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

Oleg I Volodichev,Oleg A Maksimov,Alexander I Slabunov,Tatiana I Kuzenko

doi:10.3390/min11101029

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.

Highlights

One of major problems to be resolved in modern geology is to answer the question: when did the present-day plate-tectonic processes, including subduction, begin on theEarth? [1,2,3,4,5,6,7,8,9,10,11,12,13]
The zircons extracted from massive eclogites were divided into three groups, based on their morphology and internal structure
This study has revealed that the massive eclogites from a boudin on Stolbikha Island contain three age groups of zircons: two Archean (Zrn-I and Zrn-II) and one Paleoproterozoic

Summary

Introduction

One of major problems to be resolved in modern geology is to answer the question: when did the present-day plate-tectonic processes, including subduction, begin on theEarth? [1,2,3,4,5,6,7,8,9,10,11,12,13]. There are two regions in the world where eclogite-facies metamorphism of Archean age has been described: the Belomorian Province in the Fennoscandian Shield [14,15,16,17] and the Bundelkhand Craton in the Indian Shield [18]. The presence of Early Precambrian eclogites in the Belomorian Province is an undoubted fact (Figure 1a). These rocks are divided into two age groups, Archean and Paleoproterozoic, based on field observations and petrological and geochronological

Results

Discussion

Conclusion