北上山地下の珪長質下部地殻と斜方輝石岩マントル：白亜紀スラブメルティングの証拠

抄録

  The Kitakami Belt of northeast Japan is composed of the North Kitakami Belt (a Jurassic accretionary complex) and the South Kitakami Belt (a microcontinental block). Early Cretaceous plutonic rocks with adakitic, calc-alkaline, and shoshonitic affinities intruded into both the terranes. Previous studies proposed that partial melting of a subducted oceanic slab produced the Kitakami adakitic plutonic rocks. This study presents a new view of the constituent rocks in the crust and mantle beneath the Kitakami Belt based on the P- and S-wave velocities obtained by seismic tomography and comparisons with the experimentally determined velocities of rock-forming minerals. We infer that large volumes of felsic rock exist in the lower crust beneath the Kitakami Belt, underlain by orthopyroxenite in the uppermost mantle. These inferences suggest that (1) the Early Cretaceous adakitic magmatism produced the orthopyroxenite via peritectic reactions between slab-derived felsic melts and mantle peridotite; and (2) the reacted slab melt formed thick adakitic felsic rock in the lower and upper crust. Considering the regional distribution of these unusual mantle and crustal rocks, Early Cretaceous slab melting is inferred to have occurred over a wider area than previously thought.