PETROGENESIS OF QUATERNARY BASALTS IN THE SOUTHERN HANGAY MOUNTAINS, CENTRAL MONGOLIA: MANTLE SOURCE AND MAGMATIC EVOLUTION

Alkaline volcanism of equivocal origin has occurred in central Mongolia throughout the Cenozoic. Theories for the origin of this anomalous volcanism include: a mantle plume, lithospheric delamination, or a shallow thermal anomaly heating metasomatized lithosphere. Geochemical data for Quaternary volcanic rocks in the southern Hangay Mountains in central Mongolia were used to constrain the tectonic processes that generated them, and document the fractionation history of the magmas. Three stratigraphic sections and four cinder cones were studied, along with samples from three individual flows.

The Hangay lavas range from primitive alkaline basalts (10.2 wt. % MgO) to moderately evolved trachybasalts (6.2 wt. % MgO). They are generally enriched in incompatible elements, and HREE depletion relative to LREE suggests that the mantle source includes garnet. Previous xenolith studies put the spinel-garnet transition at ~70 km below the Hangay, so Quaternary lavas in this region were produced by melting at this depth or greater. Positive anomalies of Ba, K, and Sr suggest a metasomatized source.

Olivine is the dominant phenocryst phase, with subordinate clinopyroxene, and Fe-Ti oxides; plagioclase is not present as a phenocryst phase in most samples. Fractional crystallization explains most of the variation in chemistry. Fractionation of olivine, clinopyroxene, Fe-Ti oxides, and possibly spinel was inferred from patterns of Ni, CaO, CaO/Al₂O₃, Sc, V, and Cr vs. MgO. Mass balance modeling demonstrates that the most evolved magmas could be derived from the most primitive magmas by ~30% fractionation of an assemblage of 44% plagioclase, 30% olivine, 22% clinopyroxene, 4% magnetite, and <1% spinel. That plagioclase is required as a significant fractionating phase by mass balance modeling, but is not abundant as a phenocryst in most samples suggests efficient separation of plagioclase in crustal magma chambers.

Comparison of these results with those of a parallel study of Tertiary Hangay lavas suggests little change in processes of magma generation over a period of ~30 m.y., an observation inconsistent with plate migration over a mantle plume. Our results support melting of metasomatized lithosphere, possibly by lithospheric delamination or a shallow thermal anomaly, as the cause of this anomalous magmatism.