Garnet and zircon geochronology of the Paleoproterozoic Kuru-Vaara eclogites, northern Belomorian Province, Fennoscandian Shield

Abstract

The Belomorian Province of the Fennoscandian Shield exposes numerous Precambrian eclogites, which makes it significant for the study of early tectonic processes. The age of these eclogites has been discussed for more than 15 years and regarded as either Archean or Paleoproterozoic. In the Kuru-Vaara quarry within the northern Belomorian Province, the eclogitic assemblage is preserved in concordant mafic boudins in felsic gneisses and a partially eclogitized gabbro-norite dike cutting discordantly through the gneiss fabric. Both eclogite types preserve zircon cores with a magmatic geochemical signature that yield protolith ages of ca. 2.88 Ga for a mafic boudin and ca. 2.44 Ga for the eclogitized gabbro-norite. Ca. 1.9 Ga zircon rims and grains from the eclogites show low Th/U ratio and HREE depletion, reflecting the growth of metamorphic zircon in equilibrium with garnet. In the eclogite boudin, the Archean zircon cores yield δ18O = 5.1–5.9‰ typical of mantle melts; the oxygen isotope composition of garnet (δ18O = 4.0–5.0‰) is in equilibrium with that of the 1.9 Ga zircon (δ18O = 4.5–5.4‰). Garnet Lu-Hf geochronology coupled with U-Pb zircon geochronology constrains prograde metamorphism for the Kuru-Vaara eclogites at 1.92–1.89 Ga. Mineral inclusions of garnet, zoisite, plagioclase, kyanite, amphibole, quartz, and low-Na clinopyroxene in ca. 1.9 Ga zircon from the eclogite boudin imply epidote–amphibolite/amphibolite facies conditions for the prograde metamorphism. All data point to a Paleoproterozoic age of the eclogite facies metamorphism.