Mid-Paleozoic arc granitoids in SW Japan with Neoproterozoic xenocrysts from South China: New zircon U–Pb ages by LA-ICP-MS

Abstract

The Kurosegawa belt in SW Japan preserves fragments of Early–Middle Paleozoic granitoids traditionally called the Mitaki igneous rocks and previously dated ca. 470–435Ma by several isotope analyses such as Rb–Sr, K–Ar and U–Pb, together with high-grade metamorphic rocks and Silurian strata, in a narrow belt. However, the timing of the oldest arc-related plutonism in Japan is constrained by laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U−Pb ages of zircon from Mitaki granitoids in 3 areas in SW Japan; i.e. Kyushu, Shikoku, and Kii Peninsula. Weighted mean U–Pb ages of these zircons are 431.8±3.9 for a granodiorite from the Mt. Mitaki area in Shikoku, 444.1±5.8Ma for a granodiorite from Kuraoka area in central Kyushu, and 444.5±7.6Ma for a quartzdiorite from the Nabaenohana area in western Kii peninsula, respectively. These ages confirm that the Mitaki igneous rocks have ca. 445–435Ma (late Ordovician to mid-Silurian) ages. They are some of the oldest subduction-related plutonic rocks in SW Japan. Particularly noteworthy is a “tonalite” from the Nabaenohana area, which has a unique spectrum of zircon U–Pb ages with distinct 3 clusters; ca. 700–500Ma (Neoproterozoic–Cambrian), ca. 1350–830Ma (Meo–Neoproterozoic), and ca. 3230–1560Ma (Paleoarchean–Mesoproterozoic). Of the 44 dated zircon grains, the youngest (possibly xenocrystic) grains are ca. 500Ma. Older zircon grains with ages >500Ma range up to 3230Ma and are interpreted as inherited xenocrysts in the “tonalite”. The Mitaki igneous rocks are interpreted to have been derived by melting of post-500Ma terrigenous sedimentary rocks that yielded a unique tonalitic S-type granitoid magma. From the presence of abundant 1350–700Ma (Meso- to Neoproterozoic) zircon grains in the “tonalite” we conclude that during the Early Paleozoic, proto-Japan was located close to the Cathaysian margin of South China.