K-A dating on the igneous rocks in Japan (II)

Granitic rocks in Kitakami massif

Abstract

In the Kitakami-massif of north-eastern Japan, there develope many granitic masses, and they have been grouped into two types from their geological relation, i.e. older type of pre-Cretaceous and younger type of Cretaceous age. From these 23 rock masses, 52 samples were collected for K-A dating experiment. As a result, it is ascertained that the granitic rocks of older type would have emplaced more than 222×10⁶ years ago at least, though the value does not show the true age, because of the thermal effect of younger granitic intrusion. On the other hand, it is clarified that most of the younger granitic rocks intruded 120×10⁶ years ago, though some of them show the ages of 128×10⁶ years and 106×10⁶ years.
In conclusion, the underground structure of the Kitakami-massif is considered to be composed of one granitic batholith.