U–Pb (SHRIMP-II) Age of Zircon from Granites of Bolshoi Tyuters Island (Gulf of Finland, Russia) and the Problem of the Ediacaran Thermal Event in the Region

Abstract

New data on the U–Pb age (SHRIMP-II) and trace element composition (SIMS) of zircons from granites of Bolshoi Tyuters Island (Outer Islands of the Gulf of Finland) are presented. The upper intersection of the discordia (1825 ± 11 Ma) is taken as the age of crystallization of granites cutting through secondary quartzites and thereby determines their youngest age. Subconcordant zircons located in the upper part of the discordia have growth oscillatory zoning and geochemical characteristics of zircons of magmatic origin. The age of the lower intersection of discordia and concordia is about 570 Ma. It is supported by the independent zircon generation represented by black CL domains and rims in magmatic zircons characterized by increased contents of non-formula elements (light REEs, Ca, P, Ti, Nb, etc.), up to anomalous values. The age of the lower intersection of discordia and concordia can be considered as related to the Timan (Ediacaran) or Finnmark (Early Caledonian) thermal activation of the Fennoscandian Shield discovered earlier based on zircons from Kola Group gneisses.