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

The Kurosegawa belt in SW Japan preserves fragments of Early-Middle Paleozoic granitoids traditionally called the Mitaki igneous rocks and previously dated ca. 470-435 Ma 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 +/- 5.8 Ma for a granodiorite from Kuraoka area in central Kyushu, and 444.5 +/- 7.6 Ma for a quartzdiorite from the Nabaenohana area in western Kii peninsula, respectively. These ages confirm that the Mitaki igneous rocks have ca. 445-435 Ma (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. 700-500 Ma (Neoproterozoic-Cambrian), ca. 1350-830 Ma (Meo-Neoproterozoic), and ca. 3230-1560 Ma (Paleoarchean-Mesoproterozoic). Of the 44 dated zircon grains, the youngest (possibly xenocrystic) grains are ca. 500 Ma. Older zircon grains with ages >500 Ma range up to 3230 Ma and are interpreted as inherited xenocrysts in the "tonalite". The Mitaki igneous rocks are interpreted to have been derived by melting of post-500 Ma terrigenous sedimentary rocks that yielded a unique tonalitic S-type granitoid magma. From the presence of abundant 1350-700 Ma (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. (C) 2014 Elsevier Ltd. All rights reserved.