Major, Trace Element, And Srnd Isotopic Geochemistry Of Cenozoic Basalts In Central-North And East Mongolia: Petrogenesis And Tectonic Implication

This paper presents new major element, trace element, and SrNd isotope data of Cenozoic basalts in volcanic provinces Hovsgol, Taatsiin Gol, and Dariganga-Abaga in Northern, Central, and Eastern Mongolia, respectively, to explore the petrogenesis of the basalts and their geodynamic setting. Cenozoic volcanic rocks in Mongolia are dominated by basalts, belonging to alkaline and tholeiitic series, and are characterized by ocean island basalts (OIB)-like major and trace element signature, such as enrichment of light REE relative to heavy REE and enrichment in both large-ion lithophile elements (LILE) and high-field-strength elements (HFSE) with positive NbTa anomalies in primitive mantle-normalized spider diagram. On the other hand, the Central-North Mongolian basalts are higher in SiO2 and Al2O3 but lower in MgO, CaO, and TiO2 than East Mongolian basalts and display positive K anomaly, which is in contrast to the negative K anomaly of the latter. Despite the evolved nature of the Central-North Mongolian basalts, the roles of crustal contamination and fractional crystallization, or assimilation coupled with fractional crystallization (AFC), are insignificant in the genesis of the basalt; instead, both the mantle heterogeneity and melting condition have significant contributions to the compositional variations of the basalts. Importantly, the SrNd isotopic signatures suggest that the mantle sources are characterized by mixing of two isotopic components, a depleted mantle (DM) and an enriched mantle (EM). Although, the EM component is variable, which can be ascribed to metasomatized subcontinent lithosphere (SCLM) and altered recycled oceanic crust (ROC) for the mantle sources of the Central-North and East Mongolian basalts, respectively. Taking these results together with published age and high-resolution tomographic data, we propose that all Cenozoic volcanism in Mongolia is related to shallow mantle (asthenosphere) upwelling stemming from the mantle transitional zone (MTZ), but the triggering causes for the mantle upwellings are different. The mantle upwelling related to generation of the Central-North Mongolian basalts was likely triggered by a far-field effect of the Indo-Asian collision, which might have caused the avalanched/subducted materials of the Mongol-Okhotsk and/or the Central Asian orogenic belt (CAOB) that sink into and stagnated within the MTZ. The mantle upwelling related to the formation of the East Mongolian basalts, which are located at the westernmost of the big mantle wedge constructed by the subducting and stagnant Pacific slab, was induced directly by the westward subduction of the Pacific Plate beneath East Asia.