男鹿半島・門前層下部(漸新統),潜岩・加茂溶岩部層のK-Ar年代  [in Japanese] K-Ar dating unravels the volcanic history of the Kuguriiwa Lava and Kamo Lava Members, Oligocene Monzen Formation, Oga Peninsula, NE Japan  [in Japanese]

男鹿半島・第三系門前層下部の潜岩・加茂溶岩部層は従来, 岩系の違い, 層位関係 (下位=潜岩溶岩部層, 上位=加茂溶岩部層) などに基づいて定義されてきた部層である。著者らは戸賀湾北方に分布する火山岩相を潜岩溶岩部層, 戸賀湾南端部〜門前間に分布するものを加茂溶岩部層と呼ぶこととし, それぞれの部層をステージI(下位)〜IV(上位)に細分した。両部層の間で, 火山岩相および記載岩石学的特徴の垂直変化に大きな差は認められない。ステージ区分に従って, 潜岩溶岩部層(試料数:5個), 加茂溶岩部層(試料数:9個)の火山岩類についてK-Ar年代の測定を行った。ステージIの試料からは良好な結果がえられなかったものの, ステージII以降の火山作用は34 Ma前後に始まり, 31 Ma頃に終了したこと, 二つの部層が同時期に形成されたこと, およそ26 Maに始まる真山流紋岩部層の形成期まで, 約500万年間の火山活動が低調な時期(休止期)が存在したこと, また, この休止期が東北日本内帯に広く認められることなどが明らかになった。

The Kuguriiwa Lava and Kamo Lava Members area thick succession of lavas and volcanic lastic rocks in the lower part of the oligocene Monzen Formation on Oga Peninsula, NE Honshu. Classical stratigraphic studies suggested that the Kamo Member overlies the Kuguriiwa Member, and that the two members have a distinct petrography. However, the Nishioga Research Group (1972) and our studies revealed that the two members are contemporaneous and have similar lithologies. We have redefined each member on the basis of the area occupied, and using lithostratigraphy recognized four stages of volcanism, I to IV, in each member. K-Ar dating was used to constrain the timing of these four volcanic episodes: 1) Stage I, before 34 Ma involved eruption of trachytic andesite and dacite to rhyolite lavas and accompanying pyroclastics. 2) Stage II, ca. 34 Ma, was characterized by the eruption of alkaline basalt to basaltic andesite lava, with ages ranging from 34.5 to 33.8 Ma. 3) Stage III, ca. 33 Ma involved eruption of calc-alkaline basaltic andesite lava, ranging in age from 33.6 to 32.5 Ma. 4) Stage IV pyroxene andesites have ages around 31.5 Ma. After along period of weak volcanism, eruption of the Shinzan Rhyolite Member of the Monzen Formation started at around 26 Ma. This long dormant period of about 5 m.y. is common along the back-arc side of the northeast Japanese arc. Dormancy can be related to drifting of northeast Japan away from the volcanic front, perhaps due to crustal thinning accompanying the opening of the Japan Sea.