ИСТИНА

We have studied reactions of orthopyroxene (Opx) – cordierite (Crd) – biotite (Bt) – plagioclase (Pl) – quartz (Qtz) metapelite (Banderlierkop Formation, Limpopo, South Africa) with H2O, H2O-CO2, H2O-CO2-NaCl and H2O-CO2-KCl fluids at 5.7 – 6.0 kbar and 850OC. Interaction of the metapelite with aqueous and equimolar H2O-CO2 fluid results in partial melting with formation of Al2O3 and CaO-rich (16-18 and 2.1-2.7 wt. %, respectively) granitic melts and peritectic spinel and Opx with Al2O3 up to 9 wt. %. In contrast, interaction of the metapelite with H2O-CO2-NaCl and H2O-CO2-KCl fluids is expressed in partial melting reactions Bt + Pl + Qtz = Opx + (amphibole) + ilmenite + melt without participation of Crd. Despite the low estimated water activity (0.52-0.60), cordierite gives place to the assemblages of various hydrous phases. Reactions with H2O-CO2-NaCl fluids result in formation of aspidolite-bearing biotite, spinel, albite after Crd or Na-bearing (up to 3.5 wt. % Na2O) gedrite after Crd+Opx. The Na2O content in biotites and gedrites is a function of the NaCl concentration in the starting fluid. The H2O-CO2-КCl fluids produce biotite+sillimanite+corundum+spinel after Crd. K-feldspar appears in this assemblage at XKCl ~ 0.02 in the fluid. In contrast to Opx equilibrated with H2O и H2O-CO2 fluids, Opx forming in presence of NaCl or KCl-bearing fluids actively loses Al2O3. Partial melting in presence of H2O-CO2-NaCl and H2O-CO2-KCl fluids produces calcic or calc-alkalic granite melts with FeO/(MgO+FeO) < 0.8 at MgO+ΣFeO = 1.5 – 2.5 wt. %. Increase of the salt concentration results in a decrease of the Al2O3, CaO content in the melts and an increase of their alkalinity driving their characteristics toward the A-type granitoids.