关键词: 高镁安山岩, 板内岩浆作用, 岩浆演化

Abstract: Cenozoic high-Mg^# andesites (HMAs) are generally distributed in the active continental margin and island-arc setting, but they are rare in the intraplate setting. In recent years, intracontinental HMAs erupting at 20.6-0.3 Ma (Neogene- Quaternary) have been found in Wuchagou of western Songliao Basin and Xunke, Menluhe and Russian Far East of northern Songliao Basin. The discovery of these intracontinental HMAs is important to understand the nature and fate of recycled crustal materials in the mantle. Here we, for the first time, report Paleogene HMAs erupting at 49.2-36.3 Ma in Mudanjiang area, eastern Songliao Basin. These volcanic rocks have SiO₂ contents of 54.75-58.95 wt% and Mg^# values of 45.18-51.72, falling into the scope of HMAs. They are mainly basaltic andesites and andesites belonging to subalkaline series with a small variation of the major element compositions. Based on petrographic observation and MELTS_Excel calculation, plagioclase and orthopyroxene phenocrysts were crystal phases at the earliest stage and mineral phases in the groundmass (such as orthopyroxene, clinopyroxene and titanomagnetite) appeared successively. The results of MELTS_Excel calculation indicate that the crystal phases have not been separated from the magma system and the composition of these rocks can represent the primary magma composition. The crystallizing pressures of the Mudanjiang HMAs are 169-570 MPa by using the orthopyroxene-melt barometer, suggesting a depth of 5.6-18.8 km. The crystallizing temperatures of these rocks are 1109-1181 ℃ by using the orthopyroxene-melt and melt thermometers, which are consistent with other Cenozoic HMAs (Xunke, Menluhe and Wuchagou) in northeast China (1132-1192 ℃) and are higher than island-arc andesites (950-1050 ℃). The pre-eruptive melt H₂O contents vary from 0.3 to 1.0 wt% calculated by the plagioclasemelt hygrometer, which are in the range of Cenozoic HMAs in other parts of northeast China (0.3-1.4 wt%), yet tend to lower H₂O than island-arc andesite (2-9 wt%). Therefore, the chemical composition, petrographic characteristics, magmatic physical properties (temperature and H₂O content) and evolutionary characteristics of the Mudanjiang HMAs are similar to Cenozoic HMAs found in other areas of Northeast China, which all belong to intracontinental HMAs. The discovery of Mudanjiang HMAs suggests that these intracontinental HMA-type magmatism continued from Eocene to Pleistocene and are distributed throughout northeast China.

Key words: high-magnesium andesites, intracontinental volcanism, magmatic evolution