Sr–Nd isotopic compositions of the Susuma–Nagaho Plutonic Complex in the San–yo Belt, Southwest Japan: Implications for the Cretaceous enriched mantle

Abstract

The Inner Zone of Southwest Japan is dominated by voluminous Cretaceous to Paleogene igneous rocks. The central part of Yamaguchi Prefecture, in the Susuma–Nagaho area, displays a dominance of the Susuma–Nagaho Plutonic Complex that consists of granodiorite and gabbro. The gabbro can be subdivided into fine–grained gabbro and coarse–grained cumulus gabbro. The fine–grained gabbro has a high Mg–number similar to primitive basalt with negative εNd values corrected to 92 Ma although Cr and Ni contents are relatively low, which indicates that the basaltic magma may have been derived from enriched mantle. In consideration of the geochemical signatures including the Sr and Nd isotopic compositions, the enriched mantle characterized by negative εNd isotopic values would have existed underneath the Inner Zone of Southwest Japan during the Cretaceous.