Origin and tectonic relationship of metagabbro of the Sambagawa Belt, and associated Karasaki mylonites of western Shikoku, Southwest Japan

Abstract

The Sambagawa Belt is a subduction-related high-pressure intermediate type metamorphic complex with its protoliths characterized by the ocean plate stratigraphy. The Iyo-nada Sea area of this belt at western Shikoku, Southwest Japan exposes lenticular bodies of metagabbro. Besides, low P/T type metamorphosed Karasaki mylonites occur in this area as an upper structural unit of the Sambagawa Belt. LA-ICP-MS zircon U-Pb dating of the metagabbros in the Sambagawa Belt revealed their crystallization ages of 139–135 Ma as a magmatic event. Trace element composition of zircon suggests that the metagabbros were formed under the arc tectonic setting. Metagabbros do not show any evidence of high P/T metamorphism, instead, they indicate low P/T metamorphism based on the chemical compositional analysis of the hornblende. From the present study, it is constrained that the 139–135 Ma metagabbros were possibly originated as the hanging-wall arc crust. Zircon U-Pb dating of the Karasaki mylonites revealed imprints of multiple tectono-thermal events of 114.3 ± 1.5 Ma (magmatic event), ∼108–105 Ma (metamorphic with or without magmatic event), and 102.4 ± 0.9 Ma (metamorphic event). The first two tectono-thermal events can be correlated with those of the Higo Belt, central Kyushu. However, the last metamorphic event is younger than the tectono-thermal events of the Higo Belt, instead, it coincides with the initial stage tectono-thermal events of Ryoke-San’yo igneous province. Detrital zircon age of the pelitic schist in Karasaki mylonites revealed their origin as Jurassic to earliest Cretaceous sedimentary rock and its age distribution pattern is likely comparable to the previously reported age spectra of Higo metasedimentary rocks and their equivalent rock units. The newly obtained dataset indicates that the Karasaki mylonites are the eastern extension of the late Early Cretaceous Higo Belt.