Mantle Dynamics beneath East Asia Constrained by Sr, Nd, Pb and Hf Isotopic Systematics of Ultramafic Xenoliths and their Host Basalts from Hannuoba, North China

We have determined the Sr, Nd, Pb and Hf isotopic compositions of clinopyroxene separated from mantle-derived ultramafic xenoliths (six spinel peridotites, two composite Cr-diopside pyroxenites, 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 petrologic history of the mantle lithosphere beneath North China Block, a crustal segment 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; eNd = 0 to +11; eHf = +10 to +38) display mixing hyperbolas between mantle compositional end members DMM and EMII on the Sr- Pb and Nd-Pb isotope correlation diagrams. This is distinctly different from the host basalt data which show a mixture of DMM and EMI components on the diagrams. We interpret this to reflect infiltration by metasomatic agents, possibly silicate melts, having an EMII-like isotopic signature, which 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; eNd = -16; eHf = -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 on the isotopic correlation diagrams by peridotites/Cr-diopside pyroxenite with their metasomatic agents, and by the host basalt. 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 magmatism. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. We observe that the relatively little-metasomatized Hannuoba peridotites define a Lu-Hf isochron of 2587 +/- 86 Ma (2s). This value is, within error, indistinguishable from the Sm-Nd isochron age of the overlying granulite terrain. We suggest, therefore, that the Lu-Hf system can be used to constrain the timing of lithospheric mantle differentiation. Preservation of the Neoarchean mantle lithosphere beneath Hannuoba, despite the protracted tectono-magmatic reactivation during the Mesozoic and Cenozoic in this area, suggests that complete removal of the lithospheric mantle beneath East Asia by wholesale delamination is unlikely.