A SILURIAN AGE FOR THE SPRINGFIELD GRANODIORITE: TECTONIC AND METAMORPHIC IMPLICATIONS FOR THE CENTRAL APPALACHIAN PIEDMONT
Published isograd maps of the Pennsylvania Piedmont show an area of pelitic migmatite that lies to the south of, and is intruded by, the Springfield Granodiorite to be within the kyanite + K-feldspar metamorphic zone. Petrographic analysis in support of recent mapping reveals that kyanite is a late crystallizing phase and is likely associated with the growth of muscovite during higher pressure and/or lower temperature metamorphism, subsequent to attainment of peak temperatures and partial melting. The pelitic migmatite contains intergrowths of kyanite and pale green, extremely low Ti biotite, which are interpreted as pseudomorphs after cordierite. These observations suggest that peak temperatures occurred under relatively low pressure conditions. Thus, the kyanite + K-feldspar zone was apparently mapped based on the occurrence of minerals from two periods of metamorphism and does not reflect the breakdown of muscovite at pressures in the kyanite stability field.
The Silurian age of the Springfield Granodiorite and the metamorphic data suggest that an elevated thermal regime was present regionally during the Silurian. The distribution of Silurian-aged high T-low P metamorphic mineral assemblages reflects overlapping thermal aureoles surrounding individual plutons, and not a thermal high centered on the Wilmington Complex as previously described. The Silurian thermal regime is interpreted to result from lithosphere thinning associated with either slab delamination, marking a shift in subduction polarity, or subsequent backarc extension above a west-dipping subduction zone.