Abstract:
Zircons from Archean (Belomorsky mobile belt and Karelian granite-greenstone province) and Proterozoic (Sumian and Jatulian) magmatic and metamorphic complexes of Karelia were studied with aim to assess influence of metamorphism upon generation and regeneration of zircons in different thermodynamic conditions. Accessory zircons were revealed to be the complicated systems very sensitive to changes of P-T metamorphic conditions. This capacity appeared in processes of their generation and regeneration in wide range of temperatures and pressures - from granulite to greenschist facies. Habits of metamorphic zircons vary from overgrowing crusts, of different size, to their normal grains with relict cores of the early generation crystals or without them. This generation of metamorphic zircons has often prismatic euhedral habit with oscillatory zoning, as in magmatic zircons. Flattened, sometimes twinned forms are typical for metamorphic zircons. It has been also revealed that the well known morphotype of metamorphic zircons, oval-shaped, rounded and ellipsoidal grains, is widespread in rocks of not only granulite, but amphibolite and epidote-amphibolite facies, too. Special attention is attracted to the fact that radiological dating of zircons should by preceded by a detailed geologic-petrological study of the host rock, and thorough mineralogical investigation of their genetic nature, including definition of morph type, peculiarities of internal structure, etc. Only in this way it may be clear: what geological process is dated.