Abstract
The Yatsushiro area in Kyushu Island, southwest Japan, forms the western tip of the Kurosegawa tectonic belt. This area is underlain by a serpentinite melange with complicated tectonic block assemblage of granitic rocks, high-temperature (HT)-type metagabbro (Grt-Cpx granulite, Grt-amphibolite and Cpx-bearing amphibolite), and high-pressure (HP)-type metagabbro (Jd-Gln rock and Gln-bearing metagabbro). The granitic rocks and the gabbros are the protoliths of the HT-type metagabbro (HT-gabbro), and supposed to be derived from volcanic arc at an active continental margin based on their major, trace, and rare earth element (REE) chemistry, while geochemistry of the gabbro protoliths of the HP-type metagabbro (HP-gabbro) indicates mid oceanic ridge basalt (MORB)-like precursors. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the granitic rocks and the HT-gabbro gave the Ordovician magmatic ages (∼ 450 Ma). On the other hand, the HP-gabbro showed the igneous age of the Late Cambrian (∼ 490 Ma). Similar rock suites collected from the Kurosegawa tectonic belt in Shikoku and in the Kii-peninsula have the same chemical and age affinities according to their rock types. These results indicate the strong geological and tectonic similarities within the Kurosegawa tectonic belt. The similar assemblage is present in the South-Kitakami belt suggesting similarities in their origin.
