Abstract
Late Cretaceous to Paleogene felsic volcanic rocks, which are correlated to the Nohi Rhyolites and allied rocks of the Inner Zone of Southwest Japan, are distributed in the northern part of the Ashio Mountains, Tochigi Prefecture. The volcanic mass that presents the largest exposure among these volcanic rocks in this area has been named as the Irohazaka Welded Tuffs distributed in the south of the Lake Chuzenji. On the basis of detailed field and microscopic observations, the Irohazaka Welded Tuffs is divided into seven pyroclastic units and three volcanic stages. Based on the petrochemical characteristics, it is concluded that the magma of erupted welded tuffs formed the zoned magma in three times indicating an upward increase in the felsic components within the magma chamber. The formative states surrounding the magma are estimated from magnetic susceptibility value to be oxidation state in the early stage and reduction state in the middle to late stage. Furthermore, based on the calculated Sr isotopic initial ratio of 0.71173±0.00028 obtained from the Stage II, it is suggested that the formation of the magma erupted during the Stage II suffered a contamination process between source materials and basement rocks before eruption to the surface. In addition, noticing the similarities in the magma mechanism between the Irohazaka Welded Tuffs and large felsic pyroclastic flows in Yellowstone and Taupo, it is inferred that the magma of main pyroclastic units of the Irohazaka Welded Tuffs was formed under a calm environment with a slow speed of crustal strain in a prominent tensional stress field. On the basis of petrological characteristics, volcanic activities of the Irohazaka Welded Tuffs are correlated to the igneous activites of ilmenite series belonging to the Magmatism Stage III (Harayama et al. 1985) corresponding to the Kasagatake Rhyolites and Ohamamiyama Group of the Inner Zone of Southwest Japan in the Chubu district.
