. afford relatively continuous exposure of this transition. In the study area, the Doe Run Schist contains the assemblage garnet + kyanite + staurolite + biotite + muscovite +/- chlorite. Peters Creek schist contains garnet + biotite + muscovite + chlorite. Metamorphic grade in the footwall increases approaching the shear zone, indicating the presence of an inverted metamorphic gradient. This suggests that metamorphism in the footwall resulted from emplacement of hot hanging wall rock. Prograde metamorphism below the shear zone would result in release of fluids, driving retrograde metamorphism in the hanging wall.

To test this hypothesis we examined garnet zoning in hanging wall and foot wall rock. Garnet in the Doe Run Schist is wrapped by shear zone fabrics, indicating pre-kinematic crystal growth. X-ray composition maps reveal a sharp increase in Mn content in the garnet rims, indicating some degree of garnet dissolution. In contrast, garnet in Peters Creek Schist contains curved inclusion trails suggesting syn- to post-kinematic growth. Mn content decreases from core to rim, typical of prograde garnet growth. These observations support the hypothesis that emplacement of the Doe Run Schist drove prograde metamorphism in the structurally lower Peters Creek Schist.

The Laurels Schist, a narrow band of relatively quartz-rich schist is mapped at the base of the hanging wall along the trace of the Embreeville Thrust. Preliminary results indicate that small garnet in the Laurels Schist exhibits prograde zoning. This suggests that the Laurels Schist should properly be mapped in the footwall.