Mantle dyanmics beneath East Asia constrained by Sr, Nd, Pb and Hf isotopic systematics of ultramafic xenoliths and their host basalts from Hannuoba, North China

Abstract

We have determined the Sr, Nd, Pb and Hf isotopic compositions of clinopyroxene separated from mantle-derived ultramafic xenoliths (six spinel peridotites, one composite Cr-diopside pyroxenite, and one discrete Al-augite pyroxenite) hosted by Cenozoic alkali basalts at Hannuoba, North China, in order to understand the nature of the mantle source for this intraplate volcanism, and the evolutional history of the mantle lithosphere beneath North China Block, a crustal fragment of the Sino-Korean Craton. Measured Sr, Nd, Pb and Hf isotopic compositions in the clinopyroxene grains separated from spinel peridotite and Cr-diopside pyroxenite (87Sr/86Sr = 0.70265 to 0.70485;206Pb/204Pb = 17.75 to 19.15;？Nd = 0 to +11;？Hf = +10 to +38) display mixing hyperbolas between DMM and EM2 on the Sr？Pb and Nd？Pb isotope correlation diagrams. This is distinctly different relative to the host basalt data which show a mixture of DMM and EM1 components on the diagrams. We interpret this to reflect infiltration by metasomatic agents, possibly silicate melts having an EM2-like isotopic signature that enriched a precursor time-integrated depleted mantle. An Al-augite pyroxenite also hosted by these basalts is characterized by highly enriched Sr, Nd, and Hf isotopic compositions (87Sr/86Sr = 0.70733;？Nd = -16;？Hf = -18) with only moderately radiogenic Pb that has a 206Pb/204Pb value of 18.23. All of these data plot outside (1) the fields for oceanic basalts, and (2) the mixing arrays defined by peridotites/Cr-diopside pyroxenite with their metasomatic agents, and by the host basalt on the isotopic correlation diagrams. These observations suggest that (1) Al-augite pyroxenite is not cogenetic with the Cr-diopside pyroxenite, (2) parental melts of the pyroxenites are not likely to be the source for the metasomatism, and (3) the thermo/mechanically reactivated pyroxenite and/or spinel peridotite, is not likely to be the source for host basalt mag