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The Rb-Sr isochron ages were determined for the granitic rocks in the Tsukuba district, Japan. The Coarse-grained granite gives a mineral isochron age of 60.2 ± 0.4 Ma and whole rock isochron age of 61.6 ± 4.3 Ma. The Porphyritic granodiorite has a mineral isochron age of 59.5 2.7 Ma. Pegmatite dikes in the Yamanoo area, which are the products of final stage granitic activity in the Tsukuba district, yield whole rock and mineral isochron ages of 59.9 ± 4.0 Ma and 58.3 ± 2.3 Ma, respectively. The Rb-Sr ages in this study and published K-Ar mineral ages for the granitic rocks in the Tsukuba district are concentrated in very narrow range, mostly 59-62 Ma, irrespective of intrusive stages, indicating that the magmatic activities in the Tsukuba district occurred during short time interval (less than 3 Ma). Concordance of the Rb-Sr whole rock age and K-Ar mineral ages suggests that the granitic magma cooled rapidly after emplacements.

Rb-Sr ages of granitic rocks from the Tsukuba district, Japan.