Chemographic relationships in sapphirine-bearing rocks from Sonapahar, Assam, India

Abstract

Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 7:9;3 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel. The consistent topology of the observed mineral assemblages with reference to the system MgO-FeO-Al₂O₃-SiO₂-H₂O, systematic partitioning of Mg and Fe in the coexisting phases, and textural relations are in accord with attainment of chemical equilibrium during the regional metamorphism. The sequence of metamorphic reactions has been deduced as a function of changing MgO/(MgO+FeO) ratio in the bulk composition and it has been shown that the topology in the SiO₂-(FeO+MgO)-Al₂O₃ diagram for more magnesian compositions is consistent with experimental data in the system MgO-SiO₂-Al₂O₃-H₂O. The stable occurrence of the spinel-cordierite and spinel-sillimanite joins in less magnesian bulk compositions suggests that these tie lines are stable to higher pressures and lower temperatures respectively as compared to the Fe-free synthetic system. Using experimental and theoretical data on garnet-cordierite, garnet-biotite, garnet-plagioclase and orthopyroxene-clinopyroxene equilibria, PT conditions have been estimated to be 750 °±50 °C/ 5±0.5 Kb for the thermal peak of metamorphism. It is suggested that the unusual bulk composition of the sapphirine-bearing rocks viz. higher Al₂O₃, MgO and MgO/(MgO+FeO) and lower SiO₂, Na₂O and CaO compared to the parent cordierite-bearing granulitic gneisses may be attributed to partial melting and removal of granitic melt. This is in agreement with the variation of oxides in the restites and parent rock. The restites consisted of the assemblage cordierite-spinel-corundum which then reacted to form sapphirine. This reaction, which in the synthetic Fe-free system is confined to extremely low-pressures (~300 bars), is likely to occur at higher pressures with MgFe substitution.