Petrogenetic implications and geochronology of middle Miocene Tannayama igneous rocks, Goto Islands, Japan Sea southern margin, northwestern Kyushu, Japan

Abstract

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.

Stratigraphic reconstruction of the lower–middle Miocene Goto Group, Nagasaki Prefecture, Japan

“…The Goto felsic volcanism and granitic magmatism was active from 16.8 to 15.4 Ma, during crustal thinning and rifting of southwest Japan (Hoshi, 2018; Suda et al, 2021). The end of Goto Group sedimentation is constrained by the age of the Goto felsic volcanic rocks, which have the oldest age of 16.8 Ma.…”

“…It also shows the period when Daijima‐type flora flourished. Data sources are as follows; 1, 6, 7, 8, 9, 14, 15, 16, and 17 (NEDO, 1990); 3, 4, and 5 (Kawada et al, 1994); 2a, 2b, 10, 11, 12, 13, 18, 19, 20, 21, 22, and 23 (Ishikawa & Tagami, 1991); 24a, 24b, 24c, 24d, and 25 (Suda et al, 2021)…”

“…Crystallization ages in the Mt. Tanna area (Suda et al, 2021) are as follows: 14.7 ± 0.3 Ma for gabbronorite (No. 24a; Table 1); 15.9 ± 0.5 Ma for granite (No.…”

“…Recently, new zircon U–Pb age data have been reported for the Mt. Tanna and Cape Yagatamezaki areas on Nakadorishima Island (Suda et al, 2021). Crystallization ages in the Mt.…”

“…25; Table 1). These ages represent middle Miocene magmatic activity due to upwelling of hot mantle diapirs during crustal thinning in an extensional setting (Suda et al, 2021). All previously determined ages of the granitic rocks are Miocene.…”