Fluid flow and alkali transport within a metamorphic terrane, western North Carolina

Abstract This paper presents preliminary results from study of a metasedimentary terrane near the Great Smoky Mountains of North Carolina. Grade of metamorphism ranges from garnet to staurolite-kyanite zone. The stratigraphic section is dominated by metasandstone but contains thick pelitic horizons over the central portion (Anakeesta Formation) of the studied interval. Metasandstone beds contain numerous isolated calcite concretions; these react to form leucocratic calc-silicate rock at variable grade of metamorphism. Metasandstones themselves are unreactive. Study of calc-silicate reaction permits the following statements regarding fluid flow and alkali transport within the studied terrane. Within unreacted concretions, the typical mineral assemblage is quartz-oligoclase-muscovite-biotite-calcite. Reaction between micas and calcite proceeded through several stages, forming as prograde minerals andesine/bytownite, clinozoisite, zoisite, garnet, hornblende, and chlorite. Reaction occurred in the presence of a fluid phase containing a H2O/CO2 ratio of 9 and resulted in massive expulsion of Na2O, K2O and CO2. Since postulated reactions would have produced, on average, a fluid phase whose H2O/CO2 ratio is only one, flow of water-rich fluid through the concretions accompanied reaction. Rough estimate suggests a volumetric ratio of external fluid to rock of at least 1.1. The metamorphic grade at which concretions undergo reaction is controlled by stratigraphic position of the host metasandstone relative to metashale. Within the Anakeesta Formation and overlying strata, extensive reaction occurs near the pelitic staurolite and kyanite isograds. Within massive metasandstones of the underlying strata, reaction is delayed to the middle staurolite-kyanite zone, 5 km upgrade. Some reaction occurs within the upper garnet zone, but this is restricted to certain thin metasandstone beds interlayered with metashale. Study of dehydration reactions within metashale demonstrates that calc-silicate reaction was coupled to production of H2O-rich fluids within pelitic strata. Transport of H2O-rich fluids beyond outcrop scale was by advection. Diffusion was limited to outcrop distance. On most outcrops, all isolated concretions show the same degree of reaction. But at larger scale separate metasandstone packets bounded by metashale show differing degrees of reaction. Also, all rocks underwent some degree of penetrative deformation during time of reaction. The suggestion is made that advective flow was in fact channelized into fractures within metasandstone, and that diffusion between fractures promoted reaction within outcrop-size volumes of rock. Overall direction of fluid transport was in part upsection and in part channelized within metasandstone strata. Downsection flow was limited. For the most part alkalis liberated by calc-silicate reaction appear to have left the terrane. Metasandstones do not appear to constitute a sink for alkalis, and no sink of any sort was found for sodium. Potassium metasomatism of pelitic rock did occur and is marked by conversion of chlorite to biotite. But such K deposition is limited to sections within which metashale and calc-silicate rock are within outcrop distance. No sink is evident for most liberated potassium. It is within conjecture that the liberated elements caused alkali metasomatism elsewhere within the metamorphic belt, but such an event has yet to be demonstrated.