Origins of Cenozoic basalts in Mongolia : a chemical and isotope study

Alkalic basaltic volcanism has occurred throughout Mongolia since the Cretaceous. Early Cenozoic lavas were erupted ~30 Ma, the in the Gobi, southern Mongolia, whereas younger lavas (6 to 0.005 Ma) were erupted in Hangai, central Mongolia (based on new 40Ar/39Ar dating). Previously proposed as plume-related, the Hangai lavas differ from Mesozoic basalts, but are remarkably similar to the 30Ma Gobi lavas. Focusing on all the Cenozoic lavas, chemical and isotopic studies (Sr, Nd, Pb, Hf and He) were undertaken to establish magmatic sources and melting processes. Analysis of crustal xenoliths indicate that the basalts have not been affected by crustal contaminations, and a strong heavy rare earth element depleted signature indicates that the lavas originated from within the garnet stability field. Petrogenetic studies show that the lavas are enriched in K, Nb, Sr and P. I propose that this enrichment is due to the break down of hydrous phases during the Cenozoic melting event. Alternatively, it originated from small degree partial melts ahead of an advancing melt front. The enrichment may have occurred at any time in the past; both ancient and modern metasomatic amphiboles occur within Cenozoic hosted mantle xenoliths. Low 143Nd/144Nd data in the basalts, support an old enrichment event, potentially in response to Palaeozoic subduction. In addition to an enriched lithospheric component, isotopic ratios suggest a depleted lithospheric mantle reservoir also contributed to melt compositions.;Melt initiation most likely occurred in response to an asthenospheric thermal input, as there is no evidence for extensive lithospheric extension during the Cenozoic. Therefore, and asthenospheric component may have contributed to overall melt compositions, but is overprinted by lithospheric contributions. A thermal anomaly underlying Mongolia need only be slightly hotter than ambient mantle temperatures, as preferential melting of hydrous-bearing assemblages in the lithosphere could lead to alkalic melt generation..