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[SCG54-P07] Formation ages of episyenites in Hakatajima Island, Geiyo Islands, Ehime Prefecture
Keywords:Episyenite, Granite, Episyenitization, whole-rock Rb-Sr isochron age, quartz dissolution, Hakatajima Island
Episyenite is a quartz-depleted alkali-feldspar-rich rock, formed from granitic rocks through the interaction with silica-undersaturated fluids under sub-solidus conditions (Suikkanen and Ramo, 2019). This interaction induces vug-formation by quartz dissolution and alkali metasomatism of felspars followed by varying degrees of vug-filling with secondary hydrothermal minerals (e.g., Cathilineau, 1986). In this study, we describe field occurrences, petrographic remarks, and geochemical characteristics of two adjacent contrasting episyenites (pearl-white episyenite and oyster-white episyenite) and host rocks of Cretaceous granitoid in Hakatajima Island, southwest Japan. We also constrain episyenitization age from the whole-rock Rb-Sr isochron. We then discuss the age of episyenitization in Hakatajima Island.
Both of episyenites expose as small discrete bodies (~30-40 meters length and ~5-10 meters width) in host granitoid and are mainly composed of alkali feldspar, clinopyroxene, garnet, and titanite. The distance between two episyenite bodies is ~40 m. Both bodies display irregular-shaped gradual transitions to the host granitoids in cm scale. The pearl-white episyenites are characterized by a vuggy texture in the outcrops. The mafic minerals in the pearl-white episyenites are mainly garnet with subordinate clinopyroxene. Mineral aggregates composed of garnet, clinopyroxene, and titanite are observed as vug-filling minerals in the thin sections of pearl-white episyenite. Alkali feldspars in the pearl-white episyenites have perthite or mesoperthite texture. The oyster-white episyenites are characterized by massive and dense lithology and lack the obvious vugs in the outcrops. The mafic minerals in the oyster-white episyenites are mainly clinopyroxene with subordinate garnet. Micro-pores are observed in the granular aggregate of clinopyroxene in the thin section of oyster-white episyenites. Alkali feldspars in the oyster-white episyenites have perthite or mesoperthite texture.
The whole-rock geochemical compositions of the two episyenite bodies are distinctive from those of the host granitoids, showing considerably lower SiO2 and higher Al2O3 contents than those of the host granitoids. Significant differences in whole-rock compositions (Al2O3, FeO(total), MgO, CaO, Sr, Zr and Ba) are observed between the pearl-white and oyster-white episyenites. The whole-rock REE compositions of the pearl-white episyenite are lower than those of host granitoid, while those of oyster-white episyenite are higher than the granitoid. The whole-rock Rb-Sr isochron age of episyenites is calculated as 82.7 ± 1.4 Ma.
The episyenites in Hakatajima Island are mainly alkali feldspar, and the other constituent minerals are clinopyroxene and garnet, which contain no Na and K. Therefore, most of Na and K in the episyenites are hosed in alkali feldspar. Because Rb is one of the alkali metal elements, Rb is also mainly hosted in alkali feldspar. Therefore, the whole-rock Rb-Sr isochron age (82.7 ± 3.2 Ma) calculated from the episyenites probably reflect the timing of alkali felspar formation through alkali metasomatism during episyenitization. Imaoka et al. (2024) reported zircon U-Pb ages ranging from 93.5 ± 1.7 to 91.5 ± 1.9 Ma for granites from the Iwagi Islet, adjacent to Hakatajima Island.
Assuming the similar formation ages for the granitoids of Hakatajima Island, the time gap between the formation of the granitoids (ca. 94-92 Ma) and episyenitization (ca. 83 Ma) is estimated as ~10 million years in the study area. On the other hand, Ar-Ar age of 91.5 ± 0.26 Ma has been reported from the katayamalite of the episyenite in the Iwagi Islet (Imaoka et al., 2024), which are interpreted as the age of episyenitization in the Iwagi Islet. Therefore, the hydrothermal activity forming the episyenites in the Hakatajima Island (ca. 83 Ma) can be distinguished as an independent later event from those of the Iwagi Islet (ca. 92 Ma).
REFERENCES: Cathelineau, 1986, Journal of Petrology, 27, 235–244; Imaoka et al., 2024, Minerals, 14, 929; Suikkanen and Ramo, 2019, Mining Metallud Exploration, 36, 861–878.
Both of episyenites expose as small discrete bodies (~30-40 meters length and ~5-10 meters width) in host granitoid and are mainly composed of alkali feldspar, clinopyroxene, garnet, and titanite. The distance between two episyenite bodies is ~40 m. Both bodies display irregular-shaped gradual transitions to the host granitoids in cm scale. The pearl-white episyenites are characterized by a vuggy texture in the outcrops. The mafic minerals in the pearl-white episyenites are mainly garnet with subordinate clinopyroxene. Mineral aggregates composed of garnet, clinopyroxene, and titanite are observed as vug-filling minerals in the thin sections of pearl-white episyenite. Alkali feldspars in the pearl-white episyenites have perthite or mesoperthite texture. The oyster-white episyenites are characterized by massive and dense lithology and lack the obvious vugs in the outcrops. The mafic minerals in the oyster-white episyenites are mainly clinopyroxene with subordinate garnet. Micro-pores are observed in the granular aggregate of clinopyroxene in the thin section of oyster-white episyenites. Alkali feldspars in the oyster-white episyenites have perthite or mesoperthite texture.
The whole-rock geochemical compositions of the two episyenite bodies are distinctive from those of the host granitoids, showing considerably lower SiO2 and higher Al2O3 contents than those of the host granitoids. Significant differences in whole-rock compositions (Al2O3, FeO(total), MgO, CaO, Sr, Zr and Ba) are observed between the pearl-white and oyster-white episyenites. The whole-rock REE compositions of the pearl-white episyenite are lower than those of host granitoid, while those of oyster-white episyenite are higher than the granitoid. The whole-rock Rb-Sr isochron age of episyenites is calculated as 82.7 ± 1.4 Ma.
The episyenites in Hakatajima Island are mainly alkali feldspar, and the other constituent minerals are clinopyroxene and garnet, which contain no Na and K. Therefore, most of Na and K in the episyenites are hosed in alkali feldspar. Because Rb is one of the alkali metal elements, Rb is also mainly hosted in alkali feldspar. Therefore, the whole-rock Rb-Sr isochron age (82.7 ± 3.2 Ma) calculated from the episyenites probably reflect the timing of alkali felspar formation through alkali metasomatism during episyenitization. Imaoka et al. (2024) reported zircon U-Pb ages ranging from 93.5 ± 1.7 to 91.5 ± 1.9 Ma for granites from the Iwagi Islet, adjacent to Hakatajima Island.
Assuming the similar formation ages for the granitoids of Hakatajima Island, the time gap between the formation of the granitoids (ca. 94-92 Ma) and episyenitization (ca. 83 Ma) is estimated as ~10 million years in the study area. On the other hand, Ar-Ar age of 91.5 ± 0.26 Ma has been reported from the katayamalite of the episyenite in the Iwagi Islet (Imaoka et al., 2024), which are interpreted as the age of episyenitization in the Iwagi Islet. Therefore, the hydrothermal activity forming the episyenites in the Hakatajima Island (ca. 83 Ma) can be distinguished as an independent later event from those of the Iwagi Islet (ca. 92 Ma).
REFERENCES: Cathelineau, 1986, Journal of Petrology, 27, 235–244; Imaoka et al., 2024, Minerals, 14, 929; Suikkanen and Ramo, 2019, Mining Metallud Exploration, 36, 861–878.