抄録
The Shimokawa greenstone-argillaceous sediment complex, which occurs as a tectonic slab in the Hidaka belt, central Hokkaido, northern Japan, consists of diabase/basalt sills and dikes, basaltic pillowed sills with minor amounts of pillow lavas, greenstone breccias and intercalated fine-grained terrigenous sediments.
Basaltic magma is thought to have injected into unconsolidated sediments and formed sills of diabase and basalt. In the case where magma ascended into unconsolidated wet sediments near the seafloor under the condition of Pm (magma pressure)-σz (lithostatic pressure of sediment) ≥τ0 (shear strength of sediment), the magma became quenched and predominantly formed pillow structures. On the contrary, where magma ascended into less wet sediments, it formed sills under the condition of Pm-σz>τ0.
Greenstone breccia is made up of lithic components derived from the diabase/basalt sill and dike, pillowed sill and lava, and is characterized by chaotic aggregate of disrupted angular clasts with subordinate pulverized greenstone and argillaceous matrix. Pillowed sills and greenstone breccias occur at three different horizons and constitute key beds. The existence of greenstone breccia stratum at different horizons in the complex implies that they were formed during the interval of basaltic magmatism and sedimentation of terrigineous sediments. A favored interpretation to account for the greenstone breccia is that they originated from the steep and unstable escarpment, transported to the lower level of the basin as mass movement in the form of debris flows and rock falls triggered by collapse of slopes, and settled during volcanism and sedimentation. Volcanogenic massive sulfide deposits occupy the lower most horizon of the complex and are hosted in either sediments or greenstones.
Volcanism and hydrothermal activity are assumed to have occurred where fine-grained terrigineous sediments were supplied continuously. Syn-volcanic and syn-sedimentary faults are also thought to provide pathways for discharging hydrothermal solutions, from which sulfide minerals precipitated and formed the Shimokawa volcanogenic massive sulfide deposits.
These phenomena support that the Shimokawa greenstone-argillaceous sediment complex was a part of sedimented spreading center located close to the continent.
