非持トーナル岩のRb-Sr, Sm-Nd年代とその意義(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] Rb-Sr, Sm-Nd ages and its significance of the Hiji tonalite in the Ina district of the Ryoke belt, Southwest Japan Arc(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

伊那地方領家帯に分布する最古期の領家花崗岩である非持トーナル岩のRb-SrおよびSm-Nd年代測定を行い, 活動時期の検討を行った.縞状構造の発達する試料は, 69.9±8.1Maと63±13Maの全岩アイソクロン年代を示すが, これらは縞場構造の形成時期を示してはいない.5試料から分離した黒雲母, フェルシックフラクションによるRb-Sr鉱物アイソクロン年代として, 59.1±0.1Ma, 64.4±0.03Ma, 61.8±0.1Ma, 64.3±0.3Ma, 63.7±0.04Maを得た.これらの年代は, 非持トーナル岩の冷却時期を示す.さらに, 普通角閃石およびフェルシックフラクションによるSm-Nd鉱物アイソクロン年代として, 161.3±1.4Ma, 124±18Ma, 164±18Ma, 86±27Ma, 125.1±8.6Maを得た.若い年代(124Ma, 86Ma, 125.1Ma)は, 新期花崗岩による熱的影響の時期を示しているかもしれない.非持トーナル岩の活動時期は, 160Maもしくはそれよりも古いと考えられる.非持トーナル岩のNd進化線は, 下部地殻の進化範囲と領家花崗岩Nd同位体比初生値の範囲内にある.

Rb-Sr and Sm-Nd geochronological analyses were carried out for the Hiji tonalite which is the oldest granitic body in the Ryoke granitic rocks in order to determine the timing of igneous activity. The samples having compositional layering give Rb-Sr whole-rock isochron ages of 69.9±8.1Ma and 63±13Ma. These ages do not indicate the time of formation of layering. Biotites and felsic fractions separated from five samples give Rb-Sr mineral isochron ages of 59.1±0.1Ma, 64.4±0.03Ma, 61.8±0.1Ma, 64.3±0.3Ma and 63.7±0.04Ma. These ages indicate timing of cooling of the Hiji tonalite. Sm-Nd isochron ages defined by hornblende, felsic fraction and whole-rock sample are 161.3±1.4Ma, 124±14Ma, 164±18Ma, 86±27Ma and 125.1±8.6Ma. The younger ages (124Ma, 86Ma, 125.1Ma) indicate the timing of thermal effect of the younger granite. The time of igneous activity of the Hiji tonalite approximates 160Ma or older than 160Ma. The Nd evolution line of the Hiji tonalite is in evolution range of lower crust estimated by granulite xenoliths included in Miocene volcanic rocks and that of initial Nd isotopic ratios of the Ryoke granite.