Greenstones in the Mino Paleozoic-Mesozoic Terrane of the EastTakayama Area, Central Japan: Evidence for MagmatismEvolution from Normal Ridge to Plume Volcanism

摘要

Greenstones of early Permian age occur extensively in the Mino terrane of the east Takayama area. Greenstones aremainly tholeiitic basalt and subordinately alkalic basalt. Tholeiitic basalt comprises a lava sequence, about 1000 mthick, and tectonically contacts alkalic basalt. Fusuline fossils in associated limestones show that alkalic basalt isslightly younger than tholeiitic basalt. Chrome spinel occurs locally in tholeiitic and alkalic basalts. It is occasionallyaluminous and similar to spinel in tholeiitic and alkalic basalts of oceanic ridge regions. Basalts in the lower portionof the tholeiite sequence are depleted in TiO2, Zr, and Nb and are characterized by extremely low Nb/Y ratios; thelower basalts appear to be normal mid-oceanic ridge basalt (N-MORB). The middle-upper basalts are enriched in TiO2,Zr, and Nb and show Nb/Y ratios that are high relative to Zr/Y; they are similar to Icelandic tholeiite. Basalt,chemically transitional from N-MORB to Icelandic basalt, occurs in the lower to middle portion. The rock type inthe greenstone body, including the tholeiite and alkalic basalt units, suggests that greenstones originally occupiedpart of a ridge-seamount sequence. The variation in rock type may be explained by magmatism evolution of normalridge volcanism, followed by plume-influenced ridge volcanism and plume igneous activity.