Tollo and Arav (in press) propose that Robertson River Formation of Allen (1963) be revised as Robertson River Igneous Suite to emphasize the lithologic diversity that characterizes this belt. Rocks range in composition from alkali syenite to alkali feldspar granite to granite. This suite is the largest group of intrusives within the Blue Ridge province of VA. Extends from near Upperville, 70 mi southwest to northern suburbs of Charlottesville. Includes at least nine mappable lithologic units, among them, Rivanna Alkali Feldspar Granite. Unit is exposed only in the southernmost part of the Robertson River outcrop belt and is typically a white to light gray, medium-grained, equigranular, biotite- and fluorite-bearing alkali feldspar granite.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Recent detailed mapping and petrologic analysis indicate rocks previously assigned to Robertson River Formation include a series of granitoids, syenitoids, and felsites emplaced sequentially during 30 m.y. episode of magmatism. Collectively, the rocks form a dike-like mass of intrusions extending nearly 100 km from vicinity of Charlottesville, VA to north-northwest of Ashby Gap, VA. Therefore, name is revised to Robertson River Igneous Suite. At least 8 units of the suite are distinguishable by mineral composition, texture, bulk chemical composition, and U-Pb-determined isotopic age, and can be mapped at 1:24,000 scale. Rivanna Granite is oldest formally named unit. Named for Rivanna, VA. Type locality is exposures south of Rivanna on south side of spillway north of filtration plant on south fork of Rivanna River, Charlottesville East 7.5-min quad, central VA. Consists of white, medium-grained, fluorite-bearing granite to alkali feldspar granite. Contacts with adjacent rocks not exposed. Age is Late Proterozoic. Report includes geologic map.

Source: GNU records (USGS DDS-6; Reston GNULEX).

U-Pb zircon age of Rivanna Granite of Robertson River Igneous Suite is 735+/-4 Ma.

Source: GNU records (USGS DDS-6; Reston GNULEX).