TWO TYPES OF LEUCOGRANITE, TWO TECTONIC ROLES, GOOCHLAND TERRANE, VA-NC

At least four granitoid rock suites play important tectonic roles in the Goochland terrane. (1) The Grenvillian bi-hbd granitic State Farm gneiss may be equivalent to but younger than the charnockite-anorthosite (AMCG) suite of the Adirondacks. (2) A second, much younger, Alleghanian, bi-hbd granite suite represented by the Petersburg in VA and the Rolesville in NC, is part of a much larger group of granites associated with compressional tectonics of the southern Appalachians. The third and fourth will be discussed here. (3) Late Proterozoic A-type to peralkaline leucogranites, represented by the Flat Rock and others close to the eastern edge of the Goochland terrane in the north, and the North View, Green Hill and Lake Raleigh peralkaline bodies within the Falls leucogneiss along the western edge of the terrane in southern VA and NC. Right lateral, late Paleozoic, translation along the Hylas-Lake Gordon-Nutbush Creek shear zone may have redistributed these bodies from a more compact grouping at the time of intrusion. Their intrusion is interpreted to document a late Proterozoic rifting event in the Goochland terrane. Textures and mineralogy indicate a series of shallow intrusions, modified by strong Alleghanian (?) metamorphism and deformation. (4) Grenvillian (?) peraluminous silli-gar or ky-gar leucogranite. Occurring as lenses, from less than one to several m thick, generally contained within pelitic gneiss of the Maidens formation. These sills and dikes are interpreted to have been produced by anatexis during the silli-Kspar granulite event that affected the entire Goochland terrane. Late Paleozoic ky-Kspar granulite metamorphism may have produced additional anatexis of pelitic gneiss in the northeastern corner of the Goochland terrane. Additional geochronology on these leucogranites will certainly help to define both compressional and extensional tectonics of the Goochland terrane.