SHRIMP dating of magmatism in the Hitachi metamorphic terrane, Abukuma Belt, Japan: Evidence for a Cambrian volcanic arc

Abstract

Ion microprobe dating of zircon from meta-igneous samples of the Hitachi metamorphic terrane of eastern Japan yields Cambrian magmatic ages. Tuffaceous schist from the Nishidohira Formation contains ca 510Ma zircon, overlapping in age with hornblende gneiss from the Tamadare Formation (ca 507Ma), and meta-andesite (ca 507Ma) and metaporphyry (ca 505Ma) from the Akazawa Formation. The latter is unconformably overlain by the Carboniferous Daioin Formation, in which a granite boulder from metaconglomerate yields a magmatic age of ca 500Ma. This date overlaps a previous estimate for granite that intrudes the Akazawa Formation. Intrusive, volcanic, and volcaniclastic lithologies are products of a Cambrian volcanic arc associated with a continental shelf, as demonstrated by the presence of arkose and conglomerate in the lowermost Nishidohira Formation. Granitic magmatism of Cambrian age is unknown elsewhere in Japan, except for a single locality in far western Japan with a similar geological context. Such magmatism is also unknown on the adjacent Asian continental margin, with the exception of the Khanka block in far northeastern China. A 'great hiatus' in the Paleozoic stratigraphy of the Sino-Korean block also exists in the Hitachi terrane between Cambrian volcanic arc rocks and Early Carboniferous conglomerate, and may indicate a common paleogeographic provenance.