U-Pb (SHRIMP-II) isotope dating of zircons from metamorphic rocks of Southwestern Transbaikalia

Abstract

256Multiage plutonic, volcanoplutonic formations, and rocks of sedimentary basins are distinguished within the Mongolia-Okhotskii foldbelt (MOFB). Many problems of the formation of this belt have not yet been solved, which is mainly explained by the defi ciency of geochronological and isotope geochemical data. In particular, information on the age of meta morphic rocks distinguished in its structure is insuffi cient. The main events in Transbaikalia are tradition ally related to Caledonian accretion and collision [1]. The upper boundary of regional metamorphism on most of the territory is basically estimated as the period from the Paleoproterozoic to the Late Riphean-Early Paleozoic. It is also accepted that Paleozoic rocks only slightly underwent metamorphic processes, which were not reflected in the Mesozoic at all [2]. However, the geochronological data obtained by E.V. Sklyarov with coauthors [3] for rocks of the Zagansky ridge pro vide evidence for a Mesozoic age of metamorphism, which, in their opinion, was controlled by the forma tion of metamorphic cores in relation to collision and subduction at the closing of the Mongolo Okhotsk ocean. Study of the age of metamorphic events in the region was not continued. Due to the importance of this problem, isotope dating of some metamorphic rocks of the Malkhansky Complex was performed. In this paper we discuss the results obtained using the iso chronous Sm-Nd, Rb-Sr, and U-Pb (SHRIMP II) methods. Three key objects were selected for dating: the Kyakhtinskaya group of sillimanite schists and kya nite bearing rocks of the Ichetuisky and Levo Chemurtaevsky Complexes. U-Pb (SHRIMP II) iso