Polymetamorphism of the Chupa Sequence of the Belomorian mobile belt (Fennoscandia): Evidence from the isotope-geochemical (U-Pb, REE, O) study of zircon

Abstract

U-Pb age and isotope-geochemical features were determined for zircon from kyanite gneisses and amphibolites of the Chupa Sequence of the Belomorian mobile belt (BMB) of the Fennoscandian shield. The cores of the zircon from the gneisses marks the Neoarchean events within 2700–2800 Ma known in the BMB, while those of the amphibolites correspond to the age of magmatic crystallization (2775 ± 12 Ma). The inner rims of zircon from the amphibolites and gneisses likely record two different Neoarchean metamorphic events (2650 ± 8 and 2599 ± 10 Ma, respectively). The outer rims record Paleoproterozoic metamorphism with an age of 1890 Ma, which formed the modern appearance and mineral assemblages of the rock association. The value of δ18O in the zircon from the gneiss is 8.6‰ in cores, slightly decreases to 8.0‰ in inner rims, and sharply decreases to 3.9‰ in outer rims. The value of δ¹⁸O in the zircon from the amphibolite is around 6.2‰ in cores, increases up to 8.6 in inner rims, and decreases to 5.2‰ in outer rims. A significant decrease of δ¹⁸O is likely related to the anomalous composition of Svecofennian metamorphic fluid restricted to local shear zones. The geochemical features of the zircons in combination with their morphology and anatomy make it possible to distinguish zircon generations of different age and change in metamorphic environments.