Tholeiitic and calc-alkaline magma series at Adatara volcano, Northeast Japan: 2. Mineralogy and phase relations

Abstract

Low alkali [low-K by Gill (1978) or island arc by Jakes and Gill (1970)] tholeiitic magma suites are closely associated with [medium-K by Gill (1978)] calc-alkaline suites in the outer volcanic zone (called Nasu volcanic zone; NVZ) of Northeast Japan. Chemistry and phase relations of phenocrysts are studied for representative 5 low-alkali tholeiitic and 8 calc-alkaline rock samples of Adatara volcano in NVZ, in order to examine the mineralogical evolutionary mechanisms of the two series of magmas. Also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series. The correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well-documented, supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases (Part 1; Fujinawa, 1988). Mineralogical observations in the calc-alkaline suite are compatible with the views based on their whole rock chemical compositions; although fractional crystallization process is dominant in the evolution of the calc-alkaline magma, intermittent mixing of (small amounts of) magnesian high-temperature magmas operates concurrently. Temperature estimates with the two-pyroxenes geothermometer range from 1080°C to 970°C in the tholeiitic suite, whereas from 990° to 850°C in the calc-alkaline suite. Temperature-oxygen fugacity trends indicate that the low-alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc-alkaline magma. In view of the fact that the hyperstene and augite phenocryst cores in the calc-alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples, it is unlikely that magma mixing played an essential role in the generation of the calc-alkaline suite at Adatara volcano.