会津盆地南西方の鮮新世博士山火山岩層

概要

論文の詳細を見る
The Late Pliocene Hakaseyama volcanic formation located at the southern part of the Sekiryo zone of the Northeast Japan arc unconformably overlies the dacitic pyroclastics of Pliocene age and is subdivided into the lower and upper units. The lower unit consists of tuffaceous conglomerates whereas the upper unit is of volcanic products, with maximum thickness of about 250 meters, which occur as numerous lava flows and pyroclastics. An augite-olivine basalt specimen from the upper unit yielded the K-Ar ages of 2.81 Ma and 2.57 Ma. The Hakaseyama volcanic rocks are composed of basalts and andesites with SiO_2 contents ranging from 49.06 to 55.74, whose essential phenocrystic minerals are plagioclase, augite, hypersthene and olivine. All the basalts and basaltic andesites belong to the pigeonitic rock series by Kuno (1950). Chemical natures of these volcanic rocks are entirely identical with those of typical island arc tholeiitic series characterized by low total alkali and low K_2O contents. The Hakaseyama volcanic rocks are slightly higher in Na_2O+K_2O and K_2O contents than the Late Pliocene basalts and andesites distributed around the Quaternary volcanic front of the Northeast Japan arc, suggesting that the Late Pliocene volcanic rocks have systematic lateral variation of Na_2O+K_2O and K_2O contents across the arc from the Pacific to Japan Sea side.
1990-05-25