Melting of the Uppermost Metasomatized Asthenosphere Triggered by Fluid Fluxing from Ancient Subducted Sediment: Constraints from the Quaternary Basalt Lavas at Chugaryeong Volcano, Korea

作者：Sakuyama Tetsuya^*; Nagaoka Shinji; Miyazaki Takashi; Chang Qing; Takahashi Toshiro; Hirahara Yuka; Senda Ryoko; Itaya Tetsumaru; Kimura Jun' ichi; Ozawa Kazuhito

来源：Journal of Petrology, 2014, 55(3): 499-528.

DOI：10.1093/petrology/egt074

• back-arc intraplate volcanism
• alkaline basalt
• Chugaryeong volcano
• Korean Peninsula
• Quaternary
• fluid flux
• subducted sediment
• mantle melting

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 center dot 50 Ma) and the Chatan (0 center dot 15 Ma) basalts. These basalts have similar MgO (9 center dot 1-10 center dot 4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO2, Al2O3, Na2O, K2O, P2O5, Cr2O3, 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 Nd-143/Nd-144 and Pb-206/Pb-204, and less radiogenic Sr-87/Sr-86 and Pb-208/Pb-204 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 similar to 7 center dot 5% for the Chongok basalt to similar 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.

• 出版日期2014-3