Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding

Abstract

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.

Comment on Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029

“…The 25% Jd content of the omphacite is similar to the lower limit for omphacite [12]. Omphacite-hosting zircon has a 207 Pb/ 206 Pb age of 2694.1 ± 8.1 Ma (Table 1, spot 13.1; [8]). The age of ~2.7 Ga, estimated from the zircon cores, was interpreted by us as the timing of granulite-facies regional metamorphism in BMB [6].…”

“…3. There are other considerable inconsistencies in the paper discussed [8]. Micronsized garnet inclusions were found only in Svecofennian ~1.9 Ga zircon rims (Figure 4, grains 4 and 11; [8]).…”

“…There are other considerable inconsistencies in the paper discussed [8]. Micronsized garnet inclusions were found only in Svecofennian ~1.9 Ga zircon rims (Figure 4, grains 4 and 11; [8]). Why have no garnets, occurring together with omphacite, been found in zircon cores of Archean age?…”

“…Therefore, the authors [8] believe that the omphacite inclusion in zircon of Archean age from an eclogite boudin found on Stolbikha Island in the Gridino area is an indisputable argument in favor of the Archean age of BMB eclogites. However, the attentive reader of the above contribution can notice that the evidence obtained by the authors, as well as the results of earlier studies, are misrepresented.…”

“…The zircon core with a micron-sized omphacite inclusion contains 275 ppm REE (131 ppm LREE and 144 ppm HREE), 126 ppm Y, 48 ppm Th and a Th/U ratio = 0.21 (Tables 1 and 2, Spot 13.1; [8]). A comparison of these parameters with median values for typical eclogite zircons [18] shows that they are inconsistent.…”