福岡県那珂川上流,白亜紀花崗岩類の年代学的,同位体岩石学的研究  [in Japanese] Chronological and isotope geological study of Cretaceous granitic rocks, upper reach of the Nakagawa river, Fukuoka Prefecture  [in Japanese]

福岡県那珂川上流域には,糸島花崗閃緑岩と早良花崗岩細粒相および主岩相が分布している.早良岩体細粒相と主岩相は,それぞれ101.2±10.9Maと96.6±5.9MaのRb-Sr全岩アイソクロン年代を示す.また,両者のSr同位体比初生値や化学的特徴は異なり,それぞれ別々のマグマから形成されたと考えられる.早良花崗岩主岩相と糸島花崗閃緑岩から得られたRb-Sr鉱物アイソクロン年代は,それぞれ88.3±0.5Maと88.9±0.2Maであり,これらは岩体が黒雲母の閉鎖温度に到達した冷却年代を示している.これらの事からこの地域の花崗岩類の活動は次のように推定される.101Maから97Maにかけて早良花崗岩細粒相と主岩相が相次いで貫入し,糸島花崗閃緑岩の温度を黒雲母の閉鎖温度以上まで加熱した.その後,これらの花崗岩類は徐々に冷却し,88Maには黒雲母の閉鎖温度を下回った.

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 101.2 ± 10.9 Ma 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 Ma 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.