Pre-collisional (> 0.5 Ga) complexes of the Olkhon terrane (southern Siberia) as an echo of events in the Central Asian Orogenic Belt

Abstract

The Olkhon terrane is a part of the Early Palaeozoic accretionary-collisional system in the northern Central Asian Orogenic Belt (CAOB). The terrane was produced by an Ordovician collision as a collage of numerous chaotically mixed tectonic units composed of rock complexes of different ages originated in different tectonic settings. The pre-collisional history of the terrane is deciphered using new data on zircon ages and chemistry of rocks from several complexes. The oldest Olkhon rocks are the 1.87–1.83Ga granulite and gneissic granites of the Kaltygey complex, which is an exotic Palaeoproterozoic tectonic slice. The next age group consists of the Ust-Zunduk orthogneisses (807±9Ma) and the Orso amphibolites and gneisses (792±10 and 844±6Ma). Samples of both complexes have negative εNd(t) values. The Ust-Zunduk and Orso complexes can have formed in active margins of continents or in crustal blocks other than southern Siberia. The Ediacaran subduction-related rocks of the Olkhon complex may have formed in an island arc setting within the Palаeo-Asian Ocean (PAO). The protolith of schists after volcanic rocks has an age of 637±4Ma and shows positive ɛNd(t) values. The Ediacaran/Cambrian Tonta mafic granulites (ca.545Ma), with OIB affinity and slightly positive ɛNd(t), were derived from an enriched mantle source and may represent a fragment of an oceanic island. The Cambrian Shebarta gneisses after continental-arc greywackes with negative ɛNd(t) values were deposited in a back-arc basin of a microcontinent within the PAO, between 530 and 500Ма. The Cambrian Birkhin metamorphics after PAO mature island-arc rocks have U-Pb ages of ca. 500–490Ma and positive ɛNd(t) values. All pre-collisional complexes in the Olkhon terrane have their analogues among the rocks formed during main events in the northern CAOB history. Thus the reconstructed milestones in the Olkhon terrane history appear to be an echo of events in the CAOB northern segment.