Geochemistry at DSDP Leg 58 Holes

Bulk chemical composition of 279 samples of argillaceous sediments from DSDP Sites 442, 443, and 444 in the Shikoku Basin (Leg 58) was determined and interpreted. Chemical features indicate that the Shikoku Basin sediments are typically terrigenous and were derived mostly from the Japanese Islands. A local contribution from oceanic islands, such as the Izu-Mariana Islands, to the basin sediments is not as large as expected from geophysical evidence. The Nankai Trough cannot be a barrier against sediment influx to the basin from the Japanese Islands. Except for Ca and Mn, the bulk chemical composition of the sediments is inherited from the source materials, such as granites and volcanics in the Japanese Islands, without remarkable modification. Basal sediments immediately on the basaltic basement, enriched in Mn and Fe, formed under the effect of hydrothermal emanations related to volcanism. From the chemical composition of the sediments, using the formula of Boström et al. (1976), the spreading rate of the Shikoku Basin at 17 to 15 Ma is estimated to have been 1.7 cm/yr, which is not inconsistent with the rate determined from geophysical data