GARNET ZONING IN THE WISSAHICKON FORMATION, PHILADELPHIA, PA: IMPLICATIONS FOR THE SILURIAN THERMAL REGIME IN THE CENTRAL APPALACHIAN PIEDMONT

The Wissahickon Formation in Philadelphia was the site of Crawford's (1977) classic study of Ca zoning in garnet, but subsequently, very little has been published on the metamorphism recorded in these rocks. This study, conducted in large measure by the West Chester University of PA petrology class, examines garnet zoning, imaged in x-ray composition maps, in ten samples from a N-S transect along Wissahickon Creek, from the contact with Precambrian gneiss to the north, and approaching the contact with the Silurian-aged Springfield Granodiorite to the south. Nearly all garnet examined exhibits high Mn cores, with Mn concentration decreasing towards the rim, where Mn concentration increases sharply. This is interpreted as primary growth zoning, with fractionation of Mn into the growing garnet. Some degree of garnet resorption during later retrograde metamorphism is indicated by the high Mn rim, consistent with the widespread occurrence of late muscovite in all samples. Ca is not strongly zoned and typically decreases slightly from core to rim. A few samples exhibit a sharp boundary between a relatively higher Ca core and lower Ca rim. Garnet from one sample, adjacent to a deformed granodiorite sill, exhibits a euhedral, low Ca core, surrounded by a high Ca overgrowth. The high Ca overgrowth in turn exhibits a low Ca rim, similar to zoning in other samples. The well defined core-overgrowth zoning, together with the sample location, indicates two periods of metamorphism: early contact metamorphism associated with intrusion of the granodiorite and subsequent regional Barrovian-style metamorphism (500-600C, 0.7 GPa), which affected all rocks in the transect. This interpretation is consistent with recent observations indicating that regional high T-low P metamorphism is spatially and temporally associated with Silurian plutons. The Wissahickon Formation south of the Springfield Granodiorite contains both Silurian (418 ± 12 Ma) and Devonian (398 ± 5 Ma to 379 ± 6 Ma) monazite (Th-U-total Pb ages), interpreted as the time of the early, contact metamorphism and regional Barrovian overprinting, respectively.