東北日本,南部北上帯のペルム系・中生界頁岩の供給地の地球化学的検討

南部北上帯の上部ペルム系登米層と中生界の頁岩について主元素,微量元素組成の検討を行い,JSl-1, JSl-2(登米層頁岩)をあわせて供給源の地質を検討した. 登米層頁岩は上部大陸地殻の組成を代表するPAAS (post Archean Australian shale)に比べ,K2O, Rb, Csそして軽希土類に乏しく,Fe2O3, Mg, V, Scおよび中・重希土類はPAASと同じ,もしくはわずかに富んでいる.Cr/Zr-Y/NiダイヤグラムおよびCo/Y-Ti/Zrダイヤグラムにおいて登米層と中生界頁岩は中性の岩石組成を示す.登米層頁岩のCIA (Cemical Index of Alteration)は68〜78であり,後背地の岩石はトリアス系のそれ(CIA=59〜67)の場合に比べ,比較的風化していたと考えられる.ジュラ系頁岩は登米層とほぼ同じCIAをもつ. 西南日本内帯の舞鶴帯や外帯の黒瀬川地帯のペルム系泥岩と比較すると,Ti/Zr-Co/Yダイヤグラムにおいては南部北上帯の試料は両者の中間的な位置にプロットされる.Th-Hf-CoおよびLa-Th-ScダイヤグラムにおいてもJSl-1は舞鶴帯と黒瀬川帯の泥岩の中間にプロットされ,登米層頁岩の供給地は発達した組成をもつ舞鶴帯よりもより初生的な組成をもつと言える.また,ペルム紀からジュラ紀の頁岩は一貫して中性の組成をもつ.

Major and trace elements analyses of Upper Permian Toyoma shales and Mesozoic shales have been carried out to clarify the provenance of the Southern Kitakami Terrane. The Toyoma shales are characterized by depletion in K2O, Rb, Cs, Th and light REE (rare earth elements) compared with PAAS. They also show similar value or slight enrichment in Fe2O3, MgO, V, Sc and the middle and heavy REE. On Cr/Zr-Y/Ni and Co/Y-Ti/Zr diagrams, the Toyoma and Mesozoic shales plot in a field suggestive of an intermediate plutonic rock source. The shales of the Toyoma Formation show higher CIA (Chemical Index of Alteration) values (=68〜78) than Triassic shales (CIA=59〜67), indicating relatively weathered rocks in the source area. The Jurassic shales again show similar CIA values to those of the Toyoma shales. Permian shales of the Southern Kitakami plot in an intermediate field, between those of the Maizuru Terrane of the Inner Zone, and the Kurosegawa Terrane of the Outer Zone of Southwest Japan on the Ti/Zr-Co/Y diagram. On Th-Hf-Co and La-Th-Sc diagrams, JSl-1 (Toyoma shale) also plots between the Maizuru and Kurosegawa shales. This suggests that the Southern Kitakami Terrane had a provenance intermediate between that of the evolved Maizuru Terrane and the less evolved Kurosegawa Terrane. This intermediate nature of the source rocks of the Southern Kitakami was retained with little variation from Late Permian to Jurassic time.