TECTONIC AFFINITY OF THE ALTAI-MONGOLIA TERRANE IN THE CENTRAL ASIAN OROGENIC BELT

The Altai-Mongolia terrane locates in the southern Central Asian Orogenic Belt (CAOB). It consists of the Chinese Altai, the Mongolia Altai and the Russia Altai, and thus is a key to the reconstruction of the lateral crustal accretion process in the southern CAOB. However, the tectonic evolution and affinity of the Altai-Mongolian terrane are hotly disputed and still not well constrained. Early Paleozoic metasedimentary rocks are extensive in the Altai-Mongolian terrane and their provenance is thus pivotal to unravel the tectonic history of this terrane. In the Chinese Altai, the metasedimentary rocks were divided into Habahe Group and Kulumuti Group. Our geochemical studies indicate that the rocks have low Chemical Index of Alteration (CIA) and high Index of Compositional Variability (ICV) values, different from PAAS, indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lack of alumina-rich minerals. Their moderately enriched light rear earth elements, relatively flat heavy rear earth element and slightly enriched Nd isotopic compositions suggest an evolved arc-related acidic-intermediate igneous source, which are evidenced by the occurrence of volcanic detritus in the rocks and the relatively euhedral and unrounded detrital zircons. Detrital zircons from metasedimentary rocks of the two groups have similar age spectra and are dominated by Early Paleozoic to latest Neoproterozoic zircons with a few older grains formed in the early Neoproterozoic and the Mesoproterozoic to Neoarchean. In the Mongolia and Russia Altai, the Early Paleozoic rocks show similar geochemical characteristics and age spectrum of detrital zircons to those of the two groups in the Chinese Altai. This suggests that the metasedimentary rocks of Altai-Mongolia terrane have same provenance and deposited in same tectonic setting. Compared with adjacent terranes, the age spectrum of the Altai-Mongolia terrane is most similar to that of the Tarim Craton, but distinct from other surrounding terranes. Therefore, the Tarim Craton most likely provided Precambrian materials for the Early Paleozoic sediments now exposed in the three subterranes of the Altai-Mongolia terrane, which suggests that the whole Altai-Mongolia terrane has a close tectonic affinity to the Tarim Craton.