Positive Eu-Anomaly in Zircon and the Problem of Identifying Poorly Manifested Endogenic Events

A positive Eu-anomaly, extremely rare for this mineral, has been found for zircon from the silicate marbles of the Khapchan terrane of the Anabar Shield and syenites of the Sukharninsky complex of granitoids of Chukotka. In the silicate marble sample, a positive Eu-anomaly was found for late generation of zircon with an increased content of non-formula elements and volatile components. The age of zircon at the upper intersection is 1954±17 Ma (granulite metamorphism), the lower intersection is 984±49 Ma (tectonothermal event). Zircon with a positive Eu-anomaly tends to the lower intersection of discordia, which indicates a connection with late exposure to fluids. Zircon from syenites is characterized by black high-uranium marginal zones with a positive Eu-anomaly and enrichment with trace elements. The entire zircon forms a cluster with a concordant age of about 105 Ma. It is likely that the occurrence of a positive Eu-anomaly in zircon from syenites occurred as a result of the evolution of a closed magmatic system, without the involvement of an outside fluid. Zircon from silicate marble appears to have recorded the evolution of fluid composition: 1) the early fluid was enriched in boron and was predominantly aqueous, probably mobilized from the primary sedimentary protolith of silicate marbles; 2) the subsequent one, presumably separated in time from granulite metamorphism, was enriched in halogens (fluorine and chlorine), which was reflected in the composition of the late zircon generations. The paper shows that the lower intersection of the discordia, based on individual zircon generations with special geochemical characteristics (the content and spectra of REE, the amplitude and sign of the Ce- and Eu-anomalies, the content of non-formula trace elements, Th/U ratio), is very likely related to a real geological event rather than an analytical artifact.

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