GEOCHEMICAL DATABASE WITH GENERALIZED INTERPRETATION FOR IGNEOUS ROCKS FROM THE CENTRAL APPALACHIANS OF VIRGINIA, MARYLAND, AND PENNSYLVANIA

When used with field observations, chemical compositions of igneous rocks provide information about the tectonic settings in which the rocks formed. Geochemical analyses of ~ 120 pristine to metamorphosed plutonic and volcanic rocks from the Central Appalachians are being organized for release by the VA DMME. Much of the sampling was done by the senior author and co-worker Vik W. Skema in search of a plutonic source for glaciogenic cobbles of island arc affinity in the Rockwell Formation of PA, MD, and WV. Units to be included in the release are: Carysbrook (8 samples), Catoctin (1), Chopawamsic (26), Columbia (5), Elk Hill Volcanic Complex (14), Ellisville (4), “Eocene” (8), “Evergreen Church” (2), Gold Vein (1), Lahore (2) Leatherwood (3), Occoquan (1), “Mt. Hermon Pyroclasts” (3), Ordovician pyroclastics (2), Poore Creek (2), Green Springs (7), Milton (3), Mount Rogers (7), Rich Acres (1), Robertson River Igneous Suite (6), Ta River (3), and U.S.G.S. Reference samples (3).

Preliminary observations suggest that the Chopawamsic (CP) of VA and James Run (JR) of MD include a range of compositions from basalt to rhyolite, but that the JR contains fewer mafic rocks. Similarly, no samples of JR contain ≤ 0.05 ppm Ta, but 12 samples of the CP do. Hence, the subduction zone for the JR arcs may have had a more outboard, i.e., oceanic interaction overall despite evidence of local Grenvillian contamination of the CP near its western margin. The Elk Hill Volcanic Complex (EHVC) is more bimodal than the CP or JR, with compositions clustering around basalt and rhyolite. It has an arc signature including both back arc N- and E-OFBs. One 21-cm-wide felsic dike at the type locality crosscuts folded compositional layering. The tightly clustered adakitic Carysbrook granodiorite and more variable Ellisville granite-granodiorite are related. The Columbia granitic gneiss is not adakitic and does not appear to be geochemically related to either the Carysbrook or the Ellisville plutons. The Rowlandsville pluton of MD ranges from granite to diorite and some samples are very similar to the Carysbrook except for the Rowlandsville not being adakitic. However, ongoing studies of the Green Springs-Poore Creek plutons indicate that they are parts of a single differentiation series in which the felsic samples are adakitic, but not the mafic ones.