Teshima pyroclastics: Onset of characteristic Setouchi magmatism induced by slab melting at 14.8 Ma

Abstract

Ar-Ar ages, and petrographical and geochemical characteristics of pyroclastics and an overlying lava from Teshima Island, southwest Japan are presented. Although previous geological and age data suggested Teshima pyroclastics were products of magmatism > 3 my prior to lava flows of Setouchi volcanic rocks generated in association with southward migration of the southwest Japan arc sliver during opening of the Sea of Japan backarc basin at 15 Ma, the present results led to the conclusion that a sequence of Setouchi volcanism, induced by slab melting and subsequent melt-mantle reactions, produced both pyroclastics and lava at 14.6-14.8 Ma. This age is oldest among those reported so far and may represent the timing of onset of characteristic Setouchi magmatism immediately posterior to and hence as a result of the mega-tectonic event including rotation of the southwest Japan arc sliver.

Zircon U–Pb ages of Miocene granitic rocks in the Koshikijima Islands: Implications for Neogene tectonics in the Kyushu region, southwest Japan

“…Adakite and ADR (arc andesite, dacite, and rhyolite) fields are after Castillo (2012) fault activity. Two candidates for the origin of this andesitic magmatism can be presumed, as previously proposed for the "14 Ma" granitic magmatism of the Koshikijima Islands: (i) rift-related igneous activity associated with the opening of the Japan Sea and (ii) the extension into this area of igneous activity caused by the hot subduction of the Shikoku Basin lithosphere (which produced the Setouchi Volcanic Rocks at 15-13 Ma; Nakaoka et al, 2021;Shinjoe & Orihashi, 2017;Tatsumi, 2006;Tatsumi et al, 2010), as inferred for the 14 Ma HMA reported from the nearby Amakusa Islands (Nagao et al, 1992). The intrusion of andesite-dominant northwestsoutheast-trending dikes coeval with F1 faulting may conform to riftrelated magmatism.…”

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

“…Based on the geochronological data and the timescale of the Japan Sea opening during the late Oligocene to late Miocene (27-6 Ma), magmatic events in southwest Japan can be divided into the four episodes in chronological order: the pre-back-arc opening (>27 Ma), the early to middle stage of the back-arc opening (27-15 Ma; Figure 9), the last stage of the back-arc opening (17-12 Ma; Figure 9), and the post-back-arc opening (<10 Ma; Figure 9). The magmatic activity during the last stage of the back-arc opening was extensive and voluminous, and extended to the island-arc side of southwest Japan (Figure 1a), as represented by the high-Mg andesite of Setouchi (14.8-12.7 Ma after Tatsumi, 2006;Nakaoka et al, 2021) and the acidic volcano-plutonic complexes along the Pacific coast (15.0-14.6 Ma after Tatsumi, 2006;15.6-13.5 Ma after Shinjoe et al, 2021).…”

“…This activity is thought to have been induced by the upwelling of mantle diapirs and the accompanying opening of the Japan Sea Basin (Uto et al, 2004). Geochronological data indicate that magmatism on the volcanic-arc side occurred mainly in the period 16-12 Ma (Nakaoka et al, 2021;Tatsumi, 2006;Tatsumi et al, 2001), whereas the magmatism on the back-arc side occurred mainly in the period 18-12 Ma (e.g., Kimura et al, 2005;Yamamoto, 2010). The high frequency of available age data from either side of southwest Japan indicates a climax at $15 Ma.…”