Oceanic island basalts in accretionary complexes of SW Japan: Tectonic and petrogenetic implications

Abstract

The paper presents the results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of early Carboniferous to Miocene volcanic rocks of the Akiyoshi, Mino–Tamba, Chichibu and Shimanto accretionary complexes of Japan in combination with an analysis of their geological relations with sedimentary units and their petrogenesis (melting modeling). Most of the samples are oceanic island basalts (OIBs) associated with Carboniferous–Cretaceous sediments of Ocean Plate Stratigraphy (OPS): carbonate “cap” limestones, hemipelagic siliceous mudstone and pelagic radiolarian chert. The basalts are enriched in TiO2, LREE (La/Smn=1.9), and Nb (Zr/Nbav.=10), have differentiated HREE (Gd/Ybn=2.0), medium to low εNd (7.8–2.2) and are characterized by clear Nb positive anomalies in multi-element plots (Nb/Thpm=1.3, Nb/Lapm=1.1). The Carboniferous–Cretaceous OIBs originated at relatively low degrees of melting (<5%) from mantle sources in the spinel and garnet stability fields and erupted in an oceanic island setting. The Shimanto AC also includes Miocene andesites having lower TiO2 (0.7wt.%) and other HFSE (Hf, Y, Zr, P2O5 and Nb), strongly negative Nb anomalies in multi-element diagrams and higher εNd values (average 6.8–7.4). They may be derived from high degree melting of depleted harzburgite and probably erupted in an intra-oceanic arc setting. The studied OIBs suggest an about 250Ma period of intra-plate magmatism in the Paleo-Pacific Ocean probably related to mantle plume activity.