THERMOCHRONOLOGY OF MINGLING DYKES IN WEST SANGILEN (SOUTH‐EAST TUVA, RUSSIA): EVIDENCE OF THE COLLAPSE OF THE COLLISIONAL SYSTEM IN THE NORTH‐WESTERN EDGE OF THE TUVA‐MONGOLIA MASSIF

Abstract

In West Sangilen (South‐East Tuva, Russia), there are outcrops of metamorphic and magmatic complexes of early Caledonides, which are related to the period of long‐term collisional and post‐collisional events in the north‐ western edge of the Tuva‐Mongolian massif. The evolution of orogenic structures in West Sangilen is an example of the collapse of folded structures in case of changes in tectonic regimes from compression and transpression (collision period) to intra‐ and marginal continental transform‐shear extension (post‐collision period). Numerous geologic fea‐ tures give evidence of changes in the kinematics and characteristics of deformations, as well as in the conditions of metamorphism and magmatism in the study region. However, thinning of the crust during the collapse of the colli‐ sional orogenic structure has not been supported by any direct data. Indicators of such events are the complexes of combined dykes, which are abundant in West Sangilen, especially in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river. The most representative object is a combined basite‐granite dyke at the foot of the Tavit‐Dag mountain. Its position is controlled by the strike‐slip fault system. The thermochronological analysis of mingling rocks shows different ages of the closure of isotope systems: 494.8±5.4 Ma (U/Pb, zircon, basites), 489.7±7 Ma (U/Pb, zircon, granitoids), 471.2±1.9 Ma (Ar/Ar , amphibole, basites), and 462.5±1.0 Ma (Ar/Ar, biotite, basites). Taking into account the parameters of the closure of isotope systems (~800–900 °C, zircon, U/Pb; ~500 °C, amphi‐ bole, Ar/Ar; ~300 °C, biotite, Ar/Ar), the cooling curve of the mingling dyke is estimated. It corresponds to lowering of the temperature by 600 °C (900 °С  500 °С  300 °C) in the period from 500 (494.8±5.4) Ma to 461 (462.5±1.0) Ma. It is shown that the recent thermal events did not affect the mingling dyke located on the Tavit‐Dag site. The sequen‐ tial changes in the age of the closure of isotope systems are indicative of thinning of the crust in the study region during the post‐collisional collapse of the orogenic structure. According to the geological and thermochronological data, the mingling dykes on the Tavit‐Dag site were moved from the deep crust (~27 km) to a more shallow level (10 km) at a rate of about 0.5 km per 1.0 Ma. This process lasted for about 32 Ma, and the temperature was decreasing by 18.6 °С per 1.0 Ma.