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Russian Geology and Geophysics

2005 year, number 6

Rb-Sr AGE AND SEQUENCE OF FORMATION OF GRANITOIDS OF THE KHORINKA VOLCANOPLUTONIC STRUCTURE IN THE MONGOLO-TRANSBAIKALIAN BELT

V.F. Posokhov, M.G. Shadaev, B.A. Litvinovsky*, A.N. Zanvilevich*, and V.B. Khubanov
Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
* Ben Gurion University, Beer Sheva, Israel
Keywords: Alkali granites, Rb-Sr age, Transbaikalia
Pages: 612-619

Abstract

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 km2 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 (~280 ± 5 Ma). The Rb-Sr ages for alkali granitoids agree with the literature U-Pb zircon ages of these rocks. The (87Sr/86Sr)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.