Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province, Russia

Abstract

Although eclogites in the Belomorian Province have been regarded as Archean in age and among the oldest in the world, there are also multiple studies that have proposed a Paleoproterozoic age. Here, we present new data for the Gridino‐type eclogites, which occur as boudins and metamorphosed dykes within tonalite–trondhjemite–granodiorite gneisses. Zircon from these eclogites has core and rim structures. The cores display high Th/U ratios (0.18–0.45), negative Eu anomalies and strong enrichment in HREE, and have Neoarchean U–Pb ages of c. 2.70 Ga; they are interpreted to be magmatic in origin. Zircon cores have δ18O of 5.64–6.07‰ suggesting the possibility of crystallization from evolved mantle‐derived magmas. In contrast, the rims, which include the eclogite facies minerals omphacite and garnet, are characterized by low Th/U ratios (<0.035) and flat HREE patterns, and yield U–Pb ages of c. 1.90 Ga; they are interpreted to be metamorphic in origin. Zircon rims have elevated δ18O of 6.23–6.80‰, which was acquired during eclogite facies metamorphism. Based on petrography and phase equilibria modelling, we recognize a prograde epidote amphibolite facies mineral assemblage, the peak eclogite facies mineral assemblage and a retrograde high‐P amphibolite facies mineral assemblage. The peak metamorphic conditions of 695–755°C at >18 kbar for the Gridino‐type eclogites suggest an apparent thermal gradient of <39–42°C/kbar for the Lapland–Kola collisional orogeny.

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

“…In such a case, the summarized metamorphic evolution of the Shirokaya Salma eclogites could provide an example of the Neoarchean E‐HPGM category. By contrast, there is the Palaeoproterozoic scenario (Li et al., ; Liu et al., ; Skublov et al., , ; Yu, Zhang, Wei, Li & Guo, ). At this time, the regional Svecofennian and/or Lapland–Kola orogenies have exerted a widespread influence over most parts of the Fennoscandian Shield.…”

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

“…It has recently been noted [37] that to finish the ongoing debate, Archean zircons, that in addition to their geochemistry characteristic of zircons from eclogites contain inclusions of eclogite-facies minerals, primarily omphacite and garnet, should be found [38,39]. This contribution deals with the first finding of Archean zircon with omphacite inclusions and new geochemical and geochronological data on zircons from retrogressed eclogites that occur in the well-studied eclogite body on Stolbikha Island in the Gridino eclogite-bearing mélange [14,[22][23][24][25]27,29]. [27]).…”

“…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]. However, many researchers believe that an Archean age of the oldest eclogites is not reliably proven [19] and that the oldest eclogites are Paleoproterozoic in age: 2.0 Ga in the Tanzanian Craton [20] and 1.9 Ga in the Atabasca Terrane in the Canadian Shield [21] and in the Belomorian Province of the Fennoscandian Shield [22][23][24][25][26].…”

Temporal and spatial evolution of orogens: a guide for geological mapping

“…Signature tectonic features that characterize Trans-Hudson orogen include mounting evidence for continental subduction during diachronous plate convergence as documented by high-to ultrahighpressure/low-temperature mineral assemblage localities, and closure of intervening oceanic domains as witnessed by obducted ophiolitic suites (Scott et al, 1992;Peltonen and Kontinen, 2004;Garde and Hollis, 2010). Within the Trans-Hudson orogen of Laurentia and Baltica well-researched Paleoproterozoic eclogite localities include: the 1.83 Ga Kovik eclogite (Weller and St-Onge, 2017), the 1.87 Ga Kuummiut eclogite (Nutman et al, 2008b;Müller et al, 2018) of SE Greenland, and the 1.9 Ga Belomorian eclogite (Yu et al, 2017) of western Russia (Fig. 5).…”