Geochemical and Isotopic Characteristics and Magma Sources of the Early Cretaceous Trachybasalts of the Gobi-Altai Rift Zone: An Example of Grabens in the Arts-Bogdo Range

The Early Cretaceous (Neocomian) basalts of the Arts-Bogdo area, southern Mongolia, are high-potassium trachybasalts with an average K₂O content of 3.1% and a Na₂O/K₂O ratio of 1 : 3. The rocks are enriched in incompatible trace elements such as Rb, Sr, Ba, La, etc., and radiogenic Sr (⁸⁷Sr/⁸⁶Sr = 0.70513-0.70620). The variations in the contents of major and trace elements of the basalts were largely determined by the fractional crystallization of the parent basaltic magma, a process that resulted in the enrichment of later derivatives in silica, alkalis (predominantly potassium), and incompatible elements. However, this process was not responsible for the overall enrichment of the basalts (and, hence, of their parent magma) in radiogenic Sr, K, and some other elements. The indicative ratios Ce/Pb = 19-33 (with an average of 28) and Nb/V = 41-56 (with an average of 48) are identical to those of mantle-derived rocks, a fact suggesting the mantle nature of the geochemical signatures of the basalts and their parent melts. These melts were supposedly produced by the interaction of an enriched mantle source, related to the mantle plume of the southern-Khingan hotspot, with the depleted lithospheric mantle. The Rb-Sr isotopic signatures of the Neocomian basalts of the Gobi-Altai rift zone, including grabens of the Arts-Bogdo Range, indicate that the depleted mantle had ⁸⁷Sr/⁸⁶Sr < 0.703 and Rb/Sr < 0.01, and the enriched mantle source had ⁸⁷Sr/⁸⁶Sr > 0.707 and Rb/Sr > 0.06.