SHRIMP U-Pb dating of detrital zircons from the Sanbagawa Belt, Kanto Mountains, Japan: need to revise the framework of the belt

Abstract

Radiometric ages of detrital zircons in three samples of psammitic schists from the Sanbagawa Belt, Kanto Mountains, were obtained from the 238 U/ 206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe (SHRIMP II). Most of the zircon ages cluster around Cretaceous, with a few ages corresponding to older zircons. The origins of the detrital zircons are mainly Cretaceous igneous rocks. The ages of the youngest zircons in samples AM48p, SnbE, and AM29p indicate Late Cretaceous time, and they are 78.8 ± 1.3 Ma, 91.4 ± 1.4 Ma, and 95.3 ±1.5 Ma, respectively. The samples AM48p and AM29p have K -Ar ages of 65.9 ± 1.4 Ma and 82.1 ± 1.8 Ma, respectively. The age difference between the youngest detrital zircon age and white mica K -Ar age is 13 Myr. The Sanbagawa Belt is believed to be a metamorphosed phase of the Chichibu Belt, which is a Middle Jurassic to earliest Cretaceous accretionary complex; however, the results of this study suggest that the protolith of the Sanbagawa belt was accreted in Late Cretaceous, similar to the Shimanto belt that runs parallel to the Sanbagawa and Chichibu belts.

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

“…They have a few Paleoproterozoic grains, and the Permian to Late Cretaceous age groups contain few or no 150–139 Ma zircon grains (Figures and ). These features are common to previous studies of detrital zircon geochronology in the Sanbagawa metamorphic complex such as the Southern unit of the Kanto Mountains (Tsutsumi et al, ) and the Iyo unit in western Shikoku (Knittel et al, ).…”

“…The Sanbagawa metamorphic complex of Southwest Japan probably represents a deeper counterpart of the Cretaceous Shimanto accretionary complex, although they might have been formed at laterally disparate parts of the same subduction zone (Aoki, Maruyama, Isozaki, Otoh, & Yanai, ; Kiminami, ; Tsutsumi, Miyashita, Terada, & Hidaka, ; S. R. Wallis & Okudaira, ). The coeval Sanbagawa‐Shimanto pair represents one of the best‐preserved examples of the structural‐stratigraphic and metamorphic architecture developed above a young and warm slab.…”

Coexistence of jadeite and quartz in garnet of the Sanbagawa metapelite from the Asemi–gawa region, central Shikoku, Japan

“…Hirajima (1984) considered Yorii jadeite-bearing rocks to be members of the Kurosegawa belt. Fukuyama et al (2013) reported U-Pb zircon ages of 162.2 ± 0.6 Ma from the Yorii samples, which are distinctly older than the Late Cretaceous Sanbagawa metamorphism (about 80-95 Ma U-Pb detrital zircon ages: Tsutsumi et al, 2009). The K-Ar phengite ages of metapelite from the Sanbagawa metamorphic belt yield about 58-84 Ma, which is comparable to the U-Pb detrital zircon ages (Hirajima et al, 1992;Miyashita and Itaya, 2002).…”

Jadeite–bearing metaigneous rocks from the Northern Chichibu belt, SW Japan: implications for the lowest–grade Sanbagawa metamorphism

“…1a). The Sanbagawa metamorphic belt is mainly derived from Cretaceous accretionary complex (e.g., Tsutsumi et al, 2009). The Chichibu belt, in regions where the Kurosegawa zone is present, is subdivided into the Northern and Southern belts.…”

“…Recent detrital zircon geochronology has revealed that the Sanbagawa belt s.s. is not metamorphosed Jurassic accretionary complex but metamorphosed Cretaceous accretionary complex (e.g., Tsutsumi et al, 2009). This implies that the Sanbagawa metamorphism of the Northern Chichibu belt (Jurassic accretionary complex) took place in the hanging-wall of the Cretaceous subduction zone.…”