Olivine-chromian spinel compositional relationships of the Cenozoic alkali basalts from Southwest Japan : implication for their mantle restites

Abstract

Basaltic volcano clusters of the Cenozoic are widely distributed in the central Chugoku district, Southwest Japan. These clusters can be divided into several zones and are considered to be produced by mantle upwelling. Most of these basaltic magmas have relatively undifferentiated characteristics and coprecipitate olivine and chromian spinel. Restites of these basaltic magmas are determined by compositional relationships between Fo content of olivine and Cr# (=Cr/(Cr+Al) atomic ratio) of chromian spinel. Each cluster has each characteristic refractoriness of their mantle restites: lherzolite (Cr# <0.5) in the Tsuyama, Kibi, Sera and Oki-Dogo volcano clusters; lherzolite to harzburgite (Cr#=0.2-0.6) in the Kuroiwa-Kogen, Yokota and Hiba volcano clusters; lherzolite (Cr#<0.4) (11 Ma) and harzburgite (Cr#=0.5-0.6) (1 Ma) in the Matsue volcano cluster; and harzburgite (Cr#=0.5) in the Kurayoshi volcano cluster. The mantle restites indicated by the Fo-Cr# relations are comparable to the residual phases deduced by melting experiments of Takahashi (1980) and Iwamori (1991) with one exception that restites indicated by the Fo-Cr# relation differ from those deduced from melting experiment for the basalt erupting at 11 Ma in the Matsue cluster. The regional distribution of possible mantle restites suggests that the relatively depleted restite (the Yokota cluster) are surrounded by the relatively fertile mantle restites. On the other hand, temporal variations of Cr# of spinel coexisting with Mg-rich olivine are recognized in each zone, i.e. refractoriness of possible restites sometimes trends to increase with time. The restites for the basalts of 1 Ma in age from the Matsue and Yokota clusters, located near the center of mantle upwelling, are relatively refractory. Therefore, generation of these basaltic cluster may be more largely affected by mantle upwelling. The change of possible restites through space and time may be caused by the repeated basaltic volcanism in central Southwest Japan due to upwelling of mantle plume.