Geology of the Bald Rock Granitic Batholith, South Carolina

The Bald Rock Batholith is a postmetamorphic granitic pluton located in the southern Appalachian Piedmont. Granites and quartz monzonites of the Bald Rock consist mainly of microcline, plagioclase, quartz and biotite, with accessory amounts of allanite, epidote, titanite, muscovite, apatite, zircon, magnetite, ilmenite, pyrite and, locally, hornblende and garnet. i Two major textural phases of the Bald Rock granite are distinguished: a medium- to coarse- grained porphyritic phase containing tabular microcline megacrysts up to 7 cm in length; and a fine-grained, equigranular phase that intrudes the former. The fine-grained granite is a more highly differentiated derivative of the porphyritic phase. Five variants of the porphyritic phase are recognized on the basis of modal and textural criteria. Most petrographic variation is concentrated near the margins of the batholith, suggesting that contamination occurred. Movement of magma is indicated by foliations of megacrysts and mafic xenoliths that are concentric about an area in the east-central portion of the batholith and are generally parallel to the margins. This alignment pattern may be attributed to resurgent deformation, whereby successive pulses of magma forced previously-intruded, plastic material radially outward from the center of upwelling. Deformed dikes and annealed protoclastic textures corroborate this model of synplutonic deformation. Swarms of elongated xenoliths are tapered in the same direction in any given pavement exposure near the margin of the batholith. On a batholithic scale the sense of tapering is in a counterclockwise manner. Convection early in the emplacement history of the batholith is a likely cause of this inferred component of magma motion, but convective flow was not vigorous enough to homogenize the contaminated marginal facies.