Cenozoic Volcanism on the Hangai Dome, Central Mongolia: Geochemical Evidence for Changing Melt Sources and Implications for Mechanisms of Melting

Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. 30Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at similar to 6 Ma and their geochemistry suggests a garnet lherzolite source region at 90-100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) (Pb-206/Pb-204 = 17.822, Pb-207/Pb-204 = 15.482, Pb-208/Pb-204 = 37.767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with Pb-206/Pb-204 = 16.991 and Sr-87/Sr-86 = 0.704704. The youngest lavas, dated as younger than similar to 8 ka, have the highest K2O contents (up to 5.2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0.7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.