Early Paleozoic subduction initiation volcanism of the Iwatsubodani Formation, Hida Gaien belt, Southwest Japan

摘要: In placing Japanese tectonics in an Asian context, variation the Paleozoic geological environment is a significant issue. This paper investigates geochemistry of lower basalt formation (Iwatsubodani Formation) Hida Gaien belt, Japan, to consider its tectonic setting. formation includes following two types rock ascending order: A with sub-ophitic texture and B porphyritic texture. Basalt has high uniform FeO*/MgO ratio, moderate TiO2, V, low Ti/V. The HFSE REE are nearly same as those MORB, all data points being “MORB-like fore-arc tholeiitic (FAB)” reported, for example, from Izu–Bonin–Mariana arc. By contrast, V It LREE-enriched trend distinct negative Nb anomaly MORB-normalized multi-element pattern moderately LREE/HREE. All these factors suggest that calc-alkaline basalt. known FAB erupted at earliest stage arc formation—namely, subduction initiation—and boninitic/tholeiitic/calc-alkaline volcanism follows supra-subduction zone (SSZ). Thus, occurrence basalts (FAB) (calc-alkaline rock) strong evidence early arc-formation initiation SSZ. Evidence SSZ also recognized Oeyama, Hayachine-Miyamori, Sergeevka ophiolites. Hence, both ophiolites Iwatsubodani Formation probably coexisted primitive system Paleozoic.