[S-GL18] 日本列島および東アジアの地質と構造発達史

[SGL18-03] U-Pb dating of detrital zircons from the Yezo Group in Sakhalin: Northern extension of the Late Cretaceous fore-arc basin of Japan

*佐藤 友彦¹、磯﨑 行雄²、堤 之恭³、重田 康成³、小玉 一人⁴、長谷川 卓⁵ (1.岡山理科大学基盤教育センター、2.東京大学総合文化研究科宇宙地球科学教室、3.国立科学博物館地学研究部、4.同志社大学文化遺産情報科学調査研究センター 、5.金沢大学理工学域地球社会基盤学類)

キーワード：砕屑性ジルコン、U-Pb年代、前弧海盆、白亜系

The Sakhalin Island exposes Late Mesozoic orogenic units, e.g. accretionary complex, blueschists, and ophiolite, along the Pacific side of Far East Asia, which represent the northern extensions of western Hokkaido. The non-metamorphosed Cretaceous Yezo Group is fairly correlated with that in Hokkaido in terms of detailed bio-, magneto-, and carbon-isotope stratigraphy. The Yezo Group in Sakhalin and in Hokkaido is all together regarded as a typical fore-arc basin of the Cretaceous arc-trench system along the eastern margin of Asia. During the Miocene opening of the Japan Sea, however, these Cretaceous units were separated from the mainland Asia. In order to reconstruct the pre-Japan Sea configuration of the Cretaceous arc-trench system, provenance analysis for the terrigenous clastics of the Yezo Group is inevitable. Using LA-ICPMS, we measured U-Pb ages of detrital zircons in the 5 sandstone samples from the Yezo Group in Sakhalin. A sandstone sample from the lowermost horizon (Ai Fm) contains abundant Early Cretaceous (110-101 Ma) zircons with minor amounts of pre-Jurassic ones. Four sandstones from the uppermost (Krasnoyarka Fm) is enriched dominantly in Late Cretaceous (101-66 Ma) grains. For all samples, their YC1σ ages are consistent with the previous fossil ages; nonetheless, a claimed Albian age of the Ai Fm is re-estimated to be the Cenomanian, whereas the uppermost part of the Krasnoyarka Fm was confirmed to be the Danian based on the present zircon dating for the first time. The stepwise younging in peak age of zircons according to the stratigraphy suggests the continuous replacement of arc granitoid exposures in the Cretaceous magmatic arc in the provenance likely developed on the continent side. This secular trend in zircon age spectra of the Yezo Group in Sakhalin is properly correlated with that in Hokkaido and also with that of coeval sandstones in SW Japan, suggesting a monotonous sedimentation in the fore-arc basin of the Late Cretaceous arc-trench system developed along the Pacific margin, from Sakhalin to SW Japan for ca. 2,500 km in length. Its spatial dimension corresponds to that of the modern representative fore-arc basin in the Java-Sumatra region.