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 an d numerous fields of volcanic derivates of alkaline-granitic magmas. Format ion of the major volume of acid magmas in the province was earlier consider ed to take place continuously from early Permian to early Triassic, about 2 80-240 Ma ago, The Rb-Sr and Sm-Nd isotope studies of the rocks of the typi cal trachybasalt-comendite bimodal series of the Tsagan-Hurtei Range showed its Late Triassic age, 212+/-5 Ma. The isotopic Nd-Sm characteristics (eps ilon (Nd)(T) +1.9...+2.8 both in trachybasalts and comendites) and a regula r distribution of REE in basic and acid rocks of the series suggest affinit y of these rocks and possible genetic relations of alkaline-granite melts w ith trachybasaltic magmas. Data on high-temperature homogenization of melt inclusions in phenocrysts of quartz from comendites (to 1100 degreesC) and on the presence of no less than 1 wt.% H2O and F in these inclusions indica te considerable depths of magma generation or injections of high-temperatur e basaltic melts into acid magma chambers.