Isotopic-geochemical study of zircons from metabasites of the Kontokki dike complex: Age of regional metamorphism in the Kostomuksha structure

Results of local isotopic-geochemical and chemical examination of zircons from metabasites of the Kontokki dike complex in the Kostomuksha structure, western Karelia, Russia, make it possible to interpret the concordant U-Pb zircon age of 2674 ± 13 Ma as the boundary between regional amphibolite-facies metamorphism and accompanying metasomatism. Zircons from the metabasites arte heterogeneous and consist of central parts with relict magmatic cores, metamorphic intermediate zones (which are pale in CL), and younger metasomatically altered zones (dark in BSE images), which development along boundaries between zones and lengthwise arrays of cracks permeable to fluids. The dark altered zones are characterized by high (for zircons) concentrations of LREE, MREE, Th, U, Ca, Sr, Ba, Fe, and Al. The REE distribution in the zircons was proved to be much less susceptible to overprinted metasomatic processes than the U-Pb system of the same zircons. Characteristics of the REE distribution in the zircons makes these zircons comparable with metasomatic zircons. Genetically, the metasomatic processes that affected the geochemistry of the zircons could be related to synmetamorphic granitoid intrusions (Bibikova et al., 2005).