EOCENE IGNEOUS ACTIVITY AND RELATED PHENOMENA, CENTRAL APPALACHIAN VALLEY & RIDGE

An Eocene dike swarm centered in Highland County, Virginia intrudes outcrops of Ordovician to Devonian strata, and a structurally-complex subsurface overthrust belt sequence. The intrusions range from felsic to mafic in composition. Mafic rocks include exposures interpreted as diatreme breccias, volcanic breccias, or lahar deposits. Some felsic exposures are breccias. Several dikes have glassy chilled margins.

Based on silica content, dike compositions are bimodal: rhyolite/rhyodacite and basalt, with very few intermediate compositions. Both rhyolites and basalts show an alkalic affinity based on K2O+Na2O and trace element trends. The bimodal composition suggests a similarity to rift sequences. Central location of rhyolites within a larger area of basalts is similar to some western U.S. volcanic fields. Ta and Th values from an obsidian are similar to ocean rise extensional systems. Trace element data for rhyolites suggest minimal involvement with sialic crust.

There are several phenomena associated with the intrusions. The dike swarm partially overlaps a geothermal area centered at Hot Springs, Virginia. Uplift is evident from regional drainage patterns that flow away from the dike swarm area, and from a high on the Schooley erosion surface. Dike swarm may fill a triangular space between a down-to-the-northeast late Silurian hingeline and the 38th parallel lineament. The igneous event may be associated with plate rearrangements in the Cordillera and Caribbean areas.