Zircon U-Pb ages and geochemical characterization of granitic mylonites in the Northern Zone of the Maizuru Belt, northern Okayama area, Southwest Japan

Abstract

The Maizuru Belt in Southwest Japan is a Permian island arc-back-arc basin system that includes fragments of continental massifs ranging in age from the Neoarchean to Triassic. The Northern Zone of the Maizuru Belt is composed mainly of fragmented granitic rocks with subordinate pelitic/psammitic metamorphic rocks, and their geochronological and geochemical characteristics are key for defining the tectonic correlation between the Maizuru Belt and Northeast Asian continents. Previously, occurrences of the Northern Zone of the Maizuru Belt have been restricted to two identified localities; however, we have identified a new Paleozoic igneous-sedimentary rock unit in the Kume area of northern Okayama, and it is confirmed to be part of the Northern Zone of the Maizuru Belt. The weighted mean zircon ²⁰⁶Pb/²³⁸U ages of the tonalite mylonite in this unit are 487.8 ± 2.5 Ma, and those of the granite mylonites are 449.3 ± 3.6, 323.0 ± 3.2, 312.2 ± 2.7, and 299.6 ± 2.6 Ma, respectively; these ages represent the timing of magmatism based on the oscillatory zoned texture of the zircon grains. Geochemically, these granitic mylonites commonly show an arc affinity. The detrital zircon U-Pb age of the pelitic slate in the same unit shows a youngest age of ∼ 278 Ma with two major age clusters of ∼ 380-280 (∼ 67%) and ∼ 530-430 Ma (∼ 28%) with very minor Precambrian detrital zircon grains. These age populations are strongly correlated with the magmatic ages of the granitic mylonites in the same unit and further represent the overall magmatic episodes of the unit. These new data indicate that the timing of magmatism in the Kume area is correlated with that of the Khanka-Jiamusi Massif, suggesting their common evolutionary history along with the Maizuru-Oe area of the Northern Zone of the Maizuru Belt.