ALBERT

Description: Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca . 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.