THE SARKHOI GROUP IN EAST SAYAN: NEOPROTEROZOIC (~770-800 Ma) VOLCANIC BELT OF THE ANDEAN TYPE
The Sarkhoi Group localized in the stratotype area is composed of various volcanics and volcanoclastics more than 4 km in total thickness. The volcanics of the lower part are greenschist facies altered rocks ranging from basalt to dacite, and those of the upper part are mainly rhyolitic ignimbrites. The composition, sequence, and facies features of these volcanics show their affinity to the active continental margin. This is confirmed by geochemical data evidencing the participation of the Early Precambrian continental crust in their formation. The U-Pb zircon dating (SHRIMP) of the Sarkhoi volcanics yielded a Neoproterozoic age of the ignimbrites (782 ± 11 Ma, concordant cluster N = 8). The tonalite-trondhjemites of the Sumsunur complex in the Gargan block are plutonic analogs of these volcanic rocks. Coeval granitoids were also revealed in other parts of the Tuva-Mongolian and Dzavhan massifs. The ~1500 km long Sarkhoi-Dzavhan volcanic belt is a fragment of the Neoproterozoic active continental margin separated from the continent. In the Middle Neoproterozoic, the geodynamic system also included the Oka accretionary prism joining the Sarkhoi continental margin and the Shishkhid island arc. Coeval analogs of the above structures are also reconstructed in the Yenisei Range and Baikal-Muya belt. This suggests that the Sarkhoi belt was initially localized along the margin of the Neoproterozoic Siberian continent. But the exotic origin of the Precambrian massifs in southern Siberia is not ruled out either.