PROVENANCE AGE DETERMINATION FOR SILICICLASTIC ROCKS OF THE PENNSYLVANIA PIEDMONT USING SM-ND ISOTOPE GEOCHRONOLOGY

The mid-Atlantic Piedmont of Pennsylvania can be divided into two very broad lithologic domains. Rocks north of the infamous Martic Line are marbles of varying composition with minimal siliciclastic components. South of the Martic Line, the Piedmont is largely underlain by siliciclastic metasediments that are interrupted by a few structural blocks containing Grenvillian gneisses, and have been laterally shuffled along late Paleozoic dextral transcurrent shear zones. The marble sequences have been interpreted to be part of the Cambrian-Ordovician Laurentian shelf-edge. The siliclastic rocks have been interpreted as being associated with A) late Proterozoic Iapetan rift sequences, B) deep water equivalents to the early Paleozoic self carbonates, C) the accretionary sequence associated with the closing of Iapetus during the Taconic orogeny, or D) all of the above. The provenance choices for the siliciclastic rocks reads like a trick multiple choice question for an exam on Appalachian Tectonics. To address this trick question, we set out to use Sm-Nd isotopic compositions for four significant siliciclastic units from the western PA Piedmont. Twelve samples of the Peach Bottom slate, Octoraro and Peters Creek formations, and the Drumore tectonite were analyzed for Sm-Nd isotopic systematics. Our results show that all four rock units are derived from Precambrian sources with TDM ages ranging from 1405 to 2805 Ma. The results indicate that Peach Bottom slate and Octoraro schist have similar Sm-Nd systematics including high Nd concentrations (>40 ppm), EpsNdCHUR values (-7.31 to 7.69 PBS; -7.83 to 9.08 OS), and TDM (1737-1780 Ma PBS; 1809-1867 Ma OS). These similarities suggest that the Peach Bottom slate and Octoraro schist share a common Precambrian source(s). The data also supports independent petrologic, structural and geochemical lines of evidence that the Peach Bottom slate was derived by dynamic retrograde metamorphism from the Octoraro schist. The Peters Creek formation has the oldest TDM ages (1927-2805 Ma) and lowest Nd concentrations (5.3-35.5 ppm), perhaps because of a greater proportion of sand in the original protolith. Rather than a juvenile source, these results suggest that these units were derived from North American crust, such as a mixture of material from Archean and Proterozoic terranes. So, the answer to the question is most likely "A".