UNDERSTANDING MINERAL TEXTURES, LITHOLOGIC RELATIONS, AND PLUTON WALL-ROCK INTERACTIONS THROUGH BULK AND MINERAL COMPOSITIONS AT HOLCOMB ROCK DAM, VIRGINIA

The basement rocks that are exposed by the incision of the James River through mountains of the central Blue Ridge anticlinorium sprawl for a few hundred meters along the riverbank just downstream from the Holcomb Rock Dam in Virginia. The “Holcomb Rock” outcrop consists mostly of complex interrelations of megacrystic granitoids in knife-edge contact with phaneritic granitoid rocks. Despite this textural disparity, the mineral, modal, and bulk compositions of the phaneritic and megacrystic lithologies, are not notably distinct from one another. This project uses evidence from rock and mineral compositions and textures to gain insight into the relationships between these different lithologies with the goal of better understanding the history and dynamics of the magma chamber before, during, and after crystallization. Both rock textures are S-type, peraluminous, subsolvus granites, granodiorites, and quartz monzonites, that were derived by the melting and subsequent magmatic differentiation of the surrounding gneissic source rocks of the lower crust along a calc-alkaline trend. Mineralogically, the two rock textures are also very similar with perthitic microcline, variable amounts of Ti-rich quartz and biotite, small amounts of albitic plagioclase, and accessory minerals that include, ilmenite with reaction rims of titanite, tiny inclusions of epidote partially replacing crystals and exsolution lamellae of albite, cordierite in the more mica-rich samples, rare muscovite, and rare chlorite partially replacing biotite. Feldspar compositions from both textures are virtually identical and suggestive of equilibration at low temperature (<400°C). The large gap in mineral size between phaneritic and megacrystic lithologies that demonstrate ample outcrop-scale evidence for coexisting contemporaneously while behaving as liquids is most likely the result of an abrupt variation in water concentration during crystallization- a difference that should be recorded in the crystallizing minerals. Fourier transform infrared (FTIR) spectroscopy was used to quantify water in feldspars from several Holcomb Rock samples. Preliminary results indicate that cores of feldspars from megacrystic samples contain water at levels slightly higher than feldspars from phaneritic samples of 149 vs. 108 ppm with ±7% error.