Rb-Sr age and sequence of formation of granitoids of the Khorinka volcanoplutonic structure in the Mongolo-Transbaikalian belt

The Khorinka volcanoplutonic structure lies in the central part of the Mongolo-Transbaikalian alkali-granite belt (MTB) extending for more than 2500 km through Northern Mongolia and Transbaikalia. The MTB formed in two stages: 280-270 and 230-210 Ma. The Khorinka structure is ca. 3000 km(2) in area. Syenites and granites prevail at the modern denudation level of its rocks. Only a minor part of volcanic comagmates ( bimodal trachybasalt-rhyolite series) has been preserved in this structure. The volcanic stage was preceded by the formation of a bimodal dike belt. Granitoid massifs resulted from the subsequent intrusion of alkali-feldspathic, alkaline, and syenogranitic magmas. Rb-Sr isotope dating has shown that the Khorinka structure was produced at the early stage of the MTB formation (similar to 280 +/- 5 Ma). The Rb-Sr ages for alkali granitoids agree with the literature U-Pb zircon ages of these rocks. The (Sr-87/Sr-86)(0) value is 0.7073 in alkali and feldspathic granitoids and 0.7033 in syenogranites, which points to different sources of their parental acid magmas.