UPPER ORDOVICIAN VOLCANIC CLASTS IN THE UPPER DEVONIAN BASAL ROCKWELL FORMATION DIAMICTITE, MARYLAND AND PENNSYLVANIA

The end-Devonian siliciclastic Rockwell Formation in south-central PA and adjacent western MD incorporates polymictic, glaciogenic diamictites with exotic volcanic to hypabyssal clasts that range in composition from basalt to rhyolite. Earlier attempts to geochemically determine the clast provenance (Skema and Smith, 2010) identified them as calc-alkaline magmatic arc lithologies, but were unable to unequivocally pinpoint the source(s).

U-Pb analyses by the ID-TIMS method were performed on single zircons from two dacitic igneous cobbles in diamictite collected at Crystal Spring, Fulton County, PA (39º 57' 11.8" N and 78º 14' 22.2" W). Preliminary results based on 3-4 zircon analyses per sample yielded coherent clusters of data with weighted mean ²⁰⁶Pb/²³⁸U dates of 452.70 ± 0.55 Ma and 453.9 ± 2.1 Ma, that are indistinguishable within uncertainties. The new, high-precision geochronology along with the available age data from the Appalachian volcanic arc units suggests the following potential sources: 453 ± 2 Ma upper rhyolite member of Ammonoosuc Formation (Tucker and Robinson, 1990), 453 ± 4 Ma Upper Metafelsite in the Chopawamsic Formation (Horton et al., 2010), 453.9 ± 1.5 Ma Millbrig Bentonite (Chatel et al., 2005), and possibly the unnamed upper felsite member of the James Run Formation of Cecil County, MD (Higgins and Connant, 1986). When compared to Rockwell exotic clasts, the central Pennsylvania Upper Ordovician bentonites B-12 and B-13 do fit the very tight Th-Hf/3-Ta trend, whereas the Chopawamsic Metafelsite contains much lower Nb and Ta (< 0.2 ppm and <0.01 ppm, respectively).

Trace element analyses on 35 volcanic and hypabyssal pebbles from Crystal Spring, as well as Sideling Hill and Lavale, MD, define REE_CN groups that are represented at each of the diamictite localities and suggest multiple and/or evolving volcanic source complexes for the pebbles. Further high-precision U-Pb geochronology on chemically compatible, potential source rocks will better identify the diamictite provenance and decipher patterns of glacial sediment transport in the Late Devonian and might set limits on lateral offset during the Alleghanian Orogeny.