Age of granites of Vitim-Kansk complex in the Baikal region (according to radiological data)

The age of porphyry-like granites of the four mountain-masses in the Vitim-river vicinity was determined by calium-argon and rubidium-strontium methods. The results of analyses are presented. To analyze discordance of the K-Ar method, the earlier developed graphic method was used, based on the different capability of various minerals to keep radiogeneous argon and on the mineral age accordance to the real age or time of intensive thermal event. Discordance of the definite age values for separate minerals shows that the rocks suffered later thermal influence though at different degrees. The graphic interpretation of discordant K-Ar mineral ages and the evolution diagram of the porphyry-like granite Rb-Sr system are given. Spread of points in the diagram in accordance to the straight line do not surpass the analytical error limits and allows to suppose that the rocks studied correspond to the isochronous model. The radiological data show that the age of the studied granites constitutes 320-340 mln years. Wide distribu.tion of granite intrusions of the coal age in the North-West Baikal region testify to the fact of the intensive tectono-magnetic activization of the area in this period