Controls of mantle source and condition of melt extraction on generation of the picritic lavas from the Emeishan large igneous province, SW China

Abstract

Like many continental flood basalt (CFB) provinces in the world, the source mantle compositions and melting conditions for generation of the Emeishan CFB are highly debated. We have carried out an integrated Sr-Nd-O-Mg isotopic study of the Emeishan picritic lavas to evaluate possible effects of mantle heterogeneity on the magma composition. Moreover, we have used PRIMELT3 (Herzberg and Asimow, 2015) to calculate the primary magma compositions and melting conditions for these picritic lavas. Picrites from Maoniuping, Tanglanghe and Wuguijing exhibit depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7038–0.7050; εNd = 0.27–3.89), normal mantle-like δ26Mg (−0.27 to −0.2‰) and δ18O (5.0–5.4‰), which can be explained by partial melting of plume materials with negligible contribution of sedimentary carbonates. The Wulongba picrites are characterized by depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7041–0.7045; εNd = 2.57–2.85), slightly lower δ26Mg (−0.36 to −0.23‰) and higher δ18O (5.6–5.7‰), which were attributed to involvement of minor sedimentary carbonates (magnesite) in their mantle source. The calculated mantle potential temperatures for the Emeishan picrites range from 1516 to 1596 °C. The initial melting pressures range from 4 to 5 GPa. The final melting pressures for the Wulongba picrites (2–3 GPa) are lower than those for the Tanglanghe, Maoniuping and Wuguijing picrites (>3 GPa). This suggests that the Wulongba picrites were generated in shallower garnet-spinel transition zone of the upper mantle. Picrites from other three localities are entirely generated in garnet stability field of the upper mantle.