METAMORPHISM OF THE MITCHELL DAM AMPHIBOLITE, CHILTON AND COOSA COUNTIES, ALABAMA

Two previously studied sections of core from the Mitchell Dam amphibolite were re-examined and comprehensive geochemical and mineral composition data were collected. Older thin sections were supplemented with new polished sections and whole-rock chemistry was determined to develop a chemical and mineralogical profile of the core. Five distinct lithologies were recognized petrographically The upper part of the care (0 to 32 feet) is leucocratic, consisting of quartz-plagioclase-biotite-garnet gneiss with or without hornblende. Intervals of completely annealed mylonite occur in the upper-most few feet. The dominant lithology in the rest of the core is massive to weakly foliated amphibole –plagioclase rock with generally MORB-like major element chemistry. The amphibolite is interrupted by some calcsilicate layers with relatively high grade metamorphic assemblages. The lowest section of core consists of amphibolite with a retrograde hydrothermal overprint.

Amphiboles in the main amphibolite unit are magnesiohornblende and the anorthite contents in plagioclase are generally An₃₀ to An₄₀. Neither shows any systematic zoning. Al-Ti thermometry and barometry and hornblende-plagioclase net-transfer and exchange reactions yield temperatures between 660 and 690 degrees Celsius and pressures between 8 and 11 kilobars for the metamorphic conditions during amphibolite development. Amphibolite in the lower portion of the core has a retrograde overprint. Actinolite has replaced hornblende and biotite is interleaved with chlorite or prehnite. Margins of altered regions have arborescent growths of fibrous zoisite with calcite, an assemblage also observed in some fracture filing veins. Some amphiboles have small calcite filled fractures and pyrrhotite is also seen to fill larger fractures in the amphiboles. The prehnite-chlorite-actinolite-zoisite assemblage most likely formed at a temperature of 300 to 350 degrees Celsius and a pressure of 1.5 to 2 kilobars. It is not clear whether sulfide mineralization is related to this retrograde process or some other event, but the lack of other alteration suggests the two are linked.