Magmatism and tectonic setting of proto-Japan during the Early Carboniferous: Constraints from the geochemical characteristics of mafic volcanic rocks in the Hida Gaien Belt, SW Japan

The Hida Gaien Belt in southwest Japan consists mainly of Paleozoic to Mesozoic shelf-facies rocks and records the tectonic evolution of East Asia. Here we report a study of the geochemistry of mafic volcanic rocks from the Early Carboniferous Arakigawa Formation of this belt. Two types of mafic volcanic rock are identified: (1) highAl basaltic lavas, which were generated by island arc tholeiitic magma that formed under high-pressure conditions near the crust-mantle boundary and in which the nucleation of plagioclase was suppressed and delayed; and (2) basaltic andesitic lavas, which were generated by fractional crystallization of the high-Al tholeiitic magma. Combining our new results with previous geological findings, we consider that the tectonic setting of proto-Japan during the Early Carboniferous was a primitive island arc characterized by active island arc tholeiitic magmatism and the absence of a granitic batholith.