FORMATION OF TRACHYTES FROM WANGTIAN’E VOLCANIC NECK OF THE CHANGBAISHAN AREA (NORTHEAST CHINA) AS A RESULT OF THOLEIITIC MAGMA CRYSTAL FRACTIONATION

Abstract

Tholeiitic basalts and trachytes of  Wangtian’e bimodal association are studied. It is shown that trachytes were formed under the leading role of crystal fractionation of the initial tholeiitic magma. Based on the method of melt and fluid inclusion studies, it was determined that hedenbergite phenocrysts from Wangtian’e volcanic neck crystallized in the temperature range of 1080–1100°С, and plagioclase phenocrysts crystallized at temperatures of 1050–1060°С. After thermometric experiments with melt inclusions in plagioclase from trachytes, in some cases relics of carbonates and carbon dioxide were observed in the inclusions. Trachyte phenocrysts contain hydrous Fe-rich silicate globules and CO2 inclusions containing carbonate phases. Fe-rich silicate globules are often intergrown with titanomagnetite and covered with films of amorphous carbon. A model the appearance of hydrous Fe-rich globules and CO2 inclusions with carbonate relics is proposed. It results due to the silicate-silicate and silicate-carbonate liquid immiscibility processes, that are caused by the evolution of the initial basaltic melt. As the trachyte melt has been risen to the surface, the ferrocarbonate liquid decomposed into magnetite, carbon, and carbon dioxide.