ALBERT

Description: Major and trace element and Sr–Nd–Pb isotope data for whole-rocks 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 (0·50 Ma) and the Chatan (0·15 Ma) 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 TiO 2 , Al 2 O 3 , Na 2 O, K 2 O, P 2 O 5 , Cr 2 O 3 , large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO 2 , and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic 143 Nd/ 144 Nd and 206 Pb/ 204 Pb, and less radiogenic 87 Sr/ 86 Sr and 208 Pb/ 204 Pb 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.