Origin of arc-like intraplate volcanism by melting of lithospheric mantle pyroxenite of the South China continental margin

Abstract

Volcanic arcs usually produce silica-rich magmas that may modify the peridotitic sub-arc mantle, but direct observations on the lithology and geochemistry of the sub-arc lithospheric mantle are rare. An Andean-type arc system is inferred on the continental margin of southeastern China during the Mesozoic time; however, direct petrologic evidence for such an ancient arc is lacking. In this study, we have analyzed whole-rock major and trace elements, Sr–Nd–Pb–Hf isotope, and high-precision olivine geochemistry of the basaltic rocks of the Leizhou Peninsula and Hainan Island, on the South China continental margin. Unlike Hainan Island with only typical ocean island basalt (OIB)-type basalts, the Leizhou Peninsula has volcanic rocks ranging from typical OIB-type basalts to island arc basalt (IAB)-type with enrichment in large-ion lithophile elements and depletion in high-field-strength elements. The IAB-like rocks of the Leizhou Peninsula have less radiogenic Nd and Hf isotopes than the Hainan OIB-type basalts and the sub-continental lithospheric mantle. The IAB-like rocks can be explained by interaction of the Hainan plume with a subduction-derived component based on the plots of Nb/Nb* vs. Ce/Pb, Ba/Nb, Pb/Pb⁎, and SiO2. Olivine phenocrysts with anomalously high Ni and low Ca and Mn contents are consistent with a pyroxenite-rich mantle source for the IAB-like rocks. We suggest that the pyroxenite-rich mantle component of the IAB-like rocks formed by metasomatic reaction between silica-rich arc melts and the sub-continental lithospheric mantle during an ancient plate subduction event. We suggest that these arc-like volcanic rocks record melting of lithospheric mantle pyroxenite triggered by the Hainan plume. Our results are consistent with an Andean-type active continental margin related to subduction of the paleo-Pacific Plate beneath the southeastern China during the Mesozoic.