ZONAL STRUCTURE OF CENOZOIC BASALTS RELATED TO MANTLE UPWELLING IN SOUTHWEST JAPAN

Cenozoic olivine-bearing basaltic rocks in the central Chugoku district from eight provinces, each 20 to 30 km in diameter. These provinces can be grouped into three zones almost parallel to the elongation of the SW Japan arc, based on geological and petrological characteristics. The San-yo zone (the closest to the Nankai trough) is characterized by a small amount of alkaline basalt. In the Sekiryo zone (intermediate zone), transitional basalt is dominant. The San-in zone along the Japan Sea coast is characterized by subalkaline basalt and a large amount of andesitic rocks. The eruption volume of these volcanics exponentially increases toward the back arc side (less than 1 km3 to more than 100 km3). Chemical compositions of 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). Melting experiments on the basalt-H2O-CO2 systems show that the pressure and temperature of magma segregation decrease toward the back arc side (17-19 kbar and 1340-degrees-1320-degrees-C to 8 kbar and 1250-degrees-C), and the coexisting phases change from a spinel lherzolite assemblage to a harzburgite assemblage. These PT conditions define a trend which can be interpreted as a PT path for adiabatic upwelling of the mantle material. Consequently, the observed volcanism and zonation may have been caused by upwelling of a plume from a deep part of the mantle.