Petrogenesis of Cenozoic basalts from Mongolia: Evidence for the role of asthenospheric versus metasomatized lithospheric mantle sources

Diffuse Cenozoic volcanism in Mongolia forms part of a widespread tectono-magmatic province that extends from NE China to Lake Baikal, Siberia. Mafic lavas from the Gobi Altai, southern Mongolia (similar to33 Ma) and Hangai, central Mongolia (<6 Ma) have remarkably similar trace element characteristics, with light rare earth element enrichment (La11/Yb11=11.2-46.6) and positive K, Nb and Sr anomalies on mantle-normalized trace element diagrams. On the basis of new crustal xenolith data, it can be demonstrated that the basalts have not experienced significant crustal contamination. Trace element and Sr-Nd-Pb-Hf isotopic data suggest that these magmas originated by partial melting of a heterogeneous metasomatized amphibole-bearing garnet peridotite mantle source at depths >70 km. Three isotopic end-members can explain the heterogeneity: (1) is similar to bulk silicate Earth with Pb-206/Pb-204>similar to17.8 and is asthenospheric; (2) is EMI-like, characterized by low Pb-206/Pb-204 (>17.062), and may represent mobilized ancient lithospheric mantle; (3) also lithospheric, is characterized by low Nd-143/Nd-144(>0.512292) and shows similarities to EM2, although decoupling of isotopic systems suggests a complex enrichment process. The timing of lithospheric enrichment is unconstrained, but may be related to Mesozoic magmatic events and/or melts mobilized during the Cenozoic responding to higher than ambient potential temperature mantle. Published geophysical studies suggest anomalous material at the base of the lithospheric mantle; however, there is no evidence to suggest a high heat flux mantle plume. Volcanism is likely to occur where localized extensional conditions are favourable.