Ultra High Temperature (UHT) Metamorphism of Mg-Al-Rich Pelitic Rocks: an Experimental Study of the System K2O-MgO-Al2O,-Si2O-H2O with the Bulk Composition 2Phl+6Sil+9Qtz

An experimental study in the Mg-Al rich portion of K₂O-MgO-Al₂O,-Si₂O-H₂O (KMASH) system was carried out using bulk composition 2Phl+6Sil+9Qtz. Experiments were done using Internally Heated Pressure Vessel (Gas Bomb) in a P-T range of 3-10 kbar and 900-1250°C with water undersaturated conditions. Three distinct fields were noticed in the given P-T plane: (a) Field-I: Crd+L (up to 1000°C), (b) Field-II: Ind+Sp+L (between 1000 and 1250°Cl up to 6 kbar) and (c) Field-III: Spr+Ind+AI.En±Os±Qtz±L (between 1000 and 1250°C above 6 kbar). The appearance of hexagonal indialite (Ind) instead of orthorhombic cordierite above 1000°C is significant. The analysis of the mineral phases shows that sapphirine is of 2:2:1 type and enstatite is highly aluminous (up to 14 wt% Al₂O₃). The anafyses of indialite lie between 2:2:1 sapphirine and ideal cordierite on the (Al₂O,+K₂O)-MgO-SiO₂, triangular diagram. The analysis of melt is found to depend on the associated restites and the overall composition corresponds to 'granitic'.