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Manuscript received, September 27, 1976.
Geological Survey of Japan, Kawasaki 213, Japan.
Department of Geology, Kyushu University, Fukuoka 812, Japan.

Abstract

Mesozoic igneous rocks from Southwest Japan are critically reviewed in terms of isotopic and stratigraphic ages, and new K-Ar and Rb-Sr age results on the Cretaceous granitic and volcanic rocks from the Kitakami Mountains and some other areas are presented and discussed. Cretaceous granites from the western half of the Inner Zone of Southwest Japan are bounded between the Albian Shimonoseki Subgroup and the Campanian and Maestrichtian Izumi Group. The K-Ar ages of the granites generally agree with the stratigraphic evidence, and indicate that the granites were uplifted and eroded soon after the intrusion. Granites and related Cretaceous formations become younger eastwards, suggesting that a site of plutonism, associated with a site of volcanism and sedimentation, migrated eastwards at a rate of 2.6 cmyear. The isotopic ages of granites from Amami-oshima and those of the Funatsu granite also indicate a rapid uplift and erosion subsequent to the granite intrusion.

The Miyako and Taro granites from the Kitakami Mountains, which are stratigraphically bracketed between the Neocomian and the upper Aptian, give Rb-Sr whole-rock isochron ages of 121 ±6 m.y. and 128 ± 12 m.y. respectively, and agree with the stratigraphic evidence. Seven mineral samples of the Miyako granite give an average K-Ar age of 113 ± 3 m.y., suggesting that the Aptian-Albian boundary may be slightly younger than 113 m.y. Cretaceous volcanic rocks from the Kitakami Mountains give K-Ar whole rock ages of 93-119 m.y., all slightly younger than stratigraphically estimated. Ages of the Harachiyama and Kanaigaura Formations are equal to those of the Miyako granite, and indicate either the contact effect of the granite or contemporaneity with the granite. Lower ages of the Niitsuki and Yamadori Formations may be attributed to the alteration. Volcanic rocks of the Sennan acid pyroclastic rocks in Kinki and those of the Mifune Group in Kyushu yield much younger K-Ar whole rock ages than estimated. A biotite from tuff in the Middle Yezo Group in northwest Hokkaido is dated at 91.4 ± 2.4 m.y., which agrees with the stratigraphically assigned Turonian age.

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