ИСТИНА

The Changbaishan volcanic area between China and North Korea is the only product of the bimodal magmatism of the Far Eastern part of the Late Cenozoic East Asian Intraplate Volcanic Province. The largest volcanoes of this area in NE China are Changbaishan Tianchi and Wang-Tian’e. In contrast to the Changbaishan Tianchi association of differentiated alkaline rocks lavas of WangTian’e volcano vary from basalt to basaltic andesite and belong mainly to the tholeiitic series and, less frequently, to the alkaline series. Silicic rocks are rare in the area. The goal of this study is to estimate the crystallisation conditions, composition and evolution path of Wang-Tian’e basaltic melts by studying melt inclusions in plagioclase from tholeiitic basalts. Plagioclase (An75-80Ab19-24Or0.8-1.3) was the first mineral which crystallised in this system. Melt inclusions in plagioclase contain a plagioclase rim and Fe-rich glass, or fine-grained aggregate of clinopyroxene, Ti-magnetite, ilmenite, apatite and sulphides. The composition of the wide rims incrusting the inclusions changes from the outer to inner part of the inclusion from plagioclase (An46-56Ab39-47Or4-7) to anorthoclase (An13-13.5Ab43-50Or36-43). The Fe-rich residual glass and the fine-grained aggregate were analysed by electron microprobe using a 10-µm defocused beam. The compositions of Fe-rich glass and the aggregate were identical and show high FeO content – up to 24 wt%, up to 10 wt% CaO, up to 12 wt% MgO, up to 6 wt% TiO2, up to 2 wt% Al2O3, 1 wt% (Na2O+K2O), 1 wt% P2O5, 0.3-0.4 wt% SO3 98% total at 40-43 wt% SiO2. Two types of silicate globules were identified in these inclusions: Fe-rich and Si-rich. Fe-rich globules contain 21-29 wt% FeO, up to 6 wt% MgO, 2-7 wt% Al2O3, 2 wt% K2O, 1 wt% CaO and more than 5 wt% H2O at 45-51 wt% SiO2. Si-rich globules contain up to 13.5 wt% Al2O3, 6 wt% K2O, 2.2 wt% Na2O at 71.5-72 wt% SiO2. The inclusions were homogenised at temperatures of 1180-1190 °C. During thermometric experiments the silicate globules and all the silicate phases dissolved in the melt. Sulphide phases occurring occasionally in the finegrained aggregate produced sulphide globules with a pure FeS composition. After the heating experiments the inclusions were either completely homogenised or contained glass+gas bubbles±sulphide globules. Their composition in general is identical to the rock composition: 10-12 wt% FeO, 15-16 wt% Al2O3, 3-3.5 wt% TiO2, 4-4.5 wt% MgO, 3-3.5 wt% Na2O, 1.3-1.5 wt% K2O, 8-9 wt% CaO, 0.6-0.8 wt% P2O5 and 0.3-0.6 wt% SO3 at 49-51 wt% SiO2. Two possible paths can be suggested for the evolution of the Wang-Tian’e tholeiitic melts. On one hand, in contrast to Bowen’s reaction series, the evolution might have followed a Fenner-type fractionation, which proposed that the Ab-An-Di system followed a crystal fractionation path controlled by clinopyroxene fractionation and accumulation of FeO in the melt. As plagioclase was the first mineral to crystallise from the tholeiitic melt of Wang-Tian'e volcano it could form large crystals, especially at relatively high water content, as can be seen in the tholeiitic basalts. Plagioclase crystallisation resulted in melt enrichment in FeO and depletion in SiO2, Al2O3 and alkalis, which eventually has led to clinopyroxene crystallisation and Fenner-type fractionation. On the other hand, the presence of Si-rich globules in Fe-rich glasses in the inclusions indicates that low temperature silicate liquid immiscibility could have played a role during tholeiitic melt evolution. This was shown by Roedder (1951) for the quartz-fayalite-leucite system. The formation of hydrous Fe-rich globules in Fe-rich glasses could be attributed to kinetic effects during melt ascent to the surface. Acknowledgement This study was supported by the Grant of the President of Russian Federation, proj. MK-2419.2019.5, and the Russian Foundation for Basic Research, proj. 17-05-00767. References Roedder E. (1951) Am. Mineral. 36:282-286.