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 7408C 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 7008C staurolite is present with garnetþ biotiteþmuscovite, replaced by Al2SiO5 at lower pressures through the reaction stauroliteþmuscoviteþ quartz1⁄4garnetþ biotiteþAl2SiO5þH2O. In K-poor bulk compositions, garnetþ biotiteþ stauroliteþgedrite coexist up to 7308C. A compositional reversal of theMg^Fe partitioning between garnet and staurolite is observed with decreasing pressure and is responsible for the singular equilibrium stauroliteþ quartz 1⁄4Al2SiO5þ garnetþH2O 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 muscoviteþanorthiteþgarnetþ quartzþH2O1⁄4meltþ biotiteþ Al2SiO5. Even though staurolite is never recovered at supersolidus conditions, the large pressure^temperature stability field of stauroliteþmuscovite and of stauroliteþgedrite suggests that staurolite can be directly involved in melt production through fluid-present or fluid-absent reactions.