Partial melting and migmatization of a metamorphic terrane in northeast Georgia
Mirante, Drew Campbell
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The Athens Gneiss is a suite of upper amphibolite grade rocks within the Inner Piedmont of NE Georgia that consists of Paleozoic and older rocks; primarily gneisses and migmatites along with lesser occurrences of pelitic schists, garnet amphibolites, calc-silicate xenoliths and quartzites. Gneisses have the composition qtz + plg + kfs + bio + ms. On the outcrop scale, migmatites often show a transition from more homogeneous diatexite to strongly foliated metatexite. This transition can be sharp or gradational. Metatexites have the mineral assemblage bio + plg + kfs + qtz +ms+ grt . Diatexites are characterized by the assemblage pl + kfs + bio+ qtz + ms. Diatexites also contain numerous decimeter-size foliated xenoliths with the assemblage bio + pl + qtz + kfs + grt+ ms. Garnet cores with included biotite from metatexites yield peak metamorphic conditions of 820 ±50 °C and 8.1±0.5 kbar. Garnet rims and matrix biotite in the same metatexites also record retrograde conditions of 720 ±50 ° C and 5.2±0.5 kbar. Amphibolites yield pressures of 9.5±1 kbar at 820°C, in good agreement with metatexites. Water activities range from ~1.0 at peak conditions to ~0.5 at retrograde conditions. Based on the calculated intensive variables, piston-cylinder melting experiments were performed at varying temperatures and added H2O contents to ascertain the melting processes that led to migmatization of AG gneisses. The results of the experiments suggest that there must have been an external water-rich fluid that enabled the widespread migmatization in the AG.