Early mesozoic bimodal volcanic sequences of the Central Mongolia: implications for evolution of Khentey segment of the Mongol-Okhotsk Belt

Volcanic sequences of bimodal basalt–trachyte–alkaline-rhyolite character with alkaline granites are widespread in the Central Mongolia. They outcrop within small sublatitudinal grabens scattered along the southern and western frame of the Khentey part of the Mongol-Okhotsk Belt. According to geochronological data, the bimodal magmatic activity occurred from the end of Triassic to start of Jurassic (220–195 Ma). Many rocks of bimodal sequences have high alkali content and rare metal signatures. Fractionation crystallization was the leading process causes an attainment of ore-level rare elements concentrations in the most differentiated melts. Mafic magmas enriched relative to the OIB in the majority of incompatible trace elements were primary melts for all rocks of these associations. At the same time, they show increased Ba and depleted Ta and Nb contents testify to participation of lithospheric mantle component in their source. The Nd and Sr isotopic ratios of the rocks corresponds to at least two magma sources identified as enriched asthenospheric mantle and subduction-modified lithospheric mantle. Bimodal magmatism in the Khentey segment of the Mongol-Okhotsk belt appeared ~30 Ma after the collision caused by the closure of the Ada-Tsag branch of the Mongol-Okhotsk Ocean at about 250 Ma. Rifting occurred along the entire frame of the Khentey segment of the belt and controlled this magmatism. It was initiated by collapse of the orogen with delamination of its keel caused the involvement of asthenospheric mantle in the Late Triassic–Early Jurassic magmatism of the region.