METATERRIGENIOUS ROCKS OF THE OLKHON TERRANE OF THE CENTRAL ASIAN OROGENIC BELT: U-Pb ZIRCON AGE, GEOCHEMICAL CHARACTERISTICS, AND FORMATION MODELS OF SEDIMENTARY PROTOLITHS

Abstract

The paper presents a petrographic, geochemical, and Sm-Nd isotopic data on the gneisses from different tectonic zones of the Olkhon terrane of the Central Asian Orogenic Belt, as well as the composition of garnets and the age of zircons in these metaterrigenous rocks. The garnet-biotite gneisses of the Anga-Sakhurta zone, as well as the garnet-bearing and garnet-free gneisses (granulites) of the Chernorud zone may result from metamorphism of immature terrigenous rocks of polymictic or greywacke compositions similar in geochemical characteristics to rocks of continental arcs. At the same time, the gneisses of these zones show both similarities and some differences in geochemical and isotopic characteristics, including variations in ɛNd(T) values from –0.2 to –9.0, which may indicate different proportions of one or another source in their protoliths. The age of most detrital zircons in the gneisses of the Chernorud and Anga-Sakhurta zones corresponds to 530–1000 Ma, and the youngest detrital zircons have an age of 522–537 Ma. The geochemical and geochronological data on the gneisses of the Chernorud and Anga-Sakhyurta zones suggests that the protoliths of these gneisses could be formed from the same sources of predominantly Neoproterozoic age. We assumed that gneiss protoliths could initially be sediments of the continental slope of Neoproterozoic composite terrane assembled to the Siberian Platform at 600‒610 Ma. The transport of clastic material being gneiss protoliths from this composite terrane, took place towards the Paleo-Asian Ocean. The youngest zircons with an age of about 530 Ma could be generated from igneous complexes intruding the Neoproterozoic composite superterrane. The garnet-biotite gneisses of the Krestovskaya zone are similar in chemical composition to immature graywacke sandstones, The ɛNd(T) value in these gneisses is –3.7. Detrital zircons in the gneisses of the Krestovskaya zone form age peaks at 780–820 and 498–515 Ma. Based on geochemical and geochronological data we assume that these gneisses could be formed in an intracontinental basin formed at the orogenic stage during accretionary-collisional events at the amalgamation the Orso block and the Birkhin island arc into the Krestovskaya zone.All terrigenous rocks being gneiss protoliths were metamorphosed at 460–510 Ma under granulite or amphibolite facies associated with accretionary and collisional events, which led to the formation of the Early Paleozoic Olkhon composite terrane.