Petrology of Myogazawa and Kazamiyamada volcanic rocks distributed in northern Utsunomiya, central Japan

Abstract

Early to middle Miocene basalt to dacite are distributed in Utsunomiya area, central Japan. These volcanic rocks are divided into two types, Myogazawa andesite (Myo-type) and Kazamiyamada basalt to dacite (Kz-type) based on their geological and geochemical characteristics. K-Ar whole rock ages of the Myo-type and the Kz-type volcanic rocks are reported as corresponds to the epoch of the opening event of Japan Sea. The Kz-type volcanic rocks belong to the tholeiitic rock series, and show Sr-Nd isotopic ratios close to the undepleted (Lithospheric) mantle. From this, it can be considered that the Kz-type volcanic rocks were originated from the undepleted mantle. However, initial Sr isotope ratios (SrI) of the Kz-type display positive correlation with SiO₂, and their initial Nd isotope ratios (NdI) are decreasing with increasing SiO₂. These features indicate that the genesis of the Kz-type volcanic rocks cannot be attributed to simple fractional crystallization of primary basaltic magma, but to an assimilation and fractional crystallization (AFC) process. An AFC model using the granitoid as the assimilant, can successfully reproduce the isotopic and chemical variations of the Kz-type volcanic rocks.
 The Myo-type volcanic rocks show intermediate geochemical characteristics of tholeiitic and calc-alkaline rock series, and have a very high SrI and low NdI. Therefore it is difficult to form the Myo-type volcanic rocks by the assimilation of the granitoid. The Myo-type volcanic rocks are plotted on mixing line between Depleted MORB Mantle (DMM) and subducted sediment beneath the northeast Japan arc in Th/Yb versus NdI and Th/Yb versus SrI diagrams. In addition, the Myo-type volcanic rocks have the Sr-Nd isotopic compositions close to the Quaternary volcanic rocks in central Japan, which have affected by slab-fluid derived from Philippine Sea plate. Accordingly, it is considered that the Myo-type volcanic rocks were also influenced to the slab-fluid. However, the Quaternary volcanic rocks affected by addition of slab-fluid derived from the Philippine Sea plate are absent in this area. The Utsunomiya area was affected by the slab-fluid, before northeast Japan arc was counterclockwise rotated by the opening of Japan Sea.