ABSTRACT

In order to evaluate the role of sediment subduction in the magma genesis in the Setouchi volcanic belt, SW Japan, bulk rock compositions of both pelagic sediments from the Philippine Sea (GDP 15-12) and trench-filled sediments from the Nankai Trough (DSDP site 582) were determined. The analytical result shows that trace element concentrations, particularly Pb, Ba and Th, of the pelagic sediments are much higher than those of the terrigenous sediments. Mixing calculations using these elements indicate that the terrigenous sediments rather than the pelagic sediments played a major role in producing the Setouchi magmas. To obtain further constraints on the origin of Setouchi magmas, a model of melt-mantle reaction was examined based on the assimilation fractional crystallization formulation (AFC). The result also indicates that the terrigenous sediment is a dominant metasomatic agent. In addition, the model suggests that the fractionation of orthopyroxene with minor hornblende would be essential to produce the Setouchi magmas. Taking the hornblende fractionation into account, Pb/K₂O and Pb/Rb ratios on MgO variation diagrams also indicate similar contributions. Since the breakdown of mica, which is included both in the pelagic and terrigenous sediments, is inferred to induce dehydration melting of the sediments, above geochemical characteristics suggest that both sediments would melt simultaneously beneath the Setouchi area.