STRUCTURAL CONTROLS ON THE METHOD OF EOCENE MAGMATIC INTRUSION IN BLUE GRASS VALLEY, HIGHLAND COUNTY, VIRGINIA

A concentrated episode of volcanic activity (47-49 Ma) occurred in Virginia and West Virginia during the Eocene epoch, producing the youngest igneous rocks along the tectonically passive Eastern North American Margin. The Eocene magmatism is perplexing because 1) the last significant tectonic activity in this region was the rifting of Pangea, which occurred ~200 Ma and produced the ~190-200 Ma Central Atlantic Magmatic Province and 2) the mechanisms that permitted the magma to ascend through the lithosphere and erupt are poorly understood. We conducted a case study of igneous intrusions located in the Blue Grass Valley, Highland County, Virginia, to test the hypothesis that the Eocene magmas exploited preexisting Alleghenian structures to ascend to the surface. Igneous intrusions were mapped using GPS units to mark the location of outcrops in the field area. Orientation measurements of the country rock – middle-Ordovician carbonates associated with the Beekmantown formation – were obtained and mapped to interpret the local-scale structural features found at the site. Phenocryst assemblages and textural features of the igneous rocks and contact zones were documented. Two compositionally distinct igneous rocks are present in the field area: a massive amygdaloidal basalt containing phenocrysts of pyroxene and olivine, and a fine-grained trachyte containing phenocrysts of feldspar, biotite, and hornblende. The contact between the felsic dike and the country rock is brecciated and the chilled margin in the dike is glassy. These two pieces of evidence indicate rapid, explosive emplacement of the dike. The felsic dike in our field area strikes ~017, consistent with the NE-SW orientation of other Eocene dikes in the region. The dip measurements of the carbonate bedding are highly variable, but are generally steeply dipping to sub-vertical. On the north side the bedding planes of the middle-Ordovician carbonates dip shallowly to the east and may be overturned. Orientation measurements are consistent with parasitic overturned folds within the larger Hightown Anticline. The dikes in our field area are located along the hinge of the parasitic overturned fold, supporting the hypothesis that these small-scale structures provided the shallow crustal conduits for the Eocene magmatic event.