- 著者
- Raiki YAMADA Toshiro TAKAHASHI Yasuhiro OGITA
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- vol.118, no.1, pp.221219a, 2023 (Released:2023-11-24)
- 参考文献数
- 98
Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc, that were formed during back-arc spreading in the Japan Sea, were examined to reveal their petrogenesis and temporal change of arc volcanism during the Japan Sea opening. The arc volcanism in the Toyama basin initiated with rhyolitic pyroclastic flows (Tori Formation) containing hecatolite (moonstone) in 23-22 Ma. Enriched Sr-Nd isotope (SrI = 0.70769-0.70944; NdI = 0.51203-0.51224) suggests that contemporaneous andesitic magma (Kamiwazumi and Matsunagi Formations) mixed or assimilated basement granitoids and gneisses of the Hida belt to generate rhyolitic magma. Subsequently, andesitic volcanism (Iwaine Formation) occurred in 18-17 Ma after magmatic hiatus. Andesitic lavas of the Iwaine Formation are composed of high magnesian andesite (HMA), high-Sr andesite and tholeiitic andesite. HMA has Mg# > 64, high Cr and Ni concentrations, not so high Th/Yb and (La/Sm)N ratios, and slightly enriched Sr-Nd isotope (SrI = 0.70482; NdI = 0.51279). High-Sr andesite has relatively low SiO2 content (<60 wt%), high Sr (>2000 ppm) and K2O contents (3.98 wt% in the maximum), indicating that it is low-SiO2 adakite. These geochemical characteristics suggest that HMA and high-Sr andesite were produced by partial melting of the mantle wedge saturated by H2O derived from slab fluid and metasomatized by slab melt, respectively. Although chemical variation diagrams suggest tholeiitic andesite seems to have been generated from basaltic magma, it has enriched Sr-Nd isotope (SrI = 0.70713-0.70756; NdI = 0.51237-0.51241). Thus, tholeiitic andesite is considered to have been produced by AFC (assimilation and fractional crystallization) after generation of basaltic parental magma. Andesitic magmatism of the Iwaine Formation was followed by rhyolitic magmatism of the Iozen Formation in 17-16 Ma. The petrogenesis of the rhyolite from the Iozen Formation can be explained by low-rate mixing between andesitic magma (Iwaine Formation) and the Hida belt. The petrogeneses of the andesites, especially HMA and high-Sr andesite, are related to slab melting. Because the old and cold Pacific plate was subducting beneath the Toyama basin during the Japan Sea opening, additional heat source such as upwelling of the asthenospheric mantle into the mantle wedge is required. Moreover, back-arc spreading in the Japan Sea was driven by upwelling of the asthenospheric mantle into the mantle wedge.
- 著者
- Raiki YAMADA Hikaru SAWADA Shinnosuke AOYAMA Wataru OUCHI Sota NIKI Mitsuhiro NAGATA Toshiro TAKAHASHI Takafumi HIRATA
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- pp.201125, (Released:2021-02-25)
- 被引用文献数
- 8
The Hida granites, classified into the pre–Jurassic and Jurassic plutons in this study, are important components of the Hida belt, which is a Paleozoic–Mesozoic basement of the Japan arc and underwent Permian to Triassic metamorphism during the collision between the North and South China blocks. This study performed zircon U–Pb dating and whole–rock geochemical analyses for the Hida granites from the major plutonic bodies to reveal the geotectonic history and the origin of the Hida belt. Obtained 238U–206Pb weighted mean ages exhibit 239.1–238.3 Ma for the Katakaigawa body (augen granite) and 200.5–180.9 Ma for the other bodies (non–deformed granitoids), and these ages can be correlated to the pre–Jurassic and Jurassic plutons, respectively. Geochronological results suggest that the mylonitization forming augen granites of the pre–Jurassic plutons occurred during its intrusion and indicate that the Jurassic plutons are distributed widely in the Japan Sea side of the Hida belt. Meanwhile, geochemical characteristics of whole–rock major and trace element compositions indicate that the pre–Jurassic and Jurassic plutons seem difficult to distinguished geochemically and suggest that both of them are adakitic and non–adakitic granites generated in subduction zone.