PROTEROZOIC CRATONIC FRAGMENTS IN MONGOLIA: SIGNIFICANCE IN THE INITIATION OF FORMATION OF THE CENTRAL ASIAN OROGENIC BELT

The Central Asian orogenic belt (CAOB) occupies a vast area between the Urals, Far East Asia, Siberian craton, North China, and Tarim cratons and is considered one of the largest regions of continental crustal growth on Earth. Despite extensive work on its Paleozoic tectonic reconstructions, the Proterozoic to early Paleozoic history especially the initiation mechanism and geological context of crustal growth are poorly constrained. Here we present new zircon U-Pb geochronologic data to constrain Proterozoic to early Paleozoic tectonic evolution of the Proterozoic cratonic fragments in the Zavkhan terrane and the Tuva-Mongolia zone of Mongolia and propose a tectonic scenario for the initiation of orogenesis in the CAOB. These regions host similar Proterozoic basement ages and late Neoproterozoic to Cambrian overlap assemblages. The ~ 2 Ga Gargan basement of the Tuva-Mongolia terranes, its overlying ~ 750 Ma Sarkhoi Group volcanics, and Neoproterozoic carbonate dominated strata of the Khuvsgul Group are all comparable to units exposed in the Zavkhan terrane. We suggest that the two regions formed a contiguous margin throughout the Proterozoic and that orogenesis began in the CAOB around these terranes in the latest Neoproterozoic with the collision of an exotic arc followed by slab reversal and accretion. Later oroclinal bending trapped supracrustal material between these terranes, which may have been an effective mechanism of continental crustal growth.