K-AR DATING ON ACIDIC ROCKS FROM THE WESTERN AIZU DISTRICT, FUKUSHIMA PREFECTURE

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  • 西会津グリーンタフ地域における酸性岩類のK-Ar年代
  • ニシ アイズ グリーン タフ チイキ ニ オケル サンセイガンルイ ノ K A

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Abstract

K-Ar age determinations were carried out on twelve samples of various acidic rocks (six volcanic rocks, two pyroclastics and four granitic rocks) which were obtained from the western part of Aizu district. The district studied is one of the important acidic petrographic provinces in the Green tuff region of Northeast Japan, and is widely covered by the acidic volcanic rocks and pyroclastics of Neogene period.<br> The ages of six volcanic rocks range from 8 to 23m. y., and they are generally correlated to the stratigraphic units of the Neogene in Northeast Japan. Dating results on four granitic rocks from the Tagokura granitic body showed the age range of 39 to 65m. y., corresponding to the Late Cretaceous to Eocene. A sample of dacitic welded tuff from the Miyako River area gave an age of 44m. y. It is pointed out that the welded tuff may be correlated to the Late Cretaceous to Paleogene acidic igneous rocks such as Nohi rhyolites, Asahi rhyolites, Tagawa acidic rocks and others, on the basis of the age and lithofacies of the rock. However, further geological and geochronological data are necessary to settle the problem.

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