OT4 zircon collected from Oto‐dani quartz diorite in Unazuki, in the northeastern part of the Hida Belt, Southwest Japan, was evaluated using microbeam analysis for its qualification as a Mesozoic reference zircon based on U–Pb dating. The characterization of OT4 zircon was carried out using a sensitive high‐resolution ion microprobe (SHRIMP II). Chemical zoning was revealed in some of the grains with a relatively high concentration of U and Th (up to 988 and 1054 ppm, respectively). The weighted mean 206Pb/238U age corrected by 207Pb was 191.1 ± 0.3 Ma (95% confidence limit) and was found to be almost constant for all the grains. The high crystallinity, the constant REE abundance patterns, and the homogeneous Hf concentrations (8651 ± 466 ppm) indicate that the grains were not affected by any geological events after crystallization. Furthermore, the homogeneous Hf concentrations and consistency of the Ti‐in‐zircon temperature (733 ± 34°C) and the Zr saturation temperature (728°C) suggest that the OT4 zircon crystallized over a short time. The OT4 zircon contains enough radiogenic 206Pb (>2 ppm) and 207Pb (>0.1 ppm) to be detected by microbeam analysis, which indicates its potential of being an effective Mesozoic reference used in microbeam U–Pb dating.

A late Paleozoic crustal section of an intra oceanic island arc occurs in the Middle Unit (M Unit) of the Asago body from the Yakuno ophiolite, SW Japan. It consists of mafic metamorphic rocks and felsic to mafic later intrusions. The M Unit is composed of three lithologic zones: the Lower migmatite zone (L MZ), the Middle metagabbro zone (M MZ), and the Upper amphibolite zone (U AZ). The metamorphic grade of the lowest horizon of the M Unit reaches granulite facies conditions (P = 3.5 5.5 kbar at T = 850 860˚C) and an assemblage of plagioclase + clinopyroxene + orthopyroxene (pseudomorph) + hornblende (tschermakite or magnesiohastingsite) appears in the mafic metamorphic rocks. Structural relationships of the migmatite and related rocks suggest that a small scale leucosome (i.e. melt) was generated at the bottom of L MZ and its upward melt segregation led to the formation of medium to large scale plutons and dykes of later intrusions into the M MZ and U AZ. The bulk rock chemistry of these rocks suggests that the later intrusions were derived from the partial melting of mafic metamorphic rocks, leaving melanosome as residuum. Major and trace element compositional variation of the later intrusions indicates that the leucosome (SiO 2 48 71 wt%) may be a source of the plutons and dykes of medium silica content (SiO 2 52 67 wt%), whereas chemical compositions of the plutons and dykes of high silica content (SiO 2 72 76 wt%) and those of the leucosomes do not overlap. The fact indicate that the plutons and dykes of high silica content may have been derived from either compositionally more differentiated magma or a completely exotic one. IntroductionThe petrogenesis of granitoid in the intra oceanic island arc (OIA) is the key to constraining the process of continental crust growth by accretion of the oceanic island arc (e.g. Taira et al., 1992). Different models have been proposed for the generation of intermediate to felsic magmas in OIA: (1) Differentiation of high Mg andesite or basaltic magma derived from the partial melting of the upper mantle (e.g. Meijer, 1983;Shirey and Hanson, 1984;Stern and Hanson, 1991; Haragchi et al., 2003); (2) Partial melting of subducting oceanic crust (e.g. Martin, 1986;1987;1993;Nelson and Forsythe, 1989;Defant and Drummond, 1990;Drummond et al., 1996); and, (3) Anatexis of mafic lower crust (e.g. Atherton and Petford, 1993; Kay and Kay, 1993;Kawate and Arima, 1998). Many experiments indicate that partial melting of amphibolite is the most effective process to generate a large volume of intermediate to felsic melts (e.g. Beard and Lofgren, 1991; Wolf and Wyllie, 1994;Nakajima and Arima, 1998). Therefore, the anatexis of mafic lower crust and/or the partial melting of subducting oceanic crust have been though to be the most important mechanisms for the genesis of granitoid magma in the OIA (Beard, 1995).The mafic migmatite and related granitoid occur in the Middle Unit of the Asago body from the Late Paleozoic Yakuno ophiolite, SW Japan. This unit has been considered to be a lower t...

The subduction of "hot" Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back-arc and island-arc sides of southwest Japan. The Goto Islands are located on the back-arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori-jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U-Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U-Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at $16-15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15-14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.

A middle to lower crustal section of Paleozoic oceanic island arc is exposed in the Asago body of the Yakuno ophiolite. Based on field occurrence, petrography, and geochemical modeling, we investigated the evolution of the Asago body and relevant magmatic processes. The Asago body consists of two stages of rocks. The first-stage rocks consist of metagabbro and schistose amphibolite that represent the basement to the Permian Yakuno paleo-island arc. The second-stage rocks are mainly arc granitoids that intrude the first-stage rocks. Mafic migmatites occur in the lower crustal section of the Asago body. Field occurrences and petrographic data suggest that the migmatites formed by the anatexis of first-stage rocks, and that segregation and accumulation of the anatectic melt resulted in morphological changes in the migmatites toward the middle crustal section. Geochemical data indicate that the first-stage rocks were derived from a basaltic magma of back-arc affinity, suggesting in turn that the Yakuno paleo-island arc was developed within a back-arc basin. Moreover, a low-K series hornblende gabbro, quartz diorite, and tonalite and high-K series quartz monzodiorite and granodiorite within the second-stage rocks were generated by high and low degrees of partial melting of first-stage rocks, respectively. We conclude that the Asago body is an example of the transition from oceanic to continental crust, related to the anatexis of mafic lower crust in an oceanic island arc setting. Abstract