Antiquity and tectonic lineage of Japanese Islands: New discovery of Archean–Paleoproterozoic Complex

Abstract

A new rock complex has been discovered from the Tsuwano area of Southwest Japan. 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. We report here the zircon U-Pb LA-ICP-MS geochronological data of the rocks of this 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 exposed 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 is the first discovery of a Precambrian basement rock complex from the Japanese main islands. Based on the present geochronological dataset, we propose for a correlation and paleo-positioning of these basement rocks on the East Asian cratonic margin and a possible tectonic model of its evolution.