Episyenites from Hakatajima Island, Geiyo Islands, Ehime Prefecture, Japan

Abstract

Quartz-depleted syenitic rocks are hosted by Cretaceous granitoids at the northern end of Hakatajima Island, Ehime Prefecture. The syenitic rocks can be divided into pearl-white and oyster-white types. The pearl-white syenitic rock is characterized in outcrops by a vuggy texture. The main mafic mineral in the pearl-white syenitic rock is garnet with subordinate clinopyroxene. Rhythmic banding of layered mafic mineral aggregates is observed locally. Vugs are filled by mineral aggregates composed of garnet, clinopyroxene, and titanite, and protolith structures are strongly modified. In contrast, the oyster-white syenitic rocks appear massive and dense and lack vugs in the outcrops. The main mafic mineral in the oyster-white syenitic rocks is clinopyroxene with subordinate garnet. Micropores occur in granular aggregates of clinopyroxene and relict biotite and hornblende in the thin section. Some structural features of the protolith, such as an aplite vein in the host granitoid, are preserved in the oyster-white syenitic rock. Whole-rock geochemical compositions of both syenitic rocks are distinct from the host granitoids, with considerably lower SiO₂ and higher Al₂O₃ contents. There are considerable differences between the whole-rock compositions (Al₂O₃, FeO_(total), MgO, CaO, Sr, Zr, and Ba) of the pearl-white and oyster-white syenitic rocks. The presence of vugs and/or micropores and lack of primary quartz suggest the occurrence of quartz dissolution, which is an episyenitization process in granitoids. These petrographic and geochemical characteristics indicate that these episyenites formed by extensive fluid–granitoid interaction.