Late triassic stage of formation of the mongolo-transbaikalian alkaline-granitoid province: Data of isotope-geochemical studies

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. Formation of the major volume of acid magmas in the province was earlier considered to take place continuously from early Permian to early Triassic, about 280-240 Ma ago. The Rb-Sr and Sm-Nd isotope studies of the rocks of the typical trachybasalt-comendite bimodal series of the Tsagan-Hurtei Range showed its Late Triassic age, 212±5 Ma. The isotopic Nd-Sm characteristics (ε_Nd(T) - +1.9...+2.8 both in trachybasalts and comendites) 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 with trachybasaltic magmas. Data on high-temperature homogenization of melt inclusions in phenocrysts of quartz from comendites (to 1100 °C) and on 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.