METAMORPHISM AND DEFORMATION OF THE GLENARM WISSAHICKON IN SE PENNSYLVANIA PIEDMONT

The Wissahickon Formation of SE PA and northern DE comprises pelitic and psammitic schist and gneiss with amphibolite. Blackmer (2005, PA Survey Open-File Report OFBM 05–01.0) informally subdivided the western part into 2 units: Glenarm Wissahickon to the N, and Mt. Cuba Wissahickon to the S. We report results of a detailed metamorphic study of a sample of Glenarm Wissahickon (C-207), collected near the southern margin of the Coatesville Quadrangle, as part of an ongoing comprehensive study of the Wissahickon.

Sample C-207 preserves a fairly extensive metamorphic history. Oriented inclusions of quartz and Fe-Ti oxide within staurolite (st) define an older foliation (S1). Small, euhedral garnet (gar1) are included within and older than st. The dominant shear foliation, S2, defined by muscovite (musc) and biotite (bio), wraps older st. Elongate and euhedral small garnet (gar2) is syn- to post-kinematic with respect to S2. Gar1 and gar2 are similar in size, shape, and composition, except that gar1 contains about twice as much Ca than gar2, based on analysis by SEM-EDX. Gar1 has slight zoning to lower Ca and Mn at the rim, while gar2 has complex zoning: a lower-Ca core grades to higher Ca and lower Mn, then a distinct zone of low Ca and slightly higher Mn, and a rim with slightly higher Ca and Mn. The increase in Mn and rims of fine-grained bio suggest limited resorption of gar2. Clusters of gar at st margins, with musc, bio, and minute needles of sillimanite (sill) suggest formation of gar2 with st resorption (higher T, lower P?). Clots of randomly-oriented sill needles also occur along grain boundaries of musc and plag. Late chlorite with musc and bio locally rimming st suggests retrograde metamorphism at lower T during cooling.

Our data are broadly consistent with earlier work, but document 2 generations of gar growth, and may suggest a more complex metamorphic history. Sample C-207 is generally representative of the mineral assemblage and textures in the Glenarm Wissahickon samples we studied, but mineral compositions may not be consistent with formation at the same P and T for all samples. More quantitative estimates of P-T conditions will be presented. Preliminary monazite data constrain formation of the matrix foliation in C-207 during the Early Devonian (~400 Ma); amphibolite-facies metamorphism is Silurian or older.