福島市飯坂町北西方地域に分布する後期中新世火山岩のK-Ar年代

抄録

The authors have examined structures, mode of occurrence, lithology, radiometric ages and arrangement of volcanic bodies to demonstrate the existence of 'Late Miocene to Pliocene volacanic front' in Northeast Japan (e.g. Yashima and Ohtake 1998; Yashima et al. 1999). Ten rock bodies of acidic volcanics have been identified in the northwestern part of the Iizaka district, Fukushima City. Because these lava domes have been weathered and eroded intensively, they are herein called as 'volcanic bodies'. K-Ar dating of six rock bodies of the acidic volcanics gives ages around 8 and 6 Ma, and indicates that the volcanic activity is correlative with the Late Miocene epoch. The Akagawa Formation, which consists largely of non-marine acidic pyroclastic deposits, is distributed around the rock bodies of acidic volcanics. It has been confirmed by field observations that the Akagawa Formation is an eruptive product of a series of volcanic activities that generated the rock bodies of acidic volcanics. Generally speaking, two volcanic processes, that is, deposition of the acidic pyroclastics and intrusion and or extrusion of the acidic volcanics, seem to have proceeded alternatively for a long period of time. The authors regard the newly aquired age data as a supportive evidence of the possible existence of the Late Miocene to Pliocene volcanic front.