Volcanic eruptions in central Mongolia during the latest Pleistocene and Holocene time preceded an initial Holocene volcanic event of 8740 ± 400 years ago in Northeast China and terminated simultaneously with that event as inferred from ¹⁴C datings. Alkali basaltoid magmatic material from a partially melted (1.5–3%) mantle source was erupted in the Taryat Basin of central Mongolia, at first along a nearly east-west line of volcanoes, and afterwards material of higher melting (up to 5%) was discharged along the north-northeast line of Khorgo edifices. A material of similar composition was erupted in the Jingpohu area, Northeast China during the period from 5430–4400 BP. Initial liquids of ∼2% beneath the Frog Pool volcanic center and ∼5% beneath Crater Forest were expressed varying liquids beneath the latter area, yielding final melts of ∼5%. The action of the decompressional and the fluid mechanism was followed by in eruptions of, respectively, isotope-homogeneous magmas in central Mongolia and isotope-heterogeneous magmas depleted in high field strength elements (Nb, Ta, Ti) in Northeast China.

Geochemical studies of the Middle—Late Cenozoic succession of volcanic rocks from the northern part of the Rio Grande Rift were conducted. The initial activation of the rift structure was coeval with voluminous eruptions of lava and pyroclastic material of mainly intermediate and acid compositions in the San Juan volcanic field 35–27 Ma. The composition of the volcanic products after the rifting was dominated by basic and intermediate lavas. It is shown that the basanites and alkali basalts of the territory had geochemical characteristics of sublithospheric slab and above–sl ab sources. The processes of the riftogenic thinning of lithosphere are expressed by geochemical parameters that reflect the interaction between the liquids from the sublithospheric mantle and the rocks from different levels of both the lithospheric mantle and lower crust. In the 35–18 Ma interval, melts of different–depth sublithospheric and lithospheric sources erupted simultaneously in the northern part of the rift. However, the products of interaction between the sublithospheric and lithospheric materials dominated later in the past 15 Ma, although the sublithospheric magmatic liquids erupted at the northern structural termination of the rift within the Yampa volcanic field at about 10 Ma.

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were...

High-Mg lavas are characteristic of the mid-Miocene volcanism in Inner Asia. In the Vitim Plateau, small volume high-Mg volcanics erupted at 16–14 Ma, and were followed with voluminous moderate-Mg lavas at 14–13 Ma. In the former unit, we have recorded a sequence of (1) initial basaltic melts, contaminated by crustal material, (2) uncontaminated high-Mg basanites and basalts of transitional (K–Na–K) compositions, and (3) picrobasalts and basalts of K series; in the latter unit a sequence of (1) initial basalts and basaltic andesites of transitional (Na–K–Na) compositions and (2) basalts and trachybasalts of K–Na series. From pressure estimation, we infer that the high-Mg melts were derived from the sub-lithospheric mantle as deep as 150 km, unlike the moderate-Mg melts that were produced at the shallow mantle. The 14–13 Ma rock sequence shows that initial melts equilibrated in a garnet-free mantle source with subsequently reduced degree of melting garnet-bearing material. No melting of relatively depleted lithospheric material, evidenced by mantle xenoliths, was involved in melting, however. We suggest that the studied transition from high- to moderate-Mg magmatism was due to the mid-Miocene thermal impact on the lithosphere by hot sub-lithospheric mantle material from the Transbaikalian low-velocity (melting) domain that had a potential temperature as high as 1510 °С. This thermal impact triggered rifting in the lithosphere of the Baikal Rift Zone

The tetrad effect is a systematic change in REE properties, which is expressed as a split of a normalized REE pattern into four rounded segments (tetrads) and caused by the formation of complex compounds. It was identified in the granitoids of the Oka zone of the Eastern Sayan by ICP determination of REE contents. The granitoids have a high F content, a prominent negative Eu anomaly, and a low K/Rb ratio. An increase in the tetrad effect in the granitoids correlates with increasing Nb/Zr, Y/Ho, and Rb/Sr ratios and SiO₂ contents and decreasing Zr/Hf, Ce/Pb, La/Sm, and La/Ta ratios and the contents of Fe₂O₃ and Na₂O. In terms of correlation of the tetrad effect with the Y/Ho, Zr/Hf, and K/Rb ratios, the Li-F and alkali rare metal granitoids of the Oka zone are correlated with, respectively, the granitoids of the northern and northwestern part of the Bohemian Massif, eastern Germany, and the Baerzhe Massif, northeastern China.