Lay summary

Mongolia is a key area of the Central Asian Orogenic Belt (CAOB) because on its territory the complete Neoproterozoic - Paleozoic history of the Paleo-Asian Ocean is preserved in a great variety of tectono-stratigraphic units (terranes). The northern belt (north of the Main Mongolian Lineament, MML) was formed by successive subduction and accretion of oceanic basement, volcanic arcs, and continental blocks against the Siberian craton during Late Proterozoic and Early Paleozoic. Subsequently, the CAOB was completed by the collision of various terranes (intra-oceanic and continental arcs, microcontinents) at the end of the Paleozoic correlating with the closure of the Paleo-Asian Ocean. The present project focuses on the Late Paleozoic history of the Paleo-Asian Ocean south of the MML. In order to develop an accurate model for the southern Mongolian mountain belt, we investigate the clastic sedimentary formations that are comprised within the various terranes south of the MML. We assume that the conglomerates and sandstones closely record with their facies and detrital content, their stratigraphic age, and the bedding relationships, the contemporaneous tectonic processes of basin formation and inversion. The analysis of detrital zircon grains plays a crucial role in our reconstructions.

Standard methods of provenance analysis (e.g. modal framework grain and heavy mineral analyses) are useful but show limited resolution because of high diagenetic overprint and the lack of age informations. Therefore, they will be complemented by geochronological and geochemical analyses of the ubiquitous detrital zircon grains. Zircon have an extremely long geological memory because very resistant, the grains survive weathering and transport as well as high temperature metamorphism and anatexis. The application of in-situ laser-ablation microprobe-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on detrital zircons reveals their igneous and/or metamorphic U-Pb age, and the chemical composition and crystallization environment of the magma from which they precipitated (essentially using P, Y, Th, U, Nb, Ta, REE and Hf concentrations in detrital zircons). 176Hf/177Hf isotope ratios, in addition, are measured to depict their mantle origin or recycled character due to crustal anatexis. As the different measurements can be performed on the same grain, the straight correlation of geological age and geochemical signatures can be achieved.