This paper summarizes the results of geochronological studies of selected rock forming minerals obtained using U-Pb (Pb-Pb) and Sm-Nd isotope systems. The objects of the isotope study were rock-forming minerals from gneisses and metaintrusive rocks of the Belomorian and Svecofennian mobile belts of the Fennoscandian Shield. Only isotope data for rock-forming minerals were selected for the analysis, as these rocks more often allow linking the age estimate directly to the mineral- and rock-forming process. It is concluded that the isotope systems of the aluminosilicate minerals are highly sensitive to the presence of xenomorphic phases and changes in the isotope-geochemical parameters of the mineral-forming environment. There are two possible causes of variance in the Pb sotope distribution: the inclusion of certain mineral phases rich in radiogenic lead or the non-uniform distribution of lead isotopes during recrystallization of the mineral. In the first case, the age based on polyphase compositions will be geologically meaningful if the xenogenic phase(s) and the host mineral were cogenetic. In the second case, it will reflect the isotopic-geochemical parameters of the mineral-forming environment during isotopic re-equilibration, which is usually associated with complete or incomplete recrystallization of mineral grains. We suggest using the isotopic heterogeneity of rock-forming minerals as an indicator of superimposed events and to identify latent mineral forming events. This approach has made it possible to identify a previously unknown event of 1.1–1.3 Ga BP based on kyanites from Chupa gneisses in the Belomorian Belt, and a metamorphic event of approximately 1.75 Ga BP based on dating of garnet from gneisses and migmatites within the Russian part of the Svecofennian Belt.