Early Precambrian retrogressed eclogites are abundant in the central and northern parts of the Belomorian Province of the Fennoscandian Shield (Gridino + Keret and Salma + Kuru-Vaara study areas, respectively). Older and younger eclogites are recognized and their Archean and Paleoproterozoic ages are argued. Archean eclogites are intensely retrogressed and occur in amphibolite boudins in the tonalite-trondhjemite-granodiorite (TTG) gneiss matrix of the Archean Gridino eclogite-bearing mélange. Less retrogressed Paleoproterozoic eclogites form patches in mafic dikes and some amphibolite boudins; their Paleoproterozoic age is supported by U-Pb/SIMS data on zircons depleted in heavy rare earth elements (REE) with omphacite, garnet, and kyanite inclusions, and Sm-Nd and Lu-Hf mineral isochrons. Archean eclogites contain Archean heavy rare-earth elements (REE)-depleted zircons with garnet and zoisite inclusions and Archean garnets. No omphacite inclusions were found in these zircons, and this fact was considered as evidence against the existence of Archean eclogites. This study reports on the first finding of omphacite (23–25% Jd) inclusions in 2.68 Ga metamorphic zircons from eclogites from the Gridino eclogite-bearing mélange. The zircons are poorly enriched in heavy REE and display a weak negative Eu-anomaly but a poor positive Ce-anomaly typical of eclogitic zircons. Thus, zircons with these decisive features provide evidence for an Archean eclogite-facies metamorphism.

A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geo chronological) of the genesis of zircons in their various populations and, correspondingly, the age of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). The three populations of zircons separated from two rock samples comprised xenogenic, magmatic ("gabbroic"), and metamorphic zircons, with the latter found exclusively in the sample of retro graded eclogitized gabbroids. The zircons of group I are xenogenic and have a Meso to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and the geochem istry of these zircons is very diverse. The zircons of group II contain mineral inclusions of ortho and clinopy roxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons were formed during the late magmatic crystallization of the gabbroids at temperatures of 1150-1160°C, and their U-Pb age of 2389 ± 25 Ma corresponds to this process. The eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that the marginal portions of the pris matic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end member. The zir cons of group III contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have an U-Pb age of 1911 ± 9.5 Ma, which corresponds to the age of the overprinted amphibolite facies metamorphism.

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