The Mongolo-Transbaikalian alkaline-granitoid province is more than 2000 km long and includes more than 350 plutons of alkali granites and syenites and numerous fields of volcanic derivates of alkaline-granitic magmas. It was earlier believed that the major volume of acid magmas in the province formed continuously from early Permian to early Triassic, about 280–240 Ma. The Rb-Sr and Sm-Nd isotope studies of the rocks of the typical trachybasalt-comendite series of the Tsagan-Khurtei Range showed its Late Triassic age, 212± 5 Ma. The isotope Nd-Sm characteristics (εNd(T) = + 1.9… + 2.8 in both trachybasalt and comendite) and a regular distribution of REE in basic and acid rocks of the series suggest affinity of these rocks and possible genetic relations of alkaline-granite melts to trachybasaltic magmas. High-temperature homogenization of melt inclusions in phenocrysts of quartz from comendites (to 1100 °C) and the presence of no less than 1 wt.% H2O and F in these inclusions indicate considerable depths of magma generation or injections of high-temperature basaltic melts into acid magma chambers.

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