Greenstones in the Mino Paleozoic-Mesozoic Terrane of the East Takayama Area, Central Japan: Evidence for Magmatism Evolution 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 are mainly tholeiitic basalt and subordinately alkalic basalt. Tholeiitic basalt comprises a lava sequence, about 1000 m thick, and tectonically contacts alkalic basalt. Fusuline fossils in associated limestones show that alkalic basalt is slightly younger than tholeiitic basalt. Chrome spinel occurs locally in tholeiitic and alkalic basalts. It is occasionally aluminous and similar to spinel in tholeiitic and alkalic basalts of oceanic ridge regions. Basalts in the lower portion of the tholeiite sequence are depleted in TiO2, Zr, and Nb and are characterized by extremely low Nb/Y ratios; the lower 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 in the greenstone body, including the tholeiite and alkalic basalt units, suggests that greenstones originally occupied part of a ridge-seamount sequence. The variation in rock type may be explained by magmatism evolution of normal ridge volcanism, followed by plume-influenced ridge volcanism and plume igneous activity.