長野県北東部烏帽子岳とその周辺の地質と火山形成史

抄録

Mt. Eboshidake is located in the western part of the Eboshi volcano group (western Eboshi volcano group), central Japan. According to the result of geological survey and the petrographic features, the western Eboshi volcano group is composed of 9 volcanic units such as; Iwayakan’non, Kokuzo, Kanahara, Omuro, Ofuji, Gotomiki, Lower Eboshidake, Narahara, and Upper Eboshidake lavas. Based on the newly measured paleomagnetic polarities and reported K-Ar radiometric ages, the volcanic activity of the western Eboshi volcano group was reconstructed as follows. The volcanic activity started about 1 Ma and during the early Pleistocene age, the volcanism was characterized by eruptions of thin basaltic andesite - andesite lavas from scattered vents, which formed small volcanic edifices. Then thick andesite - dacite lavas erupted in the initial period of middle Pleistocene, and formed clusters of lava domes. After that, the volcanic activity of the Eboshi stratovolcano started about 0.8 Ma, and after the dormant period (around 0.4 Ma) when Iwashimizu gravel bed of about 30m thick with two marker tephra beds was piled up, the basaltic andesite - andesite lavas erupted again until about 0.4-0.3 Ma. These lavas erupted from central vent and formed main edifice of Eboshi stratovolcano. The whole rock chemistry of 69 samples from the western Eboshi volcano group and eight samples from the eastern Eboshi volcano group was analyzed using XRF. The result shows that the rocks of the western Eboshi volcano group are classified into the basaltic andesite - andesite group (SiO₂ : 51-58%) and the andesite - dacite group (SiO₂ : 59-64%). The basaltic andesite - andesite group is mainly tholeiitic and more depleted in MgO content than the calc-alkaline andesite - dacite group. Most of these rocks are plotted in the medium-K field on the K₂O vs. SiO₂ diagram. This feature is similar with the other volcanoes in the eastern Eboshi volcano group and the Asama volcano.