KINEMATIC AND GEOCHEMICAL ANALYSIS OF THE LAWHORNE MILL HIGH-STRAIN ZONE, BLUE RIDGE PROVINCE, NELSON COUNTY, VIRGINIA

The Lawhorne Mill high-strain zone is a belt of heterogeneous mylonitic rock ~500 m wide that extends 15 km in the Blue Ridge province, Nelson County, Virginia. Mesoproterozoic alkali-feldspar granites and biotite granitoids are mylonitized in the high-strain zone. Tabular bodies (0.1-3 m thick) of schistose biotite-rich mylonites are common and interpreted to be derived from mafic igneous rocks of Neoproterozoic (?) age. The northeast-southwest striking high-strain zone dips steeply to the southeast and a down-dip mineral elongation lineation is common. Kinematic indicators generally record a top-to-the northwest (reverse) sense of shear. Fabric asymmetries on planes normal to foliation and lineation are consistent with a weak triclinic deformation symmetry. Quartz grain shapes and boudinaged pegmatite dikes record sectional XZ strain ratios of 4 to 6, and three-dimensional fabrics are oblate. Aspect ratios and orientations of rigidly rotated apatite grains were used as a vorticity gauge and yield Wm-values of ~0.8, consistent with general shear. However, at the relatively ‘low strains’ experienced by Lawhorne Mill mylonites, the rigid rotation vorticity gauge overestimates the simple shear component of deformation. Mylonitic rocks show a two- to three-fold increase in CaO, Fe₂O₃, P₂O₅, TiO₂, Zr, and Y, and a loss of SiO₂ and K₂O, compared to their granitic protolith. The modal abundance of biotite, muscovite, epidote, apatite, and titanite increases, while quartz and feldspars decrease in the mylonites. Chemical and mineralogical changes indicate ~50% volume loss during deformation. Even with volume loss, mylonitic rocks record true flattening strains. Quartz and feldspar microstructures are consistent with mid- to upper-greenschist facies deformation. Although the Lawhorne Mill high-strain zone may have experienced pre-Paleozoic deformation, we interpret it to be a Paleozoic contractional structure that experienced bulk general shear.