The Mongol-Okhotsk Belt in Mongolia - An appraisal of the geodynamic development by the study of sandstone provenance and detrital zircons

摘要

The Mongol-Okhotsk Belt formed in a late stage of Jurassic orogeny in the composite Central Asian Orogenic Belt. The present paper investigates the Late Palaeozoic-Mesozoic sandstones associated with the belt in Mongolia, aimed at reconstructing the time and mode of ocean formation, subduction and collision. We apply provenance analysis including (1) heavy mineral and sandstone framework grain analysis, (2) U/Pb laser ablation ICP-MS dating of detrital zircons for identifying contemporaneous volcanic arc activity and the recycling of older crustal material, and (3) on dated zircon grains, we analyze trace element contents and Hf isotopic ratios in order to characterize the rock types in which they crystallized and the magma sources, respectively.The investigated samples are derived from (1) the Adaatsag and Doschgol terranes, which represent the suture zone, (2) the Hangai-Hentei belt to the northwest, and (3) the Ereendavaa terrane and the Middle Gobi volcanic belt to the southeast of the suture. The latter two are concurrent with the northern and southern margins of the Mongol-Okhotsk Ocean. Tectono-stratigraphic arguments suggest that the Mongol-Okhotsk ocean opened during the Silurian within the Early Palaeozoic collage. In the suture zone, Permian syn-sedimentary zircon trace element contents confirm mafic rock sources, and the mantle involvement in the magmatism (epsilon Hf(t) from +. 13 up to +. 20). N and S directed, bi-vergent subduction developed as revealed by contemporaneous zircons: (1) along the northern margin (Hangai-Hentei), from Silurian-Early Carboniferous, subduction and accretion prevailed (epsilon Hf(t) from +. 3 up to +. 12 in associated zircons), which was re-initiated during the Permian, and (2) the contemporaneous Silurian-Devonian southern margin (Ereendavaa-Middle Gobi) still represented an extensional continental margin showing reworking of Neoproterozoic-Early Palaeozoic zircons from the basement. It turned into an active continental margin with starting arc magmatism in the Carboniferous (zircon mean epsilon Hf(t) +. 5.75).