U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

Abstract

We present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, started c. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Termination of intra‐arc rifting at ca 16 Ma in the Southwest Japan arc: The tectonostratigraphy of the Hokutan Group

“…The termination of graben formation can also be determined in the Kuma area, western Shikoku (Figure 1), where E–W‐trending grabens are blanketed by early middle Miocene volcaniclastic rocks (Kusuhashi & Yamaji, 2001; Ochi, Mamiya, & Kusuhashi, 2014) that have yielded a K–Ar age and zircon U–Pb ages of ca 15 Ma (Shinjoe, Orihashi, & Anma, 2019; Takehara et al, 2016; Takeshita, Tanaka, & Itaya, 2000). K–Ar and fission‐track ages of ca 16 and ca 18 Ma, respectively, have been obtained from the graben fill (Kashima & Takechi, 1996; Narita et al, 1999; Takeshita et al, 2000).…”

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

“…Miocene igneous rocks are also distributed in the present fore-arc region of the SW Japan arc. In the near-trench region of this arc (the Outer Zone), that is, z to the south of the Median Tectonic Line (MTL in Figure 1), felsic volcanic-plutonic complexes were emplaced at 13.5-15.5 Ma (Shibata, 1978;Shinjoe, Orihashi, & Anma, 2019;Sumii, 2000) into a Cretaceous to Miocene accretionary prism (Figure 1). Synchronous with this near-trench magmatism, volcanism took place in the Setouchi volcanic belt, which is characterized by the occurrence of volcanic rocks such as pitchstones, garnet-bearing dacites, and phenocryst-poor, sparsely plagioclase-phyric andesites and basalts, referred to as sanukitoids including HMAs.…”

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

“…Shinjoe et al (2019) report on the U–Pb zircon ages, the Sr–Nd isotope geochemistry, and the major- and trace-element geochemistry of 15.5–13.5 Ma (middle Miocene) granitoid intrusions in the accretionary prism complexes along a 150-km-wide and 600-km-long zone between the Kyushu Island to the SW and the Kii Peninsula of the Honshu Island to the NE in Japan (white box with number 4 in Fig. 1).…”