Synthesis of staurolite in melting experiments of a natural metapelite: Consequences for the phase relations in low-temperature pelitic migmatites

Abstract

We document experiments on a natural metapelite in the range 650-775°C, 6-14 kbar, 10 wt % of added water, and 700-850°C, 4-10 kbar, no added water. Staurolite systematically formed in the fluid-present melting experiments above 675°C, but formed only sporadically in the fluid-absent melting experiments. The analysis of textures, phase assemblages, and variation of phase composition and Fe-Mg partitioning with P and T suggests that supersolidus staurolite formed at (near-) equilibrium during fluid-present melting reactions. The experimental results are used to work out the phase relations in the system predicts the existence of a stable P-T field for supersolidus staurolile that should be encountered by aluminous Fe-rich metapelites during fluid-present melting at relatively low temperature and intermediate pressures (675-700°C, 6-10 kbar for XH2O = 1, in the KNFMASH system), but not during fluid-absent melting. The implications of these findings for the scarciy of staurolite in migmatites are discussed.