Mineralogical Abstracts from Scientific Papers Published in Japan,

Age Determination 105038 Kawano, Y. & Yuhara, M. Chronological and isotope geological study of Cretaceous granitic rocks, upper reach of the Nakagawa river, Fukuoka Prefecture. J. Geol. Soc. Japan, 114, 218-230, 8 figs., 2 tables (in Japanese with English abstract), 2008. 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 lithologically into fine-grained facies and the main facies indicates Rb-Sr whole rock isochron ages of 101.2 ± 10.9 and 96.6 ± 5.9 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 88.3 ± 0.5 and 88.9 ± 0.2 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 101 to 97 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 88 Ma. (Authors’ abstract)