THERMOBAROMETRY OF GARNET- AND BIOTITE-BEARING PYROXENE GRANULITES OF THE WESTERN BLUE RIDGE TERRANE, CENTRAL VIRGINIA

The Blue Ridge terrane of central Virginia is a complex igneous-metamorphic terrane of Mesoproterozoic age. The rocks of the eastern Blue Ridge include high-grade gneisses and granitoids that contain mineral assemblages that are indicative of extensive retrogression to amphibolite-facies. Although retrogression to amphibolite-facies occurs locally in the western Blue Ridge it is dominated by relatively pristine granulite-facies orthopyroxene-bearing quartzofeldspathic gneisses and charnockitic granitoids. Minor amounts of garnet and biotite are present in some of these high-grade rocks. Garnets typically display complex symplectic intergrowths with quartz that commonly partially rim orthopyroxene and, in some cases, biotite. Garnet compositions are Alm_65-80 Pyr_9-19 Gro_8-20 and are similar to those reported for garnets found in other high-grade granulite terranes. Biotite occurs as flakes and in symplectic forms associated with the garnet symplectites. Both types of biotite do coexist in some samples. The two biotite varieties generally have distinct compositions. Flake biotite is commonly Fe-rich (15-19 wt% FeO, 12-15 wt% MgO) while the symplectic varieties are more magnesian (12-14 wt% FeO, 16-18 wt% MgO). TiO₂ content ranges from 3-5.5 wt% and F is in excess of 1 wt% for both types of biotite. Compositions of symplectic garnets and biotites coexisting with orthopyroxene were used to estimate temperatures and pressures by the Al-in-orthopyroxene method of Pattison et al. (2003). Preliminary results yield temperatures that range between 700° C and 850° C and pressures between 6 and 9 kbars suggesting that orthopyroxene breakdown and the formation of symplectites occurred at granulite-facies conditions during the Grenville orogeny.