GENESIS OF ALKALINE AND PERALKALINE SYENITE-GRANITE SERIES - THE KHARITONOVO PLUTON (TRANSBAIKALIA, RUSSIA)

The Kharitonovo pluton is located in the central part of the large Mongolian-Transbaikalian province of Late Paleozoic alkaline syenites and granites. The province stretches for a distance of almost 2000 km and contains over 350 plutons. The Kharitonovo pluton occupies an area of 230 km.(2) It is made up of A-type granitoids forming two successive alkaline and peralkaline syenite-granite series; syenites largely predominate over granites. Rocks of both series are characterized by abundant mesoperthitic feldspar (about 90 vol % in syenites, more than 60% in granites) while plagioclase is almost absent. In the peralkaline series mafic minerals are riebeckite-arfvedsonite, kataphorite and aegirine with secondary annitic biotite. In the alkaline series edenite and iron-rich biotite are the major mafic minerals. In the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes; they indicate at least two stages of mafic magma injection into the silicic magma chamber. Major and trace element data are used to test various petrogenetic models for the origin of the syenites and granites. Of the alternatives considered, it is most likely that each series resulted from crystal fractionation of syenite magma. Mass balance calculations suggest that in the early alkaline series, crystal fractionation was probably combined with mixing of felsic and mafic magmas, a conclusion also supported by field evidence. The peralkaline series syenite parental magma could have been produced by partial melting (about 20%) of the earlier alkaline syenites. In this respect alkaline syenites can be regarded as parental rocks for the whole pluton. We suggest that the alkaline syenite magma originated via two possible petrogenetic schemes: (1) partial melting of deeply buried crustal rocks or (2) crystal fractionation of hybrid melt produced by mixing of subalkaline basaltic magma (80%) with about 20% of silicic lower crustal melt. In either case some additional input of potassium and possibly other incompatible elements is required in order to achieve the observed composition.