Assimilation and fractional crystallization of Sanukitic high–Mg andesite–derived magmas, Kyushu Island, southwest Japan An example of the Cretaceous Shaku–dake diorite body

摘要

The Cretaceous granitoid batholith is characterized by sporadically occurring small mafic bodies. Some of these mafic bodies show high–Mg diorite (HMD) compositions derived from a high–Mg andesite (HMA) magma. One of the mafic bodies, the Shaku–dake body, can be divide into two groups: Two–pyroxene diorite (TPD), diorite (Do), porphyritic fine–grained tonalite (PFT), and clinopyroxene granodiorite (CG) belong to the Group–1, but hornblende–biotite granodiorite (HBG) and fine–grained biotite granite (FBG) can be found in the Group–2. The Group–1 is influenced by the assimilation and fractional crystallization process during the ascent and emplacement of magma, whereas the Group–2 changes its whole–rock compositions via fractional crystallization. Discrimination diagrams of HMA indicate that the TPD shows geochemical signatures similar to those of the Sanukitic HMA, where the TPD is defined as Sanukitoid. On the other hand, the Do is plotted as the composition range of island arc calc–alkaline basalts and tholeiite. The Cretaceous magma activities in northern Kyushu were led by the highly thermal structure of the wedge mantle at that time, it was the primary heat source of the voluminous igneous activities during the Cretaceous in Southwest Japan and the Korean Peninsula.