A Permian brachiopod fauna (the Ishinazaka Fauna), from shale and argillaceous limestone boulders in the basal conglomerate of the Pliocene Kume Formation of Hitachi, central Japan, is here described, and the age and palaeobiogeography of the fauna are discussed. The fossil-bearing boulders are probably derived from the Permian Ayukawa Formation belonging to the Hitachi Palaeozoic rocks. Therefore, the age and palaeobiogeography of the Ishinazaka Fauna suggest age and sedimentary site of the Ayukawa Formation, respectively. The faunal composition of the Ishinazaka Fauna, which consists of 17 species in 16 genera, resembles that of the middle Permian brachiopod fauna of the South Kitakami Belt, northeastern Japan, and thus indicates a middle Permian (Wordian) age. In terms of palaeobiogeography, the Ishinazaka Fauna, which includes both Boreal (antitropical) and Tethyan (tropical) elements, belongs to the Sino-Mongolian-Japanese Province which developed around North China (Sino-Korea) during the Wordian. The Hitachi area, during the Wordian, was probably part of a continental shelf bordering the eastern margin of North China. The Hitachi Palaeozoic rocks, especially the Permian Ayukawa Formation and the underlying Carboniferous Daioin Formation, are considered to be a southern extension of the Palaeozoic rocks of the South Kitakami Belt.
The Hidaka Metamorphic Belt (HMB) is a tilted crustal section of a magmatic arc formed by magmatic activities at 37 and 19 Ma. We investigated the Sr and Nd isotopic compositions of the 37-Ma I- and S-type tonalites distributed in the central part of the HMB to clarify their origins. Emplacement of I-type hornblende-biotite tonalite magma triggered partial melting of metamorphic pelitic-psammitic country rocks, resulting in the in situ formation of migmatitic S-type muscovite-biotite tonalite. The Sr and Nd isotopic ratios of the hornblende-biotite tonalites are increasingly enriched with increasing whole-rock SiO2 concentrations, suggesting that the magma experienced fractional crystallization and assimilated gneissic country rocks The discordance of the Nd isotopic compositions of the hornblende-biotite tonalites compared with the mafic metamorphic rocks in the lower sequence of the HMB indicates that the hornblende-biotite tonalite magma was not derived exclusively from mafic metamorphic sources. Isotopic signatures show that the magma source was likely similar to that of synchronous norites in the Oshirabetsu region.
Low-grade Ryoke belt metamorphic rocks in the Iwakuni district, Yamaguchi Prefecture, are situated at a low structural level in a Jurassic accretionary complex comprising the Kuga Group. In this paper, we report on a method of sample preparation for, and results of, K-Ar dating. Samples obtained from a slate and a pelitic schist were collected for K-Ar dating from the low-grade parts of the Ryoke belt. The muscovite K-Ar dating gave ages of 99.4±2.6 Ma for the slate and 91.5±2.5 Ma for the pelitic schist. These two ages, combined with previously reported age data from this district, yield ages of approximately 99 and 92 Ma for the timing of peak metamorphism and cooling of the Ryoke belt, respectively.