Melting of the uppermost metasomatized asthenosphere triggered by fluid fluxing from ancient subducted sediment: Constraints from the quaternary basalt lavas at chugaryeong Volcano, Korea

Major and trace element and Sr-Nd-Pb isotope data for wholerocks and major element data for minerals within basalt samples from the Chugaryeong volcano, an intra-plate back-arc volcanic centre in the central part of the Korean Peninsula, are used to address the process of magma genesis in the deep back-arc region of eastern Asia.There are two lava flow units at Chugaryeong volcano: the Chongok (0·50Ma) and the Chatan (0·15Ma) basalts. These basalts have similar MgO (9·1-10·4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO₂, Al₂ O₃, Na₂O, K₂O, P₂O₅, Cr₂ O₃, large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO₂, and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb, and less radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb than the Chatan basalt. Chi-square tests for the major elements indicate that crystal fractionation can explain the chemical variations within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot result in the formation of one basalt suite from the other.The Sr-Nd-Pb isotopic compositions of the Chongok and Chatan basalts plot on mixing hyperbolae between peridotite mantle xenoliths from the area and a fluid flux derived from a mixture of ancient and recent sediments. The trace element compositions of the estimated primary melts for the two basalt suites suggest different degrees of partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated degree of melting increased with time from ̃7·5% for the Chongok basalt to ̃10% for the Chatan basalt. The source mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time, sustained by an increased sediment-derived fluid flux from the deeper upper mantle.