Geochemistry of Miocene basaltic rocks recovered by the Ocean Drilling Program from the Japan Sea

Abstract

Ocean Drilling Program Leg 127 drilling at site 797 in the Yamato Basin of the Japan Sea indicated that the basement is composed of early Miocene (−19 Ma) basaltic-doleritic rocks, whereas at site 795 in the northern Japan Basin the basement is composed of middle Miocene (−15 Ma) calc-alkali basalt and basaltic andesite lava flows. The basaltic rocks from Hole 795B are characterized by moderate amounts of large-ion lithophile elements (including K, Rb and Sr), high-field strength elements (including Zr and Ti) and light rare-earth element (Ce around 2.5 ppm). The lithology of Hole 797C basaltic rocks can be divided into upper suite (unit 10 and above) and lower suite (unit 11 and below) based on the petrology and chemistry of these rocks (Tamaki et al., 1990). The 797C lower suite basalts are higher in Fe, Ti, Na, K, P, Ba, Hf, REEs, Cu, Cr and Ni when compared with the upper suite basalts. A distinct variation gap exists between Hole 797C upper suite and lower suite basalts in the Cr vs Y plot. The upper suite basalts are confined to a restricted area, while the lower suite basalts show a systematic variation trend. The upper suite and lower suite basalts may represent two distinct magmatic series. The mantle sources for Hole 797C lower suite basalts may be more enriched in incompatible elements than those of the upper suite basalts.