Spatial variation in the Sr and Nd isotope compositions of Cenozoic alkali basalts from the Chugoku district, SW Japan.

Abstract

Cenozoic alkali basalts (1-12 Ma) in the Chugoku district, SW Japan, are divided into two types: 1) MF-type basalts characterized by high MgO, FeO and TiO2, and 2) SA-type basalts rich in SiO2 and Al2O3. They are also distinguished in terms of the TiO2/MnO/P2O5 tectonomagmatic discriminant diagram and the eNd-87Sr/86Sr systematics. The MF-type basalts are similar in petrochemistry to oceanic island alkali basalts, while most of the SA-type basalts show the features of arc basalts. The isotopic data and trace element abundances of the rocks indicate that the SA-type basalts are not derivatives from the MF-type basalt magma. These two types of the alkali basalts should be produced from the different mantle sources. The distribution of the SA-type basalts with arc signature is restricted to the back-arc side (Japan Sea side) of the Chugoku district. The spatial variation in Sr and Nd isotopic composition of the MF-type basalts show a reverse tendency to that shown by the Quaternary volcanics from NE Japan. These facts cannot be ascribed to subduction effect of the Pacific plate or of the Philippine Sea plate. The Cenozoic alkali basalts with oceanic island basalt signature from the Eastern Asia, including the MF-type basalts, show the highest 87Sr/86Sr and lowest 143Nd/144Nd ratios in the southern Japan Sea, and these ratios become lower and higher with distance from this area to both sides, respectively.