NEW CONSTRAINTS ON THE METAMORPHIC HISTORY OF THE CENTRAL APPALACHIAN PIEDMONT: PRELIMINARY EPMA U-TH-TOTAL PB MONAZITE AGES FROM THE MT. CUBA WISSAHICKON

In the Central Appalachian Piedmont of Pennsylvania and Delaware, Silurian-aged, high-T, low- to moderate-P metamorphism is centered on the granulite-facies Wilmington Complex. Sillimanite-bearing low-P assemblages in the meta-sedimentary Wissahickon Formation east of the Wilmington Complex are variably overprinted by Devonian-aged, higher-P, kyanite- and staurolite-bearing assemblages. An apparent lack of similar overprinting in high-T meta-sedimentary rock west of the Wilmington Complex indicated a significantly different metamorphic history and contributed to provisional removal of these rocks from the Wissahickon Formation and informal designation as the “Mt. Cuba Wissahickon”. Here we describe metamorphic overprinting and the results of EPMA U-Th-total Pb monazite analysis in the Mt. Cuba Wissahickon which do not support this provisional re-designation.

In the Mt. Cuba Wissahickon, the dominant foliation (regional S2) is defined by high-temperature, sillimanite-bearing assemblages. Despite variations in grain size, mineral abundances, and type of deformation fabric present (folding, S-C shear fabric), the relative timing of mineral assemblage and dominant foliation is the same: staurolite and kyanite are texturally younger than the matrix foliation. Staurolite occurs as very small grains and larger porphyroblasts with morphologies ranging from euhedral to highly anhedral. Kyanite only occurs as small grains growing over sillimanite, muscovite, or in small grain boundary reaction domains, e.g. with garnet and quartz growing at the expense of plagioclase.

Preliminary EPMA results reveal complex zoning in monazite and the presence of two age populations: 419 ± 9.5 Ma and 438 ± 9.0 Ma. Both inclusions and matrix grains contain compositional domains which yield both older and younger ages. However grains which yield only the older age occur exclusively as inclusions; those which yield only the younger age tend to align parallel to foliation and thus constrain the age of the fabric-defining, sillimanite-bearing assemblages. Compositionally distinct rims on one monazite grain yield an age of 365 ± 14 Ma. This grain is partially included in the rim garnet adjacent to a grain boundary reaction domain (described above) and may date kyanite and staurolite overprinting.