Kaushik Das

Kaushik DAS^1,*, Koske KIMURA^1,2, Yasutaka HAYASAKA^1,*

¹Department of Earth and Planetary Systems Science, Hiroshima University, Japan

²National Institute of Technology, Kure College, Japan

* Hiroshima Institute of Plate Convergence Region Research, Japan

The basement rocks of Japan keep the record of geological events related to plate convergence that occurred at the East Asian cratonic margin. The geochronological data, so far indicated that accretion of both oceanic sedimentary rocks (e.g. chert, mudstone), and siliciclastic sedimentary rocks sourced from erstwhile continental neighbors occurred at least up to the Cambrian. Intermittent long belts of ophiolitic rocks are sandwiched between these sedimentary units, possibly representing limited sliced remnants of oceanic plateau and seamounts (Wallis et al., 2020; Kimura and Hayasaka, 2019). This geological distribution is rather clear in SW Japan. However, it was enigmatic that though Precambrian detritus supply was there in some of these sedimentary rocks, Precambrian basement rocks were not found in these accretionary systems in Japan. Hence, it was difficult to do a good correlation with East Asian cratonic units, e.g. North China Craton, South China Craton which has a predominance of Precambrian rocks.

Recently, we have discovered a new rock complex from the Tsuwano area of Southwest Japan (Kimura et al., 2021). This granitic rock complex namely, Tsuwano Complex contains foliated granitic gneiss, massive granitoids, and metaquartzite, forming a distinct unit at the westernmost end of the east-west elongate Maizuru Terrane. The above-mentioned litho-assemblage is characteristically different in appearance from the typical rocks of the Maizuru Terrane as well as the surrounding Jurassic and Permian accretionary rocks. In this presentation, we shall highlight the zircon U-Pb LA-ICP-MS geochronological data of the rocks of the Tsuwano Complex. The comprehensive geochronological data indicate that this complex is much older than the surrounding rocks of the Maizuru Terrane. Zircon cores of the granitic gneiss yield possible magmatic age at ca. 2700 and 2500 Ma, while rim parts show a possible metamorphic age of ca. 1850 Ma. Associated dioritic to tonalitic granitoids also show a comparable age of crystallization at ca. 1850 Ma, indicating Neoarchean granitic rocks suffering a thermal overprinting and associated granitoid magmatism at Paleoproterozoic era. The metaquartzite also yielded Neoarchean detrital zircon grains with the calculated maximum age of deposition at ca. 2430 Ma. While, all the other studied rocks including the grayish granitoids, felsic tuff, and metagabbro yield Paleozoic to Mesozoic ages. This first discovery of a Precambrian basement rock complex from the Japanese main islands gave us a chance to re-look at the correlation and paleo-positioning of these basement rocks on the East Asian cratonic margin and a possible tectonic model of its evolution.

References:

Kimura, K., Hayasaka, Y. (2019) Zircon U-Pb age and Nd isotope geochemistry of latest Neoproterozoic to early Paleozoic Oeyama ophiolite: Evidence for oldest MORB-type oceanic crust in Japanese accretionary and its tectonic implications.

Lithos 342, 345-360

Kimura, K., Hayasaka, Y., Yamashita, J., Shibata, T., Kawaguchi, K., Fujiwara, H., Das, K., (2021) Antiquity and tectonic lineage of Japanese islands: New discovery from U-Pb zircon geochronology. Earth and Planetary Science Letters, 565, 116926.

Wallis, S. R., Yamaoka, K., Mori, H., Ishiwatari, A., Miyazaki, K., Ueda, H. (2020) The basement geology of Japan from A to Z, Island Arc, 12339