Relationship between the Shikanoshima Granodiorite and Shikanoshima basic rocks at the Shikanoshima Island, northern Kyushu, Southwest Japan: coexistence of high Mg dioritic magma and granodioritic magma.

Abstract

The Shikanoshima Granodiorite and the Shikanoshima basic rocks that have been emplaced during the Cretaceous age are exposed on the Shikanoshima Island, northern Kyushu. We found new field relationship between these two rocks. The Shikanoshima basic rocks are intruded by the Shikanoshima Granodiorite with an irregular boundary and have a chilled margin on its boundary. This occurrence suggests that the Shikanoshima basic rocks were not formed by hybridization of postdating the Shikanoshima Granodiorite magma, but generated by rapid cooling of mafic magma caused by intrusion of the coeval felsic magma. The chemical compositions of the Shikanoshima basic rocks resemble those of the Cretaceous high-Mg diorite in northern Kyushu. It means that the regional activity of high-Mg intermediate to mafic magmatism occurred in northern Kyushu during the Early Cretaceous.