Rb-Sr whole-rock and K-Ar mineral ages of granitic rocks in Japan.

Abstract

Rb-Sr whole-rock ages have been determined on 10 small-sized granitic masses in Japan having simple thermal history. The Rb-Sr ages are 4∼9m.y. older than the corresponding K-Ar biotite ages, except for two Neogene granites which show concordant ages. The Rb-Sr whole-rock ages for these masses probably represent the time of emplacement, whereas the K-Ar biotite ages indicate the time of uplift and cooling, thus the slight discordance in the two age systems may be explained as indicating the cooling period of the granitic masses. The comparison between the published Rb-Sr whole-rock and K-Ar mineral ages for Japanese granitic rocks reveals that the rocks are grouped into three on the basis of the concordance-discordance mode: nearly concordant, discordant with the age difference of about 50m.y., and grossly discordant. Discordant ages for granitic rocks generally have rather large errors in Rb-Sr whole-rock ages, and careful evaluation is needed for these ages.