MESOZOIC HYDROTHERMAL ALTERATION OF PALEOZOIC APPALACHIAN MYLONITE ZONES

One conclusion of a program of continuous basement core logging at the US Department of Energy Savannah River Site was an appreciation for the extent and intensity of metasomatic hydrothermal alteration of wall rocks along the Triassic Dunbarton Basin border fault. All lithologies were silicified and enriched in Group I cations. Alteration is pervasive adjacent to the wall rocks. Rocks at some distance from the border fault were heterogeneously altered. Altered rocks are recognizable by their pink color that crosscuts the metamorphic and mylonitic fabrics. Pinked rocks are dominated by microcline megacrysts (<40% modally); in thin section plagioclase feldspar is albitized and sericitized. The source for K⁺ and other Group I cations is interpreted to be biotite that is consistently retrogressed to chlorite. Comparable diagenetic effects within sedimentary rocks of the basin were not observed. Subsequent Cenozoic reactivation is evinced by offsets in the overlying Coastal Plain lithologies (e.g., Pen Branch fault). Several dozen examples of pseudotachylyte are found in rocks that have been pinked; in fact every pseudotachylyte example is from a rock that was pinked. Finally, retrogression of biotite schists to chlorite schists appears to localize subsequent brittle fault rocks (Pen Branch fault, soft plastic gouges recognized in core). The timing of this metasomatic event was interpreted by Dennis and others (2004) to be ca. 220 Ma (Carnian, Carnian-Norian boundary?) based on a two point isochron (whole-rock, K feldspar) from highly altered granite from core C-10 presented by Kish (1992). These observations in basement core at the Savannah River Site suggest a re-evaluation of the lithology of several sites including the quarry in the Augusta (GA) fault mylonite as well as Coles Hill (VA). The Augusta fault quarry shows extensive silicification, and pinking and chloritization are pervasive there; many of the features of the pinked Dunbarton basin border fault wall rock are replicated at or near the quarry. It has been difficult to map the highly silicified mylonite away from the quarry. Perhaps the lithology of the Augusta mylonite at its type locality is a Mesozoic hydrothermal alteration feature overprinting the Paleozoic fault rock product.