The most continent-sided occurrence of the Phanerozoic subduction-related orogens in SW Japan: Zircon U-Pb dating of the Mizoguchi gneiss on the western foothill of Mt. Daisen volcano in Tottori

Abstract Understanding the pre-Cenozoic geotectonic framework along the Japan Sea coast in southwest Japan is critical in clarifying the relationship between oceanic subduction-related and collision-related geotectonic belts along the continental margin of East Asia. Although a thick Neogene-Quaternary volcanic-sedimentary cover often conceals the pre-Cenozoic units, the Western Honshu is one of the pivotal regions especially the Mizoguchi gneiss (high-grade gneisses up to amphibolite facies with Rb-Sr isochron age of 185 Ma) exposed near Mt. Daisen in western Tottori prefecture. This unit, together with a neighboring 199 Ma granitoid, was previously correlated with the Hida gneiss in Central Japan, located nearly 250 km to the east. The present study examines this interpretation using zircon U-Pb geochronology. Ages of three gneissose rocks of the Mizoguchi gneiss reveals that the protolith is composed of and represented by granitoid, accretionary complex, and/or high pressure/low temperature (HP/LT) type metamorphosed units. In terms of protolith rock type and zircon age spectra, the Mizoguchi gneiss is different from the Hida gneiss that is interpreted to have been derived from sedimentary rocks/volcanics with continental shelf, and also from the Oki gneiss characterized by its preservation of abundant Precambrian detrital zircon grains. The high-grade metamorphism of the Mizoguchi gneiss is interpreted to have occurred after middle Jurassic as constrained by the youngest detrital grain. The protolith of the Mizoguchi gneiss belongs to the Suo belt (Jurassic HP/LT type metamorphosed accretionary complex) in southwest Japan; however, the unit is interpreted to have been metamorphosed by a Jurassic thermal event, which overlaps that of the Jurassic felsic magmatism in the Hida belt (the Funatsu episode). The Mizoguchi gneiss in western Honshu is unique, because it marks the northernmost or the most proximal occurrence of Phanerozoic subduction-accretion complex in Southwest Japan, clearly on the south of non-accretionary Oki and Hida belts.