ORIGIN AND EVOLUTION OF TERTIARY LAVAS OF THE HANGAY MOUNTAINS, CENTRAL MONGOLIA

Alkalic volcanism has occurred intermittently in isolated regions of Mongolia throughout the Cenozoic era. The origin of these lavas has been a matter of recent debate; proposed processes include a mantle plume, aftereffects resulting from the India-Asia collision, and an upper mantle thermal anomaly heating a metasomatized lithospheric mantle. In order to determine the origin and evolution of these lavas we conducted a detailed study of a sequence of 63 Tertiary lavas in the central Hangay Mountains of central Mongolia. Most of the lavas are porphyritic with a phenocryst assemblage of olivine +/- plagioclase +/- clinopyroxene. Lavas within the section range from primitive basalt (9.9 wt% MgO) to moderately evolved phono-tephrite (5.2 wt% MgO), and most are nepheline-normative. Trace element spider diagrams indicate enrichment in the fluid-mobile elements K, Ba, and Sr, suggesting a role for metasomatism in the genesis of the volcanic province. Light rare earth element enrichment and variation in slopes of rare earth element spider diagrams suggests the influence of garnet on partial melting. Melt generation modeling based on variation in La/Yb vs. La demonstrates that the lavas could have been generated by 3-7% melting of OIB-composition garnet lherzolite containing 5-8% garnet. The presence of garnet suggests melting at >70 km. Magmatic evolution was dominated by fractional crystallization, with lesser roles of magma mixing and crustal contamination. Mass balance modeling demonstrates that the evolved compositions can be derived from the most primitive composition by 31% fractionation of an assemblage of 37% plagioclase, 35% olivine (with spinel inclusions), 26% clinopyroxene, and 2% Ti-magnetite. Tertiary lavas are compositionally similar to Quaternary lavas of the central Hangay, suggesting no variation in petrologic processes over ~30 m.y., an observation that argues against a mantle plume origin for the province. The data presented here are more consistent with metasomatism as the primary cause of magma genesis.