The Early Cretaceous volcanic activity in the western part of the Gobi-Altay rift (Shiliin Nuruu, SW Mongolia)

The Cretaceous continental sedimentation in SW Mongolia was commonly accompanied by volcanic activity. Bimodal association of the Cretaceous Khulsan Gol Formation is represented mostly by calc-alkaline and rare alkaline volcanic rocks (basaltic andesite, trachybasalt to trachyandesite lavas or tuffs with lahars). The lavas were extruded mainly subaerially, although locally there is a subaqueous component present corresponding to final phases of Mesozoic sedimentation in the Gobi-Altay rift zone. Variations in chemical and isotopic composition reflect crustal assimilation coupled with fractional crystallization of parental magma generated by melting of lower lithosphere and/or uppermost asthenospheric mantle. These within-plate volcanic rocks can be explained as initial stages of Cenozoic magmatic activity in the Mongolia.