Compositional Zonation of Cenozoic Basalts in the Central Chugoku District, Southwestern Japan : Evidence for Mantle Upwelling.

Abstract

Cenozoic olivine-bearing basaltic lavas in the central Chugoku district form eight distributional provinces, each having 20 to 30 km diameter. Lavas in each province have similarities in chemical composition and mode of occurrence. Moreover, these provinces can be grouped into three zones almost parallel to the SW Japan arc on the basis of the differences in chemical composition and eruption volume of volcanics; the San-yo zone (the nearest to the Nankai trough), the Sekiryo zone, and the San-in zone (along the Japan Sea coast) from the south to the north. The eruption volume including both basalts and andesites exponentially increases northward, in other words, towards the back-arc side. The chemical compositions of the relatively undifferentiated basalts show systematic across-arc variations. SiO₂ and Al₂O₃ increase, and FeO^*, MgO and CaO decrease toward the back-arc side (44 to 54, 13 to 18, 11 to 6, 13 to 7 and 12 to 8 wt. % at FeO^*/MgO=0.85, respectively). The above compositional difference can be explained by a difference in the depth of magma segregation; the depth of magma segregation decreases towards the back-arc side. These across-arc variations of the basalts in the central Chugoku district has opposite polarity compared with that in the NE Japan arc, and cannot be explained by the process related to subduction of the oceanic lithosphere from the Nankai trough or the Japan trench. Taking into account the timing of volcanism which started at 12 Ma, the volcanism could have been caused by the upwelling of mantle materials associated with the opening of the Japan Sea.