PERALKALINE GRANITES AND VOLCANICS OF THE NEOPROTEROZOIC RIFT-RELATED ROBERTSON RIVER IGNEOUS SUITE, NORTHERN VIRGINIA, USA: ZR, NB, Y, AND REE MINERALOGY

The Robertson River igneous suite is the largest Neoproterozoic A-type granite body emplaced along the eastern Blue Ridge flank during regional Rodinia crustal extension related to the opening of Iapetus. The peralkaline products of the Robertson River igneous suite represent the final magmatic phase and preserve volcanic activity. U-Pb zircon age indicates emplacement in two magmatic pulses, 735–722 and 706–702 Ma (Tollo & Aleinikoff, AJS, 1996). The peralkaline Battle Mountain complex and Amissville alkali feldspar granite have the highest trace-element contents of the suite; Zr from 216 to 4055 ppm, Nb from 44 to 312 ppm, Y from 58 to 1510, ΣREE from 423 to 6737 ppm, and a diversity of accessory phases. Zircon, the only Zr-phase observed, ranges in habit from euhedral, mm long crystals to hydrothermal amoeboid, commonly altered masses. HfO₂ content ranges from 0.49 to 3.14 wt% and the Zr/Hf ratio (w/w) varies from 17 to 82. The major REE-bearing phases are fluorocarbonate, chevkinite group mineral, allanite, gadolinite, and a variety of Nb phases. Bastnäsite-(Ce) is a major fluorocarbonate with minor parisite-(Ce), and synchysite-(Ce). Chevkinite-(Ce) averages 11.50 wt% FeO and 2.60 wt% CaO, and is commonly altered. Allanite-(Ce) occurs in a wide variety of habits from subhedral crystals to amoeboid masses and averages 25 wt% ΣREE. Gadolinte-(Y) occurs as euhedral to subhedral crystals, sometimes zoned, and averages 52 wt% ΣREE+Y. The Nb-bearing phases, aeschynite-euxenite, fergusonite, and samarskite, occur in a wide variety of chemistry, habits, and typically show alteration, multiple generations, and deposition by late-stage fluids. Some rocks show a significant negative Ce anomaly related to late stage deposition of a La+Y+ REE-fluorocarbonate. The mafic assemblage in the peralkaline rocks is dominated by aegirine, riebeckite, and stilpnomelane; fluorite is ubiquitous. The mineralogy and petrography suggest Na-, Fe-, and F-rich fluids carrying REE, Y, Nb, and Zr were late-stage and moved through the rock during the early cooling process.