FIELD, TEXTURAL, AND GEOCHEMICAL OBSERVATIONS FROM RHYOLITES ALONG THE CONTACT BETWEEN THE MOUNT ROGERS AND KONNAROCK FORMATIONS, SOUTHWEST VIRGINIA

Geologic mapping in the Blue Ridge of southwest Virgina (VA) led to recognition of several small bodies of rhyolite near the contact between the Mount Rogers and Konnarock Formations northeast of Troutdale, VA. Two rhyolite bodies mapped within the Konnarock Formation are informally named the Jerry Creek and Ripshin Creek rhyolites; their field relationships contain pivotal constraints on the age of the Konnarock Formation. This study compares textural, compositional, and geochemical data from the rhyolites to better understand the relationship with the surrounding Konnarock Formation. The rhyolite bodies are small (<1 sq. km each), elongate to elliptical and broadly dome-shaped, and composed of porphyritic and flow banded rhyolite. Geochemical data from these rhyolites are similar to published data for Fe-rich (2.5-4 wt% Fe₂O₃), high silica (70-77 wt% SiO₂) rhyolites of the Mount Rogers Formation. Porphyritic and flow banded rhyolite from the Jerry Creek and Ripshin Creek rhyolite bodies were collected to examine textures and mineral compositions with the scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectroscopy (EDS). The Jerry Creek rhyolite contains lapilli and fiamme surrounded by iron-oxide minerals, phenocrysts of embayed quartz and perthitic potassium-feldspar in a matrix of sericite, quartz, and albite, with trace amounts of magnetite, allanite, and zircon. The Ripshin Creek flow banded rhyolite contains phenocrysts of embayed quartz and perthitic potassium-feldspar in a matrix of sericite, quartz, and albite, with accessory to trace amounts of magnetite, rutile, ilmenite, zircon, chlorite, and titanite. Flow banding in the Ripshin Creek rhyolite is defined by alternating millimeter-scale, maroon and light gray flow bands that dip steeply to moderately northwest. The rhyolite overlies laminite and rhythmite beds containing dropstones in the Konnarock Formation and flow banding in the rhyolite is locally subparallel to bedding. . The Ripshin rhyolite bodies have a TIMS zircon U-Pb age determined to be ~751 Ma. This age, and the field relationship between the rhyolite and the Konnarock Formation, suggest that the base of the Konnarock Formation and onset of glaciation is ~751 Ma.