A FIELD STUDY OF EOCENE INTRUSIVE ROCKS AND THEIR SURROUNDINGS IN HIGHLAND COUNTY, VIRGINIA

An intensive petrographic and field study of a one-square-mile area in Highland County, VA reveals some of the details of the how Eocene-age intrusions moved through the Silurian-age sedimentary country rock. Igneous rocks form a bimodal suite ranging from picrobasalt to rhyolite. Parallel plagioclase laths and lens-shaped inclusions of earlier formed igneous rocks indicate flow texture. Mapping indicates many of these intrusions cut across the regional folding of sedimentary bedding and follow two prominent regional joint sets: N40°-60°W and N60°-80°E. The joints likely served as conduits for the intrusions. Diatreme breccia bodies are interpreted as late-forming features, cutting across bedding, regional folding, and are always associated with nearby mafic or felsic rock. Diatreme bodies here can be compared with similar bodies elsewhere. Diatremes contain numerous rounded xenoliths of both mafic and felsic igneous rocks and also sedimentary country rock. Xenocrysts of olivine, plagioclase, clinopyroxene, biotite, and hornblende are interpreted in most cases to be from disaggregated phenocrysts from igneous xenoliths. The breccia bodies reveal details that are consistent with a hydrovolcanic-maar-type origin. Weakly developed internal bedding is consistent with pyroclastic layering. A close examination of sedimentary xenoliths reveals that some are from formations not found in the immediate wall rocks. Devonian-age Millboro Shale clasts are found in one breccia that intrudes the Silurian-age Wills Creek/Tonoloway Formations. These could be xenoliths moved up from lower in the crust indicating complex structure placing older rock over younger rock at depth. Alternatively, a feature of diatremes observed elsewhere is that since they are collapse structures, xenoliths from younger rock that has since been eroded away may fall into the pipe, thus being preserved in older rock. If such a mechanism operated here, the Millboro clasts would have fallen through a minimum of approximately 1100 ft (330 m) of stratigraphic section to its present level. This could provide a minimum estimate as to how much bedrock erosion there has been in Highland County since the Eocene.