Neoarchean-Paleoproterozoic granulite-facies metamorphism in Uzkaya Salma eclogite-bearing mélange, Belomorian Province (Russia)

Abstract

A TTG gneiss of Uzkaya Salma, Belomorian Province and retrogressed eclogites embedded in it have been thoroughly analyzed. The gneiss is made of plagioclase, K-feldspar, biotite, and quartz with accessory epidote, chlorite, titanite and zircon. The retrogressed eclogite is featured by garnet, clinopyroxene, plagioclase, amphibole, and quartz with subordinate orthopyroxene, muscovite, epidote, and melt phase exhibiting a typical symplectic texture. Diversity of compositional zonation in garnets depends on different garnet diameters and is related to ion diffusion between garnet and clinopyroxene/amphibole during retrogression. Clinopyroxene is mostly diopside, although a few Jd-enriched varieties (Na-rich clinopyroxene) have been found at some sites, indicating an omphacite precursor. Plagioclase formed in both symplectites and aggregates displays a wide compositional range, revealing alteration at certain PT conditions. K-bearing ternary feldspar casually occurs in association with muscovite, zoisite/epidote, and melt phase, denoting a partial melting product. Phase equilibria modeling, combined with mineralogical constrains yielded a clock-wise PT path with peak conditions of P≥17–18kbar, T=700–750°C, followed by granulite-facies at P=13–15kbar, T=800–850°C, and, eventually into the (upper) amphibolite-facies of T≤700–750°C at lower pressures. A geothermal gradient of ∼12–13°C/km is inferred accordingly, rather common for the Belomorian eclogites and some other eclogite/high-pressure granulite belts (EHPG) on Earth. Zircon U-Pb dating shows that the TTG gneiss formed at 2.73–2.74Ga, while the protolith of the eclogites may have experienced magmatism/anatexis at ≥2.77Ga. There is no implication of any Archean eclogite-facies metamorphism in this work. Nevertheless, a granulite-facies metamorphism, featured by a diopside+plagioclase+amphibole±garnet, orthopyroxene assemblage, has been determined at 2.7Ga that also superimposed the host gneiss. Afterwards, due to regional plume activities, repeated thermal events of granulite-facies metamorphism, occurred at least locally at 2.4–2.5Ga. An eclogite-facies metamorphism at 1.9–2.0Ga was dated only that corresponds to the Lapland-Kola orogeny. Further retrogression into the (upper) amphibolite-facies was documented at 1.8Ga. The proposed metamorphic PT path corresponds to the late Paleoproterozoic eclogite-facies event, while before that, only (repeated) granulite-facies metamorphism has been recorded by zircons.