Departmental Bulletin Paper 南部北上帯,氷上花崗岩と大野層のCHIMEモナザイト年代

鈴木, 和博  ,  柴田, 賢  ,  浅原, 良浩  ,  アブドゥルザーラ, イマド カディム  ,  Suzuki, Kazuhiro  ,  Shibata, Ken  ,  Asahara, Yoshihiro  ,  Abdulzahra, Imad Kadhim

27pp.46 - 51 , 2016-03 , 名古屋大学宇宙地球環境研究所年代測定研究部
The geologic relation between the Hikami Granite and the fossiliferous Siluro-Devonian strata in the South Kitakami terrane has long been an open problem. Detailed CHIME dating and elemental mapping of monazite from the southern part (Hinokamiyama type) of the granite disclose ubiquitous development of Late Devonian (365±34 Ma) mantles on Silurian cores. Monazite-free granite samples (Ono type) from the northern part give a 373±35 Ma Rb-Sr whole-rock isochron age with R1=0.70669±0.00037, suggesting to be a possible heat source for the Late Devonian overprint on the Hinokamiyama type. The CHIME monazite dating also revealed (1) the granite at Kusayami-zawa, believed as the part of the Hikami Granite and interpreted to be covered unconformably by the Silurian arkose, is of Cretaceous (123±6 Ma), (2) the Silurian arkose contains Ordovician monazite without Post-Silurian age signature, (3) an arkose bed intercalated in the Devonian Ono Formation contains unequivocal Cretaceous (113±12 Ma) monazite grains, and (4) a porphyroblastic facies, being ascribed sometimes as mylonite, within the Hikami Granite contains Silurian-Late Devonian and Silurian-Late Devonian-Permian zoned monazite grains together with chronologically uniform Silurian and Cretaceous monazite grains. The porphyroblastic facies is likely to be a granitic arkose. These lines of geochronological evidence suggest that the Hikami Granite and the fossiliferous Silurian and Devonian strata are exotic blocks enclosed within a Cretaceous mélange. 南部北上帯の氷上花崗岩とシルル-デボン系の関係を明らかにするためにCHIMEモナザイト年代測定とRb-Sr全岩アイソクロン年代測定を行った.これにより,氷上花崗岩と称されてきた岩体は4.4億年花花崗岩とこれに熱変成を与えた3.6億年花崗岩(大野型の一部),シルル-デボン系境界部付近の1.2億年花崗岩および1.2-1.1億年のアルコースの集合体であることが判明した.くさやみ沢でシルル系アルコースに直接するのは白亜紀の花崗岩であるので,その不整合関係を根拠にした従来の氷上花崗岩=先シルル紀基盤岩説は成立しない.デボン紀大野層下部のアルコースは白亜紀(1.1-1.2億年)の砕屑モナザイトを含む.これらの新知見は氷上花崗岩やシルル紀・デボン紀の化石を含む堆積岩が白亜紀メランジェ中の異地性岩塊である可能性を示唆する.

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