PETROGENESIS OF CENOZOIC BASALTS FROM MONGOLIA: EVIDENCE FOR THE ROLE OF ASTHENOSPHERIC VERSUS METASOMATIZED LITHOSPHERIC MANTLE SOURCES

Abstract

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 (33 Ma) and Hangai, central Mongolia <6 Ma) have remarkably similar trace element characteristics, with light rare earth element enrichment (Lan/Ybn = 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 206Pb/204Pb >17·8 and is asthenospheric; (2) is EM1-like, characterized by low 206Pb/204Pb (>17·062), and may represent mobilized ancient lithospheric mantle; (3) also lithospheric, is characterized by low 143Nd/144Nd (>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.