Mineral and geochemical characteristics for Jurassic volcanic rocks from ODP Site 801C in the Pigafetta Basin, Western Pacific Ocean: Implications for magmatic evolution at the oldest fast-spreading ridge

Abstract Despite their importance, no detailed mineral analyses have been conducted for volcanic rocks of Ocean Drilling Project Site 801C in the Pigafetta Basin, Western Pacific Ocean. From the eight volcanic sequences that comprise Site 801C, we performed comprehensive studies on the alkali basalts from sequence 1 (Upper Volcanic Group; UVG) and the tholeiitic basalts from sequences 3, 4, 6, and 8 (Lower Volcanic Group; LVG). To understand the magmatic evolution occurring at fast-spreading ridges, we studied the petrography, major and trace element compositions of minerals, mineral texture, and major and trace elements and Nd isotopic compositions for whole rocks. Alkali basalts from the UVG are characterized by enriched light rare earth elements (LREE) and large ion lithophile elements (LILE), relatively depleted heavy REE, and slightly low Nd isotopic ratios (ɛNd = 5.8–6.4), similar to the composition of ocean island basalts. Tholeiites from the LVG display significant depletions of LREE and LILE resembling the geochemistry of normal mid-ocean ridge basalts, and show much lower degrees of REE fractionation ((La/Yb) N  = 0.50–0.74) and relatively high Nd isotopic ratios (ɛNd = 9.8–11.2). Plagioclase, alkali feldspar, clinopyroxene, and olivine in the UVG basalts display obvious normal zoning texture, indicating the slow ascent and magma evolution. The UVG alkali basalts were likely produced by the simple binary mixing of melts from a chemically and isotopically heterogeneous mantle. The comprehensive analyses suggest that the basaltic magma from Site 801C is the result of N-MORB fast-spreading ridge tholeiitic volcanism, which was followed after a certain interval (approximately 7 Ma) by within-plate alkaline volcanism from a plume influenced fertile mantle. The fast-spreading rate of the Pigafetta Basin basement was approximately 38–58 mm/y, and the within-plate alkaline volcanism was located ~ 400 km away from the active axis.