RETROGRADE METAMORPHISM OF THE GLENARM WISSAHICKON IN SOUTHEAST PENNSYLVANIA

The Wissahickon Formation is located across the SE portion of PA to northern DE, from Philadelphia to Maryland. It is made up of pelitic and psammitic schist and gneiss that contains amphibolites. The Glenarm Wissahickon is a provisional sub-unit informally separated by Blackmer (2005, Pa Survey Open-File report OFBM 05-0.10). In most of the Glenarm Wissahickon, garnet, staurolite, and kyanite are porphyroblasts, with biotite and muscovite forming the matrix foliation. Previous work revealed high-T metamorphism following regional D2 deformation that resulted in staurolite breakdown and formation of late garnet and sillimanite needles. However, some rocks in the Glenarm Wissahickon appear to be distinctly lower grade (Wyckoff, 1952, GSA Bull., v. 63, p. 25-58); they are fine-grained and contain chlorite forming the matrix foliation instead of biotite. We are investigating the relationship between the higher- and lower- grade rocks in the Glenarm Wissahickon using field and structural relationships, petrographic analysis of mineral assemblages and microstructures, and SEM-EDS analysis of mineral compositions and zoning patterns.

In some of the higher-grade samples retrograde chlorite and muscovite are found. Biotite, garnet, chlorite, and staurolite compositions, and garnet zoning patterns are similar in both lower- and higher-grade rocks. This suggests that they may have similar bulk compositions and metamorphic histories. The lower-grade rocks are found near D2 thrust faults and also are associated with widely-distributed D3 shear zones showing oblique transtensional motion. A younger S3 foliation wraps around the retrograde porphyroblasts, indicating the retrograde metamorphism was post-S2 and pre- to syn-S3. The evidence can be explained by alteration of higher-grade assemblages by significant, post-D2 retrograde metamorphism. Different bulk rock compositions also probably influenced the mineral assemblages and reactions. Spatial distribution makes it unlikely that the higher- and lower-grade rocks are separated by a tectonic contact.