Parentage of low‐grade metasediments in the Sanbagawa belt, eastern Shikoku, Southwest Japan, and its geotectonic implications

Abstract

This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.

U–Pb ages of detrital zircons from the Sanbagawa Belt in western Shikoku: Additional evidence for the prevalence of Late Cretaceous protoliths of the Sanbagawa Metamorphics

“…113-88) of the Shimanto accredionary wedge, respectively, which suggests that the Minawa protoliths were formed during an extended period of time. Otoh et al (2010) found zircons dated at 88 ± 10 and 91 ± 9 Ma at the base of the Minawa Formation in roughly the same area thus confirming the general conclusions of Kiminami (2010). Late Cretaceous Sanbagawa metamorphics also occur in the Kii Peninsular (Otoh et al, 2010), the Chubu part of the Sanbagawa Metamorphic Belt (Tsutsumi et al, 2012), in Kyushu (parts of the Nagasaki Metamorphics, Kouchi et al, 2011) and in north-western Shikoku (this study).…”

“…It is recognized that not all Sanbagawa metamorphics have the same protolith age (Tsutsumi et al, 2009;Kiminami, 2010;Tsutsumi et al, 2012) and thus, we suggest to refer to the various units as 'Jurassic Sanbagawa Belt' and 'Early' and 'Late Cretaceous Sanbagawa Belt', respectively. Likewise, the available age data suggest that the Sanbagawa metamorphics have been subjected to metamorphism during a period of time and not at a specific point in time.…”

“…These belts could not be distinguished prior to the advent of single zircon dating due to similar lithologies and metamorphic grade, though Kiminami et al (1999), Kiminami and Ishihama (2003), and Kiminami (2010) already recognized similarities in the geochemical composition of metasediments now considered to constitute the Shimanto Metamorphic Rocks and sediments of the nonmetamorphic Shimanto Belt.…”

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

“…Whole‐rock major element compositions of quartz eclogites and mafic clots from the Gongen area and other lithologies of the Sanbagawa metamorphic belt. Data sources are as follows: Quartz eclogite and mafic clot (Miyamoto et al, ; Utsunomiya et al, ; Yokoyama, unpublished data; this study), metabasalt and metagabbro (Aoya et al, ; Banno, ; Endo et al, ; Ernst, Seki, Onuki, & Gilbert, ; Goto & Banno, ; Nozaki et al, ; Okamoto et al, ; Utsunomiya et al, ; Weller, Wallis, Aoya, & Nagaya, ; Enami, Iwata, and Yokoyama, unpublished data), metasediments (Aoya et al, ; Banno, ; Ernst et al, ; Fujiwara, Yamamoto, & Mimura, ; Goto et al, ; Kiminami, ; Kiminami & Ishihama, ; Kiminami & Toda, ; Utsunomiya et al, ; Zaw Win Ko et al, ; Enami, unpublished data) of the Sanbagawa metamorphic belt. Mfc, mafic clot; Qpd, quartz‐poor domain; Qrd, quartz‐rich domain in quartz eclogite (GE1501a) shown in Figure .…”

“…Whole‐rock minor element compositions of quartz eclogites and mafic clots from the Gongen area and other lithologies of Sanbagawa metamorphic belt in (a) Ti/50‐Zr‐Sr/2 and (b) V–Ba diagrams. Data sources are as follows: Quartz eclogite and mafic clot (Miyamoto, Enami, Tsuboi, & Yokoyama, ; Utsunomiya, Jahn, Okamoto, Ota, & Shinjoe, ; this study), metabasalt and metagabbro (Aoya, Tsuboi, & Wallis, ; Endo, Wallis, Tsuboi, Aoya, & Uehara, ; Nozaki, Nakamura, Awaji, & Kato, ; Okamoto, Maruyama, & Isozaki, ; Uno et al, ; Utsunomiya et al, ; Enami, unpublished data), metasediments (Aoya et al, ; Goto, Higashino, & Sakai, ; Kiminami, ; Kiminami & Ishihama, ; Kiminami & Toda, ; Utsunomiya et al, ; Enami, unpublished data). Mfc, mafic clot; Qpd, quartz‐poor domain; Qrd, quartz‐rich domain in quartz eclogite (GE1501a) shown in Figure .…”