PETROLOGIC AND TECTONIC SIGNIFICANCE OF IGNEOUS CHARNOCKITES, BLUE RIDGE PROVINCE, VIRGINIA

Charnockites characterized by Opx-bearing granitic mineral assemblages are a common constituent of cratonic basement terranes worldwide; however, the relative importance of igneous versus metamorphic processes involved in their origin is locally debated. Charnockites and related granitoids constitute most of the Mesoproterozoic Blue Ridge basement in northern Virginia and preserve evidence of an extended period of Grenville-age orogenesis that occurred during amalgamation of Rodinia. Charnockites in the area define high-silica (69-74 wt. % SiO2) and low-silica (55-65 wt. % SiO2) plutons containing the assemblage Qz + K-Fsp + Plag + Opx ± Cpx ± Bt with accessory ilmenite and apatite. Primary edenitic amphibole formed after pyroxene occurs locally in the charnockitic rocks and is abundant in associated silica-rich (± magnetite) differentiates. The plutons exhibit variable, but recognizable, effects of Paleozoic recrystallization at greenschist-facies conditions. Field evidence for igneous origin includes: (1) granitic textures (2) observed order of crystallization of primary minerals, (3) cross-cutting charnockite dikes, (4) presence of enclaves and xenoliths, (5) in-situ development of pegmatoids, and (6) inclusion of map-scale inliers of older rocks. Compositional characteristics including decreasing FeO, CaO, TiO2, P2O5, and Sr with increasing SiO2 are likewise consistent with fractionation of observed primary minerals. These field characteristics, compositional trends, and chemical similarities to igneous charnockites recognized in other cratons, strongly suggest that Blue Ridge charnockites formed by crystallization of relatvely anhydrous magams emplaced during local Grenville orogenesis. The charnockite plutons contrast both physically and compositionally with associated older, deformed orthogneisses and younger, largely post-tectonic, relatively undeformed leucocratic granitoids. The charnockites indicate that peak-Grenville conditions promoted development of high-temperature, relatively anhydrous magmas that were subsequently emplaced within previously deformed crust.