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Rocking Mantle”Group

IGR paper: Magma-magma interaction recorded by megacrysts from Cenozoic basalts in NE China


Recently, besides magmarock and rockrock reaction, magmamagma interaction at mantle depth has been proposed as an alternative mechanism to produce diverse compositions of mantle. Clinopyroxene and garnet megacrysts can be formed at this condition since this process is suggested to trigger the high-pressure crystallization of these minerals. Studying on this type of megacrysts provides us important information on the genesis of intraplate basalts and the chemical heterogeneity of mantle, which has not been reported before. Here we present major, trace elements and Sr isotopes of clinopyroxene and garnet megacrysts hosted by  from Penglai, Shandong province of eastern China. The megacrysts are suggested to beformed by crystallization from magma because of their moderate Mg# (74.079.9 for clinopyrox-ene and 58.865.0 for garnet) and good correlations between Mg# and other elements (e.g. CaO,TiO2, Nd and Lu). The potential crystallized temperature and pressure are estimated to be ~1156°C at 2.63.2 GPa, which should occur at the top of asthenosphere or lithosphereasthenosphere boundary based on the lithospheric thickness in this area (~6070 km). Since the megacrysts show variable Sr isotopes, and their primary magmas show negative correlation between 87Sr/86Sr and Hf/Sm ratios, as well as positive correlation between Ba/Th and Nb/U for clinopyroxenes, it indicates a mixing origin. Cenozoic basalts from Shandong show a mixing trend, and high-pressure fractionation of clinopyroxene and garnet is suggested to occur during the mixing process because some basalts show significantly higher Sm/Yb and lower Ca/Al ratios than others, which again supports our interpretations. When compared to megacrysts and host basalts fromother locations of eastern China, similar geochemical variations and a deviation trend relative to the mixing trend are also observed. It indicates that magmamagma interaction can be a commonprocess for formation of intraplate basalts and basalt-borne megacrysts.

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Plot of Ba/Th versus Nb/U and Sm/Yb versus Ca/Al for Cenozoic basalts from Shandong and representative basalts fromeastern China.