207Pb–206Pb age of sources of Late Cenozoic volcanic rocks of the crust-mantle transition in relation to the age of ophiolites and ancient blocks exposed on the surface of the crust: transect Kitoi–Baydrag of the Baikal-Mongolian region

Abstract

The results of determinations of Pb isotope ratios in Late Cenozoic volcanic rocks of the southwestern part of the Baikal rift system along the Kitoi-Baidrag transect are presented. The obtained 207Pb–206Pb estimates of the restite material incubation time in the springs are in good agreement with the Precambrian dating of ensembles of ancient continental blocks and ophiolite belts exposed on the Earth's surface. The sources are distinguished along the transect: the ancient Gargan block of the Tuva-Mongolian microcontinent (massif) with the Hadean protolith and Archean-Neoproterozoic events; Khangai and, possibly, Dzhida-Khangai slabs, and the ancient Baidrag block, a fragment of the Tarbagatai-Dzabkhan microcontinent. According by the dating of the sources, the tectonosphere of the early and middle mantle geodynamic epochs in the Baikal-Mongolian region included 3 layers: lower (proto-mantle), middle (mantle evolved), and upper (crust-mantle transition, CMP). In the Khangai continental domain, the protoliths of a wide Precambrian age range of sources were generally modified about 660 Ma ago. At the end of the Neoproterozoic and in the Early-Middle Phanerozoic, the mantle stabilized against the background of transformations of the ILC zone. In the tectonosphere of the latest geodynamic stage, the processes of transformation of three layers became active again: the proto-mantle, the evolved mantle, and the IMF.