Petrogenesis and U-Pb zircon geochronology of alkali volcanic rocks in the Akiyoshi Terrane, SW Japan and their origin

Abstract

The present study reports the geochemical characteristics and new precise zircon U-Pb age of alkali basalt to trachyte that form the basement of the reef limestones of the Akiyoshi Terrane, Southwest Japan. The rocks of the Yamano Group (part of the Permian Maizuru Terrane) in the Yakawa area, eastern Hiroshima Prefecture are unconformably overlain by the Inakura Formation of the Cretaceous Kanmon Group. The latter contains limestone blocks and is accompanied by underlying volcanic rocks that are extremely vesicle-rich and highly altered. We carried out detailed petrographic studies on one trachytic rock, and three basaltic rock samples from the Yakawa area. Apart from that, we also have studied two basaltic samples from the Akiyoshi-dai area, one from the Atetsu-dai area, one from the Oga-dai area, and one associated with the Hina Limestone. Geochemically, except for one trachyte-phonolite sample, all other samples from Yakawa, Hina Limestone, Oga-dai, and Atetsu-dai areas are alkali basalt, while samples from Akiyoshi-dai are basalt. The U-Pb zircon age of the trachytic sample from the Yakawa area yielded a crystallization age of 335.8 ± 1.6 Ma. The tectonic discrimination diagram indicates that the samples of the Yakawa area and Hina Limestone are of French-Polynesia-type superplume origin, while the rest are of normal hotspot region affinity. Zircon geochemistry also corroborates the fact that their origin was in the hotspot-type oceanic island.