Sources and geodynamics of the Late Cenozoic volcanism of Central Mongolia: Evidence from isotope-geochemical studies

In the Late Cenozoic, the volcanism of the South Khangai Volcanic Region (SKhVR) spanned the Khangai Range and its framing. Geochronological, petrochemical, geochemical, and isotope studies were performed for volcanic rocks of this region, which are represented by high-K basic and intermediate rocks of OIB affinity. Initial Sr, Nd, and Pb isotope ratios in the volcanic rocks of the SKhVR are close to those of the volcanic rocks of Pitcairn Island and form trends between PREMA, EMI, and EMII sources. The petrochemical, geochemical, and isotope zoning is unraveled in distribution of the Late Cenozoic associations within SKhVR. Volcanic sequences of the Vodorazdel'nyi graben occupying the watershed part of the Khangai Range and adjacent valley lava flows are located in the central part of the area. The peripheral part is made up of the volcanic associations formed within the Lake Valley and Taryat grabens and the Orkhon-Selenga area. Compositional zoning is characterized by an increase in contents of alkalis, Ti, P, and some other lithophile elements, as well as systematic changes of isotope composition of the rocks from central part toward periphery. Taking into account gravimetric and seismotomographic data marking asthenospheric rise beneath Central Khangai, it was concluded that the studied volcanism is related to mantle plume activity. Revealed compositional zoning of the volcanic region presumably reflects the plume heterogeneity. The volcanism of the watershed part of the Khangai Range was controlled by plume channel, which was presumably fed by PREMA-type lower mantle. The isotopic enrichment of lavas in the peripheral parts of the volcanic region was not related to participation of lithospheric components, but reflects the distribution of compositionally different mantle sources in plume structure. The most probable source of enriched components in the Late Cenozoic rocks of SKhVR was Early Precambrian recycled crustal material, which was isolated from upper mantle convection after subduction and transported by the ascending mantle jet to the lithosphere base only in the Late Cenozoic.