AUGITE-INVERTED FERROPIGEONITE THERMOMETRY IN "CHARNOCKITIC" ROCKS OF THE GRENVILLIAN BLUE RIDGE, CENTRAL VIRGINIA

The Grenvillian-age rocks of the Virginia Blue Ridge consist predominantly of pyroxene granulite gneisses and metamorphosed two-pyroxene granitoids. Individual rock types range from relatively unaltered low-Ca pyroxene quartzofeldspathic gneisses and farsundites/quartz mangerites to altered and deformed biotite, ±garnet quartzofeldspathic gneisses. This terrane is similar to other Grenvillian terranes found along the length of the Appalachian Mountains. However, despite geochemical and geochronological characterization of these rocks, the nature of their metamorphism and relationship between the granulites and “charnockites” is relatively poorly understood. In this study we report the presence of inverted ferropigeonite (now orthopyroxene hosts with augite lamellae) in the Grenvillian basement rocks of the Blue Ridge and present results of two-pyroxene thermometry using reintegrated compositions of exsolved low-Ca and high-Ca pyroxenes. Inverted pigeonite is the more abundant of the two pyroxenes, making up about 10% of the mode, while augite is generally less than 5%. Exsolution lamellae in both pyroxenes (especially in the ferropigeonite) are evident in both false-color electron microprobe compositional images and in optical observations of these pyroxenes in thin section. Compositions of the original high-temperature pyroxenes were reintegrated using estimations of the relative amounts of lamellae and host from electron microprobe images of Ca concentration. Reintegrated compositions of ferropigeonite range from Wo_13.6 En_37.0 Fs_49.4 to Wo_20.5 En_25.0 Fs_54.5 and those of reintegrated augite range from Wo_37.6 En_32.7 Fs_29.7 to Wo_36.6 En_26.6 Fs_36.9. Temperatures were determined by applying the reintegrated compositions of coexisting pyroxenes to the QUILF thermometer (Anderson et al., 1993, Comp. and Geosci.). Most temperatures ranged between 920°C and 980°C. These temperatures are quite a bit higher than typical temperatures reported for granulite facies metamorphism, and along with relict igneous textures preserved in these rocks, suggest that they represent magmatic intrusion temperatures rather than temperatures reached during Grenvillian granulite-facies metamorphic event.