Primary relationships of basaltic intrusive rocks and siliceous sedimentary rocks in the Northern Chichibu Belt, Shikoku Island, southwest Japan

摘要

Understanding the primary relationships between basaltic rocks and fossiliferous sedimentary rocks provides direct information on the timing and geologic setting of past oceanic igneous activity. Though the basalts associate with cherts in the Northern Chichibu Belt have been petrographically identified as OIA/ 01T or MORB, such primary relationships of them are not well understood. This paper provides the primary intrusive relation of basaltic rocks contact with chert and calcareous siliceous shale of Sugyo area in the Northern Chichibu Belt on Shikoku Island. The sediments are included within the basalt as xenoliths and in the contact zone with the basalt they have been recrystallized and contain metamorphic minerals possibly produced by contact metamorphism due to the basalt intrusion. These indicate that the basalts are sills or dykes that have intruded the sediments. The cherts are radio-larian chert which alternate with the calcareous siliceous shale and include minerals of oceanic volcanic origins. Asselian radiolarian fossils are yielded from a subangular chert xenolith that was intruded by the basalt possibly before the chert had undergone complete diagenesis. The petrography of the sills indicates they are alkali basalts. X-ray fluorescence analyses indicate that the basaltic rocks include subalka-line basalts and oceanic island tholeiite or oceanic island alkaline basalts. We have concluded that the basaltic dykes and sills were products of Asselian or younger intra-plate volcanism at an oceanic area covered by a pile of pelagic siliceous sediments. These Asselian cherts are earlier than the Sakmarian, Wordian and Capitanian limestones that overlie intra-plate basalts of Ohnogahara part of the Northern Chichibu Belt. The basalt sills may therefore represent the early stages of intra-plate igneous activity in the paleo-Pacific Ocean.