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

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.