Radiolarian occurrence and origin of basaltic rocks in the Hebiki Unit, Northern Chichibu accretionary complex in the Kanto Mountains, Central Japan

Abstract

Reconstructing ocean plate stratigraphy (OPS) and revealing the places of origin and ages of oceanic rocks of the Jurassic–Early Cretaceous accretionary complex in Eastern Asia are important to clarify the accretion process and the paleogeography and tectonic setting of the Panthalassa Sea during the Paleozoic to Mesozoic times. In this context, many geological investigations of the Middle Jurassic–Early Cretaceous units of the Northern Chichibu accretionary complex (except for an Early Jurassic one) and Mikabu Unit have been performed. In this study, geological mapping, faunal analysis of radiolarians, and geochemical analysis of basaltic rocks were conducted for the Hebiki Unit of the Northern Chichibu accretionary complex to obtain prime geological information such as its stratigraphy, ages, and origin of basaltic rock. The Early Jurassic Hebiki Unit consists of chaotic mélange, which contains sandstone, siliceous shale, chert, and basaltic rocks as blocks in a highly sheared shale matrix. Late Permian and Late Triassic radiolarians were newly identified from chert blocks in this study. Based on the radiolarian evidence from this study and previous ones that identified Jurassic radiolarians from siliceous shale, the OPS of the Hebiki Unit are reconstructed as follows: Upper Permian and Upper Triassic (upper Carnian to Rhaetian) chert, and Lower Jurassic (Pliensbachian) siliceous shale in ascending order. Meanwhile, the basaltic rocks of the Hebiki Unit are divided into two types in terms of the major and trace element composition and REE profiles: low and high Nb/Zr types. The low and high types are interpreted as oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB), respectively. Considering the reconstructed OPS of the four tectonostratigraphic units (Kashiwagi, Kamiyoshida, Sumaizuku, and Hebiki) of the Northern Chichibu accretionary complex, the four units are considered to have accreted successively.