An experimental determination of the effect of bulk composition on phase relationships in metasediments at near-solidus conditions

Phase relationships in metapelites were experimentally investigated in the model system CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O on four synthetic compositions differing in their K2O content and K2O/Al2O3 ratio. Experiments were carried out at pressures ranging from 0·8 to 1·4 GPa and temperatures from 620 to 740°C under different fluid-buffered conditions. The stability of the assemblage garnet–biotite–staurolite/Al2SiO5 at the investigated conditions largely controls the absence of muscovite in K-poor compositions. At 1·2 GPa and 700°C staurolite is present with garnet + biotite + muscovite, replaced by Al2SiO5 at lower pressures through the reaction staurolite + muscovite + quartz = garnet + biotite + Al2SiO5 + H2O. In K-poor bulk compositions, garnet + biotite + staurolite + gedrite coexist up to 730°C. A compositional reversal of the Mg–Fe partitioning between garnet and staurolite is observed with decreasing pressure and is responsible for the singular equilibrium staurolite + quartz = Al2SiO5 + garnet + H2O that governs the high-temperature breakdown of staurolite. Melting is found to depend both on bulk composition and on fluid speciation. The wet solidus is represented by the reaction muscovite + anorthite + garnet + quartz + H2O = melt + biotite + Al2SiO5. Even though staurolite is never recovered at supersolidus conditions, the large pressure–temperature stability field of staurolite + muscovite and of staurolite + gedrite suggests that staurolite can be directly involved in melt production through fluid-present or fluid-absent reactions.