福島市南西部に分布する鮮新世「笹森山安山岩」のK-Ar年代

抄録

Pliocene "Sasamoriyama andesite" is distributed in the southwestern margin of the Fukushima City, consisting of the volcanic front of northeastern Honshu arc at that period. We have obtained nine K-Ar ages from the "Sasamoriyama andesite". Three samples located in the southeastern part of the distributed area are about 3.7 Ma and 3.0 Ma in K-Ar ages. The other six samples situated in the northwestern part are all younger, having K-Ar ages between 2.4 and 2.0 Ma. Consequently, Pliocene volcanic activity had been started in southeastern part at 3.7 Ma. And after that, volcanic activity of 2.4 to 2.0 Ma moved northwestern direction to the back-arc side. The older volcanic rocks mainly consist of pyroxene andesite lavas containing large anorthite phenocrysts, whereas the younger volcanic rocks consist of pyroxene andesite without anorthite phenocrysts. Both of them are classified into medium-K, low-alkali suites. However, the older rocks contain both of tholeiite and calk-alkali rocks, while the younger rocks contain only calk-alkali rocks.