Melting relations in hydrous systems revisited: application to metapelites, metagreywackes and metabasalts

A petrogenetic model involving two hydrous minerals is developed in a simpl e binary system to clarify the nature of melting reactions in presence or a bsence of a fluid phase. This model shows the changes in shape and location of the solidus in P-T diagrams as a function of the bulk H2O content. The role of divariant equilibria on the beginning and progression of melting is emphasized. In agreement with this model, a generalized scheme for partial melting of metasediments is proposed involving muscovite and biotite as hy drous phases. A P-T projection; a liquidus diagram and its isobaric-isother mal sections are combined in a KN(FM)ASH system. Then, CaO is added as a co mponent and it is demonstrated that the anorthite and grossular components in plagioclase and garnet, respectively, play an important role in the melt ing of biotite + plagioclase with increasing pressure. The reactions involv ing these components have large DeltaV, low dP/dT slopes and generate S-sha ped solidi. This study shows remarkable analogies in melting behaviour of metapelites, metagreywackes and metabasalts, including the presence of different hydrous minerals at the wet solidus. At intermediate pressures, muscovite (phengit e) coexists with biotite in metapelites, and epidote (zoisite) coexists wit h amphibole in metabasalts. Both pairs of hydrous phases display analogous features: a single volumetrically important hydrous phase (amphibole or bio tite) is stable at low pressures, two main hydrous phases are present at in termediate pressures, whereas a single hydrous phase occurs again at higher pressures (zoisite or phengite). Intermediate compositions such as greywac kes or andesites may have both pairs of hydrous phases. In most cases, the beginning of fluid-absent melting at pressures above approximately 10 kbar can be attributed to re-actions involving muscovite/phengite or epidote:zoi site, two minerals that melt and disappear 150 to 200 degreesC lower than b iotite or amphibole alone, respectively. Rocks that underwent high-pressure metamorphism at moderate temperatures are excellent illustrations of the p rogressive replacement of a hydrous phase (e.g., biotite) by others (phengi te + zoisite), and of the major role of anorthite and grossular components in plagioclase and garnet for phase changes and location of the solidus.