MAGMA DIFFERENTIATION PROCESSES IN THE PR ODUCTION OF ALKALINE AND SUBALKALINE SYENITE-GRANITIC SERIES (KHARITONOVO MASSIF, TRANSBAIKALYE)

Kharitonovo massif is located in a central part of the largest Late Pa leozoic Mongolia-Transbaikalian province that extends for about 2000 k m and is comprised of over than 350 massives. It occupies 230 km2 and is composed of magmatic rocks of two successive alkaline and subalkali ne syenite-granitic series. In the Kharitonovo massif the syenites of both series largely predominate over the granites. The rocks of the Kh aritonovo massif are completely corresponding to granitoids of the A-t ype. The analysis of chemical data and mass-balance calculations sugge st that each evolutionary series was caused by processes of syenite ma gma crystal fractionation. In the early alkaline series magmatic diffe rentiation was complicated by the admixture of basaltic and syenitic m elt provided by the geological data. The alkaline syenitic magma could be formed by the partial (25%) melting of preceding subalkaline syeni tes. In this respect the latter can be regarded as a parental rock for a massif as a whole. Discussion of subalkaline syenitic magma generat ion models lead to a conclusion that the following two models are like ly: (i) partial melting of deeply subducted crustal blocks and (ii) cr ystal fractionation of hybrid melt produced by mixing of subalkaline b asaltic magma with small amount (about 20%) of silicic melt from the l ower crust. In both cases the additional excess of potassium and possi bly other incompatible elements is necessary.