Stages of Formation of the South Altai Metamorphic Belt (Central Asia)

The Hercynian mobile belts in Central Asia include the proper Hercynian and late Hercynian (Indo-Sinian) belts, whose formation is associated with the evolution of the South and Inner Mongolian basins with oceanic crust. Within the South Altai metamorphic belt (SAMB), rock complexes compose tectonic slivers of different ranks. At the early stages, their metamorphic alteration occurred under conditions of the high-temperature subfacies of the amphibolite and, in places, granulite facies. Structurally, the band of the outcrop of these complexes is confined to the Caledonian North Asian continental margin and stretches along the southern slope of the Gobi-Mongolian-Chinese Altay Mountains from southeast to northwest (East Kazakhstan), where they occur in the Irtysh strike-slip zone. We assign these complexes to the Hercynian SAMB running for more than 1500 km. The latter comprises poly- and monometamorphic complexes. Late metamorphic granitoids of the Tseel tectonic sliver (Gobi Altay) in the southeast of the SAMB have been dated at 374 +/- 2 and 360 +/- 5 Ma. The previous data and these results show that the early (similar to 390-385 Ma) low-pressure and late (375-360 Ma) high-pressure metamorphism proceeded almost along the entire belt. The interval between them was a short tectonic lull. These processes took place during the closure of a Tethyan basin of the South Mongolian Ocean (Paleo-Tethys I). The spatial position of the SAMB was controlled by the structural asymmetry of the basin, with an active continental margin at its northern edge and a passive one at the southern edge (in the present-day coordinates).