The Robertson River Igneous Suite (Blue Ridge Province, Virginia) — Late Proterozoic anorogenic (A-type) granitoids of unique petrochemical affinity

Stratigraphic notes, 1992: Four short papers propose changes in stratigraphic nomenclature in Vermont, Virginia, Maryland, and the District of Columbia

“…I propose that each of these predominantly intrusive, nonstratiform bodies represents a lithodemic unit. Furthermore, mineralogic and geochemical analyses demonstrate that all of the Robertson River lithologic units are petrologically similar to A-type granitoids recognized from other localities (Loiselle and Wones, 1979;Whalen and others, 1987;Eby, 1990;Tollo and Arav, 1992). All of the Robertson River rocks were emplaced in a rift-related continental setting and collectively define a large, dikelike mass formed by intrusion in a predominantly extensional regime.…”

“…Recent detailed mapping and petrologic analysis indicate that rocks previously assigned to the Robertson River Formation include a series of granitoids, syenitoids, felsites, and rhyolites that were emplaced episodically during an extended period of magmatism lasting more than 30 million years (Arav, 1989;Hawkins, 1991;Lowe, 1990;Tollo and others, 1991;Tollo and Aleinikoff, 1992;Tollo and Arav, 1992;Tollo and Lowe, 1994 ). These studies demonstrate that the Robertson River includes at least eight units that are distinguished by characteristic mineral composition, texture, bulk chemical composition, and age as determined by U-Pb isotopic analysis.…”

Geology and geochemistry of granitic and charnockitic rocks in the central Lovingston massif of the Grenvillian Blue Ridge terrane

“…Grenville-age rocks of the Lovingston massif were intruded by numerous Neoproterozoic A-type granitoids and mafic dikes that are related to Iapetan extension (e.g., Rankin, 1975Rankin, , 1976Bartholomew and Lewis, 1984;Bartholomew, 1992;Tollo and Arav, 1992;Tollo and Lowe, 1994;Tollo and Aleinikoff, 1996;Tollo and Hutson, 1996;Tollo et al, 2004). This granitic magmatism is part of the first pulse of a two-stage rifting cycle in the central Appalachians involving pulses dated at 760-702 Ma and ca.…”

A Gallium Anomaly Utilized in Palaeogeographic Reconstruction of Battle Mountain, Rappahannock County, Virginia

“…Within the Robertson River, however, the peralkaline units are characterized by higher Ga/Al ratios (average = 7.7) than the metaluminous units (4.7). The high Ga/Al ratio of the Robertson River (Tollo and Arav, 1992) and other similar suites (see references given above) has been attributed to the fractionation of plagioclase, which has a low distribution coefficient for Ga (Goodman, 1972) and/or ionic complexing in the melt (Collins and others, 1982). For the Robertson River, the enrichment in Ga may also be a result of alkali feldspar accumulation according to evidence preserved at Reference Locality 15 (F3) by rocks of the metaluminous Hitt Mountain Alkali Feldspar Syenite.…”

“…These and other granitic rocks that were either contiguous in outcrop exposure or could be correlated on a regional basis were later mapped as the Robertson River Pluton or Formation (Lukert and Nuckols, 1976;Lukert and Halladay, 1980;Clarke, 1984). However, Tollo and Arav (1992) proposed the term Robertson River Igneous Suite in order to emphasize the lithologic, petrologic, and geochemical diversity of the formation and Tollo (in press) presented detailed descriptions and nomenclature for the constituent units.…”