Detailed developing processes of the northeast Honshu magmatic arc associated with Kuroko formation

Abstract

The authors have been studying the detailed evolution history of the northeast (NE) Honshu magmatic arc in the late Cenozoic age. The magmatic arc was constructed by back-arc spreading between 21-18 Ma (Yamato basin), the subsequent rifting between 19-13.5 Ma (Northern Honshu rift system, NH rift) and island-arc uplifting up to the present. The Yamato basin is entirely composed of sea floor basalt lavas with dolerite sheets. The NH rift is subdivided into the Aosawa, Babame, Ani and Kuroko volcanic belts going from west to east. These volcanic belts consist of bimodal volcanism with increasing felsic activity to the eastern volcanic belt through time. The island-arc volcanism is characterized by caldera forming intermediate to felsic activity accompanied by island-arc uplifting. The NH rift volcanism started from high-Ti basalt at early stage to low-Ti tholeiitic basalt at the main stages similar to the Yamato back-arc basin. The chemical composition of the NH rift basalt changes laterally from back arc basin type (BABB) in the west (Aosawa volcanic belt) to island-arc tholeiite type (1AT) in the cast (Kuroko volcanic belt). The Kuroko volcanic belt consists of two units of bimodal volcanism. The rift volcanism (pre-ore unit) began at 16.5 Ma as preceding basaltic activity and was followed by extensive felsic hyaloclastite mainly composed of dacitic pyroclastics with rhyolite lava domes. Kuroko deposits were generated at about 14.3 Ma subsequent to the rhyolite intrusion. The island-arc volcanism (post-ore unit) started intermittent basalt eruptions intercalated with mudstone at 13.5 Ma and was followed by felsic pyroclastics with several dacite lavas up to 10 Ma. A pumice explosion with caldera formation characterizes the post-ore felsic activity. Pre-ore felsic volcanics in the Kuroko volcanic belt are petrographically characterized by aphyric or plagioclase phyric aphanitic features, whereas post-ore dacite often contains quartz phenocrysts. The variation of the normative composition on Qz-Ab-Or-An diagram indicates that the pre-ore dacite was generated by a deeper origin (equilibrated at 10-15 km) than that of the post-ore dacite (probably less than 5 km). Nb contents of the pre-ore rocks including basalt and andesite are plotted on a linear trend on Nb-SiO/sub 2/ diagram, whereas those of the post-ore felsic rocks are concentrated within an independent domain characterized by Nb depletion. All petrographical and petrochemical signatures suggest that the pre- and post-ore volcanism, in other words, rift and island-arc volcanism, were caused by a magma with different origins. The difference would result in the transition from back-arc rift to island-arc tectonic setting. Kuroko deposits were generated at the transitional period in relation to the final stage of rifting and felsic dominant bimodal volcanism in the farthest volcanic belt from the back-arc spreading axis. Moreover, the deposits are genetically related to the Nb enriched aphyric dacite.