PHASE-RELATIONS IN THE LIQUIDUS PART OF T HE FLUORINE-BEARING GRANITESYSTEM

Phase relations have been studied in the part of the system SiO2-Al2O3 -Na2O-K2O-H2O-F exceeding ratios of these components in any granite ro cks. A wide area of coexistence of two liquating melts has been found: aluminosilicate and alkaline-aluminofluorine ones. Aluminofluorines o f alkalis are the independent components of the system and structure u nits of the melts. Its coexistence gives a new explanation to the pref erred quartz crystallization (as compared to feldspar) from fluorine-b earing compositions due to decreasing of the content of [AlO4] tetrahe dra in the melt. The liquation of the aluminofluorine melt leads to in creasing of alumina to total alkali ratio in the coexisting aluminosil icate melt and to increasing of silica content, with these tendencies being independent on the temperature minima location in the system. An abrupt increasing of potassium to sodium ratio in silicate magma caus ed by nonuniform fractionation of alkali metals between coexisting mel ts. The petrochemical tendencies discussed here are found in the natur al granitic series and sometimes they can not be explained in term of common mechanisms of differentiation.