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, Amissville Alkali Feldspar Granite (Arav, 1989: George Washington Univ. M.S. thesis). Unit is exposed discontinuously along the eastern edge of the northern Robertson River segment. Typically contains quartz phenocrysts.

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. Amissville Alkali Feldspar Granite is named for Amissville. Type locality is series of roadcuts on US Hwy 211 at intersection with VA Hwy 640, 4.3 km (2.7 mi) west of Amissville, Massies Corner 7.5-min quad, northern VA. Consists of light-gray, medium-grained, porphyritic, aegerine+riebeckite-bearing alkali feldspar granite (lithic nomenclature after Le Maitre and others, 1989), characterized by small prisms of green aegerine and black riebeckite (particularly diagnostic field criteria). In fault contact with Middle Proterozoic basement gneiss along eastern border of Robertson River outcrop belt (Lukert and Halladay, 1980). Dikes of fine-grained Amissville rocks cut Cobbler Mountain Alkali Feldspar Quartz Syenite and Laurel Mills Granite (both new) in northern portion of Robertson River outcrop belt. Amissville is cut by fine-grained felsite dikes of Battle Mountain Alkali Feldspar Granite (new) near Massies Corner (Arav, 1989; Hawkins, 1991). Age is Late Proterozoic. Report includes geologic map.

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

U-Pb zircon age for Amissville Alkali Feldspar Granite is about 700 Ma.

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