ABSTRACT

KUNO (1950, 1959) first identified the existence of two rock series in the Japanese volcanic arc and proposed the origin of one from the other by assimilation of granitic material. In the Nasu zone in northeastern Japan, both the tholeiitic suite with pigeonite in the groundmass and the calc-alkalic suite with hypersthene as the Ca-poor pyroxene in the groundmass have been shown to have erupted from one volcano and to have differences in major element chemistries (KAWANO and AOKI, 1960; KAWANO et al., 1961). MASUDA and AOKI (1979), using mainly trace elements, showed that the calc-alkalic suite has higher Mg, K, Co, Cr, Ni, Th, and U than the tholeiitic suite, and also that each suite had a separate fractional crystallization. A model is presented here which is intended to explain both the mineralogical and chemical differences. KUNO's idea of assimilation is considered from a different point of view; that is, calc-alkalic basaltic andesite is formed by the partial assimilation of mafic rather than granitic material by a silicic tholeiitic andesite. Changes in the chemistry, mainly in the Mg/Fe ratio in the contaminated magma, prevents the crystallization of pigeonite; a drop in temperature as KUNO suggested is not as important although lower temperatures will help. It is demonstrated how such elements as K, Th, and U, which tend to be concentrated in granitic material, can be increased in the calc-alkalic rocks by addition of mafic xenoliths. This process is offered as an alternative to magma mixing, since it is believed that the mineralogical features used to support magma mixing could be explained equally well by assimilation.