Geochemical and Mineral Characteristics of Jurassic Volcanic Rocks from ODP Sites 304, 1149, and 801: Implications for Magmatic Evolution in the Northwest Pacific

Abstract

The regular variations in magmatic activities along the Northwest Pacific plate have been little studied in spite of their importance. In this contribution, systematic analyses were conducted on tholeiitic basalts from three Ocean Drilling Program sites (Sites 304, 1149, and 801), including the petrographic features, major and trace elements, Nd isotopic compositions, and mineral structure and compositions of whole rocks. Volcanic rocks from Sites 304, 1149, and 801 belong to tholeiites and exhibit depleted light rare earth elements (LREE), large ion lithophile elemental contents (LILE), and relatively depleted Nd isotopic ratios (143Nd/144Nd=0.513139–0.513211), similar to those of normal mid‐ocean ridge basalts (N‐MORB). Comprehensive data on mineral compositions, whole‐rock geochemistry, and geochronology demonstrate that a regular variation trend exists in the north‐south direction along the Northwest Pacific plate. The 143Nd/144Nd values (0.513139–0.513211) and trace‐element ratios for whole rocks (Sm/Th=15.35–30.00; Zr/Hf=28.53–35.76; Zr/Y=2.58–3.67; Th/La=0.04–0.06; Th/Y=0.33–0.70), as well as the trace‐element ratios (Zr/Hf, La/Yb, Ti/Zr) of clinopyroxenes from Sites 1149 and 801 tholeiites show larger variations compared to those from Site 304 tholeiites (143Nd/144Nd=0.513185–0.513195; Sm/Th=18.19–20.58; Zr/Hf=31.07–33.26; Zr/Y=2.62–3.03; Th/La=0.05–0.06; Th/Y=0.48–0.57). Mineral zoning textures were obvious in tholeiites from Sites 1149 and 801 but were rarely observed in Site 304. These regular features were likely attributed to the differences in the heterogeneity of the magma source, the process of magmatic evolution, the plate‐spreading rate, and the effective and ineffective mixing.