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| Abstract |
Rb and Sr concentrations, and Sr isotopic compositions were determined for granitic cobbles in conglomerates of Middle and Upper Permian formations in Central Kyushu, Southwest Japan. Rb/Sr ratios in ...these granitic cobbles range extensively from those in granites dredged from the Kyushu-Palao ridge and also in granites associated with serpentinites of the Nagato tectonic zone to those in batholithic granites in North Kyushu. Sr isotopic compositions of these cobbles with 87Rb戸Srratios less than 0. 3 are homogeneous, being limited in a range of 0. 70545士0.00015. The cobbles from the Upper Permian Mizukoshi Formation give an average whole rock age of 168±28m.y. with initial ratios of 0. 70508±0.00044. Data for other cobbles are plotted near this isochron of 168m. y. in the 87Rb房Srversus 87Sr /86Sr diagram. This radiometric age is in conflict with the previously reported fossil evidence, about 60 m. y. younger than the paleontological estimation.show more
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