Cretaceous granitic rocks, consisting of Itoshima granodiorite and Sawara granite, are occurred at upper reach of the Nakagawa river, Fukuoka Prefecture. The Sawara granite that is divided lithologicaly into fine-grained facies and the main facies indicates Rb-Sr whole rock isochron ages of . . Ma and . . Ma, respectively. Both granites also suggest different initial Sr isotopic ratios and geochemical features, indicating they have been originated from different source magmas. Rb-Sr mineral isochron ages using biotite of the main facies of the Sawara granite and the Itoshima granodiorite are .. Ma and . . Ma, respectively. These ages represent the time when the rocks cooled down to closure temperature of biotite. As a result, activity of the Cretaceous granitic rocks in this area is estimated as follows. The magmas of the Sawara granite fine-grained and main facies have been intruded into the Itoshima granodiorite during to Ma, and temperature of the granodiorite have been heated to over the closure temperature of biotite Rb-Sr system. After that, the rocks in the area gradually cooled down to the closure temperature of biotite around Ma.
Abstract
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