The Interpretation of Discordant Ages of Some Granitic Bodies

Abstract

Comparison of mineral ages in some granitic bodies obtained by the K-Ar, Rb-Sr and fissiontrack methods with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved.
The discordant radiometric ages are obtained from the central parts of Rokko Nunobiki and Mikumo plutons, Tanakami stock and Kumano acidic rock body. These granitic plutons and stock have begun to cool slowly compared with Kumano acidic body. The granite porphyry facies of Kumano acidic body is cooled more rapidly than the other granitic bodies, but this body has begun to cool slowly at first as uplift and then progressively quicker as the surface is approached.
Intrusion of the Kurobe granitic pluton 50 Ma ago caused the age of marginal facies (Nozawa, 1975). K-Ar ages for Kurobe granitic pluton were determined to resolve the cooling history of this pluton. Comparison of K-Ar ages with elevations of sampling sites for the cooling history has made the conclusion that the high temperatures in the tunnel near Sennin Dam along Kurobe River are caused by residual heat from the granitic body to be reached.