Generation of intermediate and felsic magmas of the Iwanoyama–Iyuzan (Ioyama) volcanic chain in the Higashi–Izu Monogenetic Volcano Field, northern Izu–Bonin volcanic arc, Japan

The Higashi–Izu Monogenetic Volcano Field (HIMVF) in the northern Izu–Bonin volcanic arc, Japan consists mainly of mafic with minor intermediate and felsic monogenetic volcanoes. The Iwanoyama–Iyuzan (Ioyama) volcanic chain, which formed approximately 2.7 ka, is an unique example of bimodal volcanism involving intermediate (andesite) and felsic (dacite and rhyolite) rocks. Both rock types in the chain are characterized by disequilibrium assemblages and bimodal chemistry of phenocrysts. These results and the mineral chemistry of three types of inclusion (from cumulate–crystal-mush-like rocks to mafic rocks) in the felsic rocks suggest that parental mafic and felsic magmas underwent magma mixing and/or mingling. Whole-rock major- and trace-element (including rare-earth-element) data are consistent with magma mixing process. The 87 Sr/ 86 Sr ratios of the rocks have a restricted range (0.70329–0.70349) but differ slightly among different volcanoes. The parental mafic magmas are inferred to have been basaltic to basaltic andesite in composition and were slightly modified by fractional crystallization, or assimilation and fractional crystallization process with lower–upper crustal rocks, whereas the parental felsic magmas (rhyolitic in composition) could have been generated by partial melting of upper crustal rocks. The mafic and felsic magma types mixed and/or mingled to some degree, and the resultant magmas were erupted independently to form different volcanoes with a linear distribution. The magma supply structure of the studied volcanic chain, which shows similarity to that of polygenetic volcanic systems, formed in the central part of the HIMVF, which has an outer zone dominated by basaltic volcanism.