Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity of the Hokuroku District, NE Japan

摘要

Felsic volcanism taken place pre- and post-Kuroko mineralization in the vicinity of the Hokuroku district have been chronologically studied based upon dating data and microfossil data. The evolution of the volcanic activities was compared with those in northern part of Honshu. It is suggested that the volcanic sequence in the Hokuroku district is correlated to those of Northern part of Honshu in relation to the general evolution of the arc tectonics and Kuroko deposits were generated in the transitional stage from the volcanism under back-arc environment to that of island-arc environment. The evolution history of the Neogene volcanic activities in northern part of Honshu including the Hokuroku district is summarized as follows: The preceding period to 17Ma is characterized by the terrestrial volcanism composed of the former basaltic activity (25 approx 22Ma) and the later granitoid intrusions (22 approx 17 Ma). The second period during 17 approx 8Ma is subdivided into intermediate to mafic lava and the pyroclastics in the pre- to initial rifting stage (17 approx 16Ma), submarine dacitic lava and massive pyroclastics in the back-arc depression prior to the Kuroko generation (16 approx 13Ma), and intermittent felsic pyroclastics under the up heaving environment of the Kuroko basin and contemporaneous shallow to terrestrial felsic volcanism in the surrounding area (13 approx 8Ma). The following period to 8Ma is characterized by a limited extended acidic volcanism under environment along the marginal part of Kuroko basin and extensive andesitic volcanism under terrestrial condition in the surrounding mountains.