Geochemistry of zircons from the proterozoic weathering profile of granitoids in Northern Karelia

Abstract

For many years zircon has been used in geology as the main mineral-geochronometer, since it is consid� ered to be maximally stable in secondary processes. However, the methodological base of U-Pbzir� conometry (6) postulates the possibility of transforma� tion of the isotopic system in zircon during hypergen� esis. Modern experimental investigations provide evi� dence for significant evolution of the morphology, structure, and composition of zircons during their interaction with aqueous solutions and magmatic flu� ids (3-5). At the same time, ideas about the behavior of accessory zircons during continental weathering are contradictory and not systematized. In this paper we present the first results obtained for zircons from the Paleoproterozoic weathering profile in Northern Karelia. The area of investigations is located in the eastern flange of the PanaKuolayarvinskaya structure, near Kiskinlampi Lake, where Archean granite gneiss sep� arated by the crust of weathering is overlain by sedi� mentary and volcanogenic-sedimentary Jatulian for� mations (2). The basement is composed of medium� granular pink granite gneiss intruded by veins of red microcline granite with a thickness of up to 3 m. The weathering crust developed after granitoids of the basement has a thickness of ~10 m. Its structure includes a zone of physical disintegration of the sub� strate, a zone of eluvial breccia with terrigenous-car� bonate cement, which composes the main part of the hypergene profile, and a clay zone (1).