Presentation Information
[T1-O-11]K-Ar geochronology of Abukuma metamorphic belt in NE Japan: towards a formation model of the plutonic metamorphic belt over 1,000km
*Tetsumaru ITAYA1 (1. Hiruzen Institute for Geology & Chronology)
Keywords:
Abukuma metamorphic belt,NE Japan,Low P/T type of metamorphism,K-Ar ages of biotite & muscovite,middle Cretaceous plutonic metamorphic belt,subduction roll-back,high Nb basalt,adakite,high Mg andesite
The Abukuma Mountains is one of the important geological units in NE Japan, and most of it is occupied by middle Cretaceous plutonic rocks. In the southern region, metamorphic rocks are distributed over a fairly wide area along with the plutonic rocks. The Abukuma metamorphic belt in the Gosaisyo-Takanuki region is well-known as a type belt that underwent the andalusite-sillimanite progressive metamorphism (Miyashiro 1958). Based on the lithology, the Abukuma metamorphic rocks can be divided into the Gosaisyo series on the east side and the Takanuki series on the west side. The Gosaisyo series are mainly composed of metabasites and metacherts with subordinate amount of meta-ultrabasites, metapelites and limestones. Based on the geochemical analysis of the metabasites and the finding of radioralian fossils in siliceous metamorphic rocks, the Gosaisyo series are thought to have been formed by the metamorphism of the upper part of the Jurassic oceanic crust (Hiroi and Kishi, 1989). On the other hand, the Takanuki series consists dominantly of pelitic-psammitic gneisses with small amounts of coarse-grained metabasites and marbles containing lenses of metalaterites. The host rocks of the Takanuki series are thought to be essentially of terrigenous origin (Hiroi and Kishi, 1989).The K-Ar analyses of muscovite and biotite from the two types of series were carried out, resulting in the two different types of series were the same in age of ca. 100Ma. This indicates that both series underwent the low P/T type of metamorphism together in middle Cretaceous after the different geological bodies juxtaposed tectonically with each other. This Abukuma metamorphism and coeval pluton formation relates to the middle Cretaceous low-P/T type metamorphism with coeval plutonic rocks in the Higo region of Kumamoto Prefecture and the Yanai Ryoke region of Yamaguchi Prefecture in SW Japan. In this way, the middle Cretaceous low P/T type of rocks with coeval plutonic rocks can be traced over 1,000 km from Higo through Yanai Ryoke to Abukuma. The crust of east Asian continental margin, which consist of various types of rocks, has undergone the low P/T type of metamorphism with pluton formation in middle Cretaceous. This means a plutonic metamorphic belt over 1,000 km in middle Cretaceous. The subduction rollback in middle Cretaceous cause the high-T asthenosphere introduces into the wedge mantle and lead the specific magma such high Nb basalt, adakite and high Mg andesite (Imaoka et al., 2014; 2017) beneath the accretionary complex. The magmas heat the lower part of complex, resulting in the low P/T type of metamorphism of the complex and the ilmenite series granite formation by the partial melting of the lower complex.
References
Hiroi, Y. and Kishi, S. (1989) Staurolite and kyanite in the Takanuki pelitic gneisses of the Abukuma metamorphic terrane, northeast Japan. Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 84, 141-151 (in Japanese with English abstr.).
Imaoka, T., Nakashima, K., Kamei, A., Itaya, T., Ohira, T., Nagashima, M., Kono, N., Kiji, M. (2014) Episodic magmatism at 105 Ma in the Kinki district, SW Japan: petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications. Lithos 184–187, 105–131.
Imaoka, T., Kawabata, H., Nagashima, M., Nakashima, K., Kamei, A., Yagi, K., Itaya, T. and Kiji, M. (2017) Petrogenesis of an upper Cretaceous lamprophyre dike from Kyoto Prefecture, Japan: Implications for the generation of high-Nb basalt magmas in subduction zones. Lithos, 290-291, 18-33.
Miyashiro, A. (1958) Regional metamorphism of the Gosaisyo-Takanuki district in the central Abukuma Plateau. Journal of the Faculty of Science, the University of Tokyo, Sec. 2, 11, 219-272).
References
Hiroi, Y. and Kishi, S. (1989) Staurolite and kyanite in the Takanuki pelitic gneisses of the Abukuma metamorphic terrane, northeast Japan. Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 84, 141-151 (in Japanese with English abstr.).
Imaoka, T., Nakashima, K., Kamei, A., Itaya, T., Ohira, T., Nagashima, M., Kono, N., Kiji, M. (2014) Episodic magmatism at 105 Ma in the Kinki district, SW Japan: petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications. Lithos 184–187, 105–131.
Imaoka, T., Kawabata, H., Nagashima, M., Nakashima, K., Kamei, A., Yagi, K., Itaya, T. and Kiji, M. (2017) Petrogenesis of an upper Cretaceous lamprophyre dike from Kyoto Prefecture, Japan: Implications for the generation of high-Nb basalt magmas in subduction zones. Lithos, 290-291, 18-33.
Miyashiro, A. (1958) Regional metamorphism of the Gosaisyo-Takanuki district in the central Abukuma Plateau. Journal of the Faculty of Science, the University of Tokyo, Sec. 2, 11, 219-272).