Polymetamorphism in the Geological Development of the South Altai Metamorphic Belt, Central Asian Folded Belt

Abstract

The crystal complexes of the Hercynian South Altai Metamorphic Belt (SAMP), which is a part of the Central Asian Folded Belt, with a length of more than 1500 km, compose tectonic plates of different scales, in which the level of metamorphism in the early stages reached the conditions of high-temperature subfacies of amphibolite and, in places, granulite facies. In tectonic terms, the band of their exits is confined to the outskirts of the North Asian Caledonian continent, stretching from southeast to northwest along the southern slope of the Gobi, Mongolian and Chinese Altai to Eastern Kazakhstan, where they are represented in the Irtysh shear zone. Poly- and monometamorphic complexes have been established as a part of the SAMP. The age of granitoids of the late episode of metamorphism was determined for the Tsel tectonic plate of the Gobi Altai in the southeastern part of the SAMP: from 374 ± 2 to 360 ± 5 million years. These and previously obtained results show that the early episode of low-pressure metamorphism and the late episode of high-pressure metamorphism occurred in the intervals of ~390–385 and 375–360 MA, respectively, almost throughout this belt. In the interval between them, a short-term stabilization stage is fixed. These processes occurred during the closure of the basin with the oceanic crust of the Tethys series of the South Mongolian Ocean (Paleothesis I). The spatial position of the SAMP is due to the asymmetry of the structure of the basin, in which the active continental margin is represented along its northern part, and the passive one along the southern (in modern coordinates).