DEFORMATION AND MAGMATISM IN THE IVY CREEK NATURAL AREA, EASTERN BLUE RIDGE, CENTRAL VIRGINIA

The Ivy Creek Natural Area (ICNA), is a 5 km² preserve located in the eastern Blue Ridge province ~2-km northwest of Charlottesville, Virginia. Rocks exposed in the ICNA include granitoid gneisses of the Mesoproterozoic basement complex, meta-sandstones and conglomerates of the Neoproterozoic Lynchburg Group, and dikes of mafic and felsic igneous rocks.

Lynchburg Group metasedimentary rocks crop out in a narrow (<1 km) NE-trending belt in the INCA, and may correlate to either the Thorofare Mountain or Ball Mountains formations of Wehr (1985). Lynchburg Group rocks rarely preserve primary structures, but are characterized by a steeply dipping penetrative NE-striking foliation and gently plunging elongation lineations. Kinematic analysis is consistent with a significant component of dextral shear across this metasedimentary belt. The western boundary of the Lynchburg belt is interpreted to be an unconformity, whereas the eastern boundary may be a reactivated and inverted Neoproterozoic fault.

Two types of mafic dikes intrude the basement complex, these include poorly exposed potassic dikes and porphyritic metabasalts. A steeply dipping, WNW striking dike (<10 m wide) of rhyolite and rhyolitic breccia cut both the basement complex and Lynchburg Group. The groundmass includes fine-grained alkali feldspar and quartz with both phenocrysts, and a diverse assemblage of clasts/xenoliths. The age and origin of this rhyolite is uncertain, but its unmetamorphosed character may indicate it is a post-Paleozoic intrusion.