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

Abstract

Detailed field mapping and petrochemical analysis indicate that the previously defined Robertson River Formation encompasses a suite of A-type granitoids and felsites that intruded Middle Proterozoic gneisses in the core of the Blue Ridge anticlinorium in northern Virginia. Constituent granitoids in the northern portion of the suite include the herein named (from oldest to youngest): Laurel Mills Granite (LMG), Cobbler Mountain Alkali Feldspar Quartz Syenite (CMAS), Amissville Alkali Feldspar Granite (AAG), and Battle Mountain Complex (BMC). The Robertson River Igneous Suite includes both subsolvus (LMG) and hyper-solvus assemblages, ranges from predominantly metaluminous (LMG, CMAS) to peralkaline (AAG, BMC) in composition, and displays mineralogic and bulk chemical affinities to A-type granites described from other localities. The suite exhibits a broad range in Ga/Al, Nb, Y, Zr, and Zn that corresponds closely to published values for the Younger Granites of Nigeria, which have been shown to represent anorogenic magmatic activity related to extensional tectonics. The extreme enrichment in Ga, Zr, Nb, and Zn in both suites is probably indicative of a petrologic history involving extensive F complexing, consistent with the presence of abundant fluorite and F-bearing amphiboles. Marked depletion in Sr and variable Rb/Sr ratios suggest extensive plagioclase fractionation, which is consistent with the presence of this mineral as a liquidus phase in the earliest intrusive unit of the suite. Although the generation of A-type melts does not necessarily indicate an anorogenic or rifting tectonic environment, the unique geochemical characteristics and marked similarity to the demonstrably anorogenic Younger Granites of Nigeria are consistent with the interpretation that the Robertson River Igneous Suite records an extensive period of anorogenic magmatism associated with the early phase of Late Proterozoic rifting of Laurentia.