ROWLANDSVILLE GRANODIORITE GNEISS, MD, THE APPARENT SOURCE OF VOLCANIC COBBLES IN THE GLACIOGENIC DIAMICTITE OF THE ROCKWELL FORMATION OF PA, MD, AND WV

An Upper Devonian diamictite displaying glaciogenic features was deposited in a 400 km long belt in PA, MD, and WV during the Late Famennian (Brezinski et al., 2010). The southern part of this deposit, where it is in the basal Rockwell Formation, contains exotic volcanic to hypabyssal clasts that have arc compositions ranging from picrobasalt to rhyolite. Zircons from two dacite cobbles collected at Crystal Spring, PA, yielded a U-Pb age of 453 Ma. On the basis of geochemical analyses, the Rowlandsville Granodiorite Gneiss of Cecil County, MD, is proposed as the source pluton for the dated cobbles. The Rowlandsville and possibly associated Baltimore Mafic Complex probably accreted to Laurentia at ca 450 Ma in the Pennsylvania embayment during the Taconic orogeny (Wise et al., 2007). Obduction of arc stratiform volcanoes could have yielded lofty peaks which may have been enhanced by an event closer to the ca 360 Ma age of glaciation. The Rowlandsville is on the north side of the Rock Run shear zone and remained attached to Laurentia. William D. Orndorff in 1999 noted that the Rock Run shear zone yielded dextral, strike-slip motion of Alleghanian age, transporting outboard portions of the Rowlandsville and associated units to the southwest. Indeed, the multiphase Carysbrook pluton and portions of the multiphase Ellisville pluton (443.7 ± 3 Ma main phase, Hughes et al., 2013) of central Virginia are decent geochemical matches to the Rowlandsville. Within the Baltimore Mafic Complex to the north of the Rowlandsville, baddeleyite in a late, refractory phase yielded a U-Pb age of 441.7 ± 7 (Smith, 2008). The Carysbrook pluton (sensu stricto) disconformably underlies the Arvonia Formation, a correlative of the Quantico Formation which contains a ca 448 Ma metatuff (Horton et al., 2010). Despite previous usage, the ca 457 Ma Columbia pluton (Wilson, 2001 and Sinha et al., 2012) of central Virginia, is geochemically unrelated to the Carysbrook, Ellisville, and Rowlandsville plutons. In summary, after having sampled and analyzed approximately 50 felsic to intermediate igneous outcrops of presumed Ordovician age in MD and VA, we are of the opinion that the Rowlandsville Granodiorite Gneiss is an excellent match to the exotics in the Rockwell Formation, and that the Carysbrook and portions of the Ellisville plutons are also quite similar.