CONTRASTING METAMORPHIC HISTORIES WITHIN THE WISSAHICKON FORMATION: EVIDENCE FOR SUBDIVISION AND TECTONIC IMPLICATIONS

The Wissahickon Formation, comprising pelitic and psammitic schist and gneiss with amphibolite, extends across SE PA and N DE from Philadelphia to MD. Recent mapping defines provisional sub-units based on lithology, associated units, metamorphic and structural histories; from E to W: type Wissahickon Fm (sensu stricto); Mt Cuba Wissahickon; and Glenarm Wissahickon. In both type Wissahickon Fm and the Mt Cuba, high-T low-P metamorphism was associated with Silurian magmatism at ~ 430 Ma, followed by higher-P lower-T metamorphism (up to 0.75 GPa) in the Devonian (~385-372 Ma). The metamorphic history of the Glenarm Wissahickon is different.

Work by WCU petrology students with the first author show that several Glenarm Wissahickon samples have similar mineral assemblages and porphyroblast-fabric relations, with no evidence of early high-T metamorphism. Metamorphic history is described relative to the dominant matrix foliation (S2), formed at ~ 410-400 Ma (EMP U-Th-total Pb age on pre- to syn-kinematic monazite). Pre-S2 garnet growth at higher P (highest Ca and lowest Mg) was followed by resorption, probably due to staurolite-in reaction at T ~ 600 ºC and P > 0.7 GPa. Continued St and later Grt growth was pre- to syn-S2, with Grt zoned to lower Ca and Mn and higher Mg. Post-S2 Grt growth with sharply lower Ca, higher Mn and Mg, was associated with breakdown of St + muscovite --> Grt + sillimanite, at T ~ 640 ºC and P ~ 0.7 GPa. These preliminary data are consistent with a clockwise P-T path, as suggested by Alcock (Ph.D. thesis, U.Penn., 1989), but reaching higher T than previously recognized.

Since both Glenarm and Mt Cuba Wissahickon are in contact with Precambrian gneiss and Cockeysville and Setters Fms, the tectonic significance of our results is murky. Ages of syn-deformational monazite from all Wissahickon units indicates shortening at ~410-400 Ma. Could loading of Glenarm Wissahickon onto Mt Cuba by back-thrusting on the Street Rd Fault explain both decompression of the Glenarm and the high-P overprint in the Mt Cuba? Or were the units separate during metamorphism and juxtaposed by younger tectonism? Timing and extent of vertical offsets are constrained by 40Ar/39Ar cooling ages of white micas which are about 365 Ma in all units (Blackmer, Kunk, and Bosbyshell, this meeting), implying a similar crustal level by that time.