LA-ICP-MS zircon U-Pb geochronology of Paleozoic granitic rocks and related igneous rocks from the Kurosegawa tectonic belt in Kyushu, Southwest Japan

Abstract

岩石鉱物科学 43, 71 99, 2014 九州・黒瀬川構造帯における古生代花崗岩類および 関連火成岩類の LA ICP MS ジルコン U Pb 年代 LA ICP MS zircon U Pb geochronology of Paleozoic granitic rocks and related igneous rocks from the Kurosegawa tectonic belt in Kyushu, Southwest JapanThe 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 a‹nities 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.

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

“…Osani et al (2014) reported a provisional U-Pb zircon age probably from the quartzdiorite of this area; however, detailed locality information was not given.…”

“…Yoshikura, 1985;Umeda et al, 1986;Kobayashi et al, 2000;Osani et al, 2014), suggesting their origin in a subduction-related sub-volcanic arc setting. The extremely low Sr isotope ratio (0.7032 and 0.7037) of the Mitaki rocks in western Kyushu (e.g.…”

“…These ages are mostly consistent with the previously reported U-Pb zircon ages of ca. 470-435 Ma (e.g., Aitchison et al, 1996;Hada et al, 2000;Osani et al, 2014), in particular, with the TIMS U-Pb upper intercept ages of 445-435 Ma for the zircon grains from the type Mt. Mitaki area (Hada et al, 2000).…”

“…Kii Peninsula by Osani et al (2014); however, information about the samples locations were not provided. Thus, all previously reported zircon U-Pb ages are loosely constrained to the period ca.…”

White mica K–Ar ages from lawsonite–blueschist facies Hakoishi sub–unit and from prehnite–pumpellyite facies Tobiishi sub–unit of the Kurosegawa belt, Kyushu, Japan

“…Osanai et al (2014) reported 492.4 ± 3.6Ma U-Pb concordia age of zircons extracted from a glaucophane bearing metagabbro from the Hakoishi sub-unit and they interpreted that this age represented the crystallization timing of the midoceanic-ridge type basaltic magma in the oceanic crust. If so, there is a huge time gap of about 200 Ma between the oceanic crust formation and the subsequent subduction which caused LBS facies metamorphism.…”