PERALKALINE AND METALUMINOUS GRANITOIDS AND VOLCANICS OF THE NEOPROTEROZOIC RIFT-RELATED ROBERTSON RIVER IGNEOUS SUITE, NORTHERN VIRGINIA, USA: NB-TA OXIDES AND THEIR ALTERATION

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. Eight plutons constitute the elongate (110 km by ≤5 km) batholith emplaced in two magmatic pulses: about 735–722 and 704–697 Ma. Metaluminous magmas were emplaced during both pulses and form most of the batholith, whereas, peralkaline magmas comprise the Amissville granite and the Battle Mountain volcanic center that erupted unknown quantities of rhyolite during the final phase of activity. The plutons and volcanics exhibit composition variations that include a striking enrichment in HFSE such as Zr, Nb, Y, and REE, high Ga/Al and FeOT/MgO, and low Ba and Sr. Both metaluminous and peralkaline rocks host a plethora of accessory minerals such as chevkinite mineral group, allanite, fluorcarbonates, gadolinite, zircon, pyrophanite, monazite, xenotime, apatite, calcite, siderite, cassiterite, and sphalerite; fluorite is ubiquitous. The major REE-bearing phases are fluorocarbonate, monazite, chevkinite group mineral, allanite, gadolinite, and a variety of Nb phases. The Nb-Ta oxides occur in a wide variety of chemistry, habits, and typically show multiple generations, and alteration and deposition by late-stage fluids. To date, five major groups have been identified by microprobe analysis; columbite group, fergusonite group, euxenite group, aeschynite group, and the pyrochlore super group. Alteration in all the Nb-Ta groups except columbite is pervasive. The fergusonite, euxenite, and aeschynite groups appear to alter mainly by hydration with addition and loss of major constituents. Si, Al, and Fe typically increase with Ti, Th, U and Pb increasing in some grains. Losses in Y and REE are major, although Nb tends to be less affected in some grains. Pyrochlore group alteration is by hydration and A-site cation loss and addition, especially Ca and Pb. The alteration systematics of the Nb-Ta minerals suggests that a late-stage, highly reactive aqueous fluid moved though the rock during the early cooling process. There are distinct differences in Nb/Ta w/w values between many of the Nb-Ta phases and their host rocks. Ta appears to have been relatively mobile, although the details of its movement and precipitation are yet unknown.