南部阿武隈山地の宮本複合岩体のRb-Sr年代

抄録

Rb-Sr chronological study has been carried out on the Miyamoto tonalitic complex to the east of Iwaki city, Fukushima prefecture, northeastern Japan. The rock facies of the complex can be grouped into three in terms of petrological understanding: they are gabbro group, tonalite group, and adamelite group in order of intrusion. The Rb and Sr abundances and Sr isotopic compositions were analyzed for gabbro group rocks (14), tonalite group rocks (8), and adamelite group rocks (5). The mineral separates from one of the gabbro group rocks were also analyzed.
The Rb-Sr age of the tonalite group is 120 Ma, and the initial ⁸⁷Sr/⁸⁶Sr ratio is 0.7052; the results are similar to those of the tonalitic plutonic complexes distributed nearby. The adamelite group yielded 119 Ma as the solidification age with teh inital ⁸⁷Sr/⁸⁶Sr ratio 0.7049. The age is identical to that of the tonalite group, but the initial ⁸⁷Sr/⁸⁶Sr ratio is different between them. Although the tonalite and adamelite group magmas intruded successively and solidified, the origin could largely differ between the two groups.
The gabbro group gave no consistent whole-rock age, and neither did the mineral separates. Excluding biotite, the remaning mineral separates, plagioclase, hornblende, and the whole rock make an isochron with an angle indicating the age of 100 Ma with an uncertainty of 13 Ma. The significance of the obtained mineral age, however, is unclear. The Rb-Sr system of the gabbro group seems considerably disturbed by alteration.