Abukuma plutonic rocks in the central to western Abukuma plateau have been divided into ‘older' (weakly foliated and intermediate) and ‘younger'(massive and felsic) granitic rocks, as well as small amounts of gabbroic to dioritic rocks, based on field occurrences and their lithofacies. Although many radiometric ages have been reported for these rocks, it remains unclear whether the gabbroic to dioritic rocks represent the first stage of granitic magmatism or are pre-Cretaceous basement rocks. Moreover, the geochronological discrepancy in K-Ar biotite ages between ‘older' and ‘younger' granitic rocks are still ambiguous. These problems are expected to be resolved by the U-Pb dating of zircon, which has a significantly higher closure temperature, making this one of the best ways to estimate the crystallization age of the plutonic rocks.
We have determined U-Pb zircon ages for six samples of Abukuma plutonic rocks using laser-ablation ICP mass spectrometry. We found that these ages were 104.9 ± 0.9 Ma for the Utsushiga-take gabbroic body, 100.4 ± 0.7 Ma for the Nagaya body, 113.4 ± 0.5 Ma for the Shikayama body, 106.7 ± 0.8 Ma for the Ishimori body, 118.0 ± 0.7 Ma for the Miharu body and 101.9 ± 1.6 Ma for the Hatsumori body.
These results suggest that the Utsushiga-take gabbroic body did not result from first stage granitic magmatism or from magma contaminated by pre-Cretaceous basement rocks, but rather that gabbroic magmatism in this district occurred during the same stage as granitic magmatism. Although our results showed clear geochronological contrast among each granitic rock, there was no significant difference between ‘older' and ‘younger' granitic rocks. These findings indicate that the previous classification system, based only on lithofacies, should be re-examined based on other criteria, such as further field observations of intrusive relationships and/or U-Pb dating of zircon.
The cooling histories of each granitic rock were also estimated by K-Ar, Ar-Ar and U-Pb age. We found that the minimum cooling rate of the Utsushiga-take gabbroic rock at relatively higher temperatures (750-530 °C) was more rapid (~ <200 °C/m.y.) than at lower temperature (530-310 °C) and the other granitic samples (∼ 10-70 °C/m.y.).
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