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The Mesozoic-Cenozoic volcanic rocks are widely distributed in the interior of the East Asia and document the tectonic transition of East Asia. We present new geochronology and geochemistry data of late Cretaceous-early Cenozoic basalts in Bayantsagaan and Han-Uul volcanic provinces in South Mongolia, in order to explore their petrogenesis and geodynamic settings. The volcanic rocks in the Bayantsagaan and Han-Uul field yielded K-Ar ages of 90.55±1.93 Ma and 55.49±1.49 Ma, respectively. The volcanic rocks in South Mongolia can be subdivided into to alkaline basalts and tholeiitic series, and are characterized by ocean island basalts (OIB) trace elements features, such as enrichment of light REE relative to heavy REE and enrichment in large ion lithophile elements (LILE) with positive K anomaly. Compared with the late Cretaceous, the early Cenozoic basalts show a decrease in the contents of HREE and an increase of Nb and Ta. Crustal contamination and fractional crystallization are insignificant in the genesis of late Cretaceous-early Cenozoic basalts South Mongolia. The available Sr-Nd isotope results indicate that a mixing depleted (DM) and enriched mantle (EM) signature characterize in late Cretaceous volcanic rocks, which derived from magmas from the asthenosphere with some contributions of metasomatized subcontinent lithospheric mantle, whereas the early Cenozoic basalts are ascribed to contributions from the asthenospheric mantle. We propose that the generation of the late Cretaceous-early Cenozoic volcanism (90-40 Ma) in Mongolia is probably related to the shallow mantle upwelling (asthenosphere) induced by the edge convection along the northern margin of the North China Craton (NCC), triggered by a far-field effect of Indo-Asian collision.

This paper presents new major element, trace element, and SrNd 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 NbTa 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 SrNd 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. [ABSTRACT FROM AUTHOR]

This paper presents new geochemical and geochronological data of the Late Mesozoic volcanic rocks in East Mongolia. These volcanic rocks belong to high‐K calc‐alkaline and shoshonitic series and exhibit features of bimodal rocks. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb, Ta, and Ti anomalies, with 87Sr/86Sr(i) and ƐNd(t) values of 0.70502–0.70572 and −1.72459 to +1.720736, respectively, which suggest that the mafic magma was derived from a lithospheric mantle source that might have been metasomatized by subduction‐derived fluids, experienced fractional crystallization, and was contaminated by crustal materials. The felsic rocks have similar rare earth element patterns to the mafic rocks but show much more significantly negative Eu and Sr anomalies. The felsic rocks have higher 87Sr/86Sr(i) values of 0.706496–0.71104 than the mafic rocks but similar ƐNd(t) values (−0.28003 to +2.928506) to the mafic ones. These data indicate that the felsic rocks originated from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K‐Ar dating, together with previous data, shows that the Late Mesozoic volcanism in East Mongolia took place during the Late Jurassic‐Early Cretaceous between ca. 155 and 99 Ma. The model of back‐arc extension, possibly induced by slab rollback of the westward subducted Pacific Plate and the subduction zone retreat, can explain the geodynamic setting and the eastward younging trend of the Late Mesozoic volcanism in East Mongolia and in adjacent NE China. [ABSTRACT FROM AUTHOR]