Nurmes paragneisses in eastern Finland, Karelian craton: Provenance, tectonic setting and implications for Neoarchaean craton correlation

The Neoarchaean Nurmes paragneiss belt, situated between the Ilomantsi and Kuhmo granite–greenstone terrains in eastern Finland comprises mainly turbidite wacke-derived, migmatised paragneisses with minor amphibolite intercalations and younger granitoid intrusions. The average chemical composition of typical biotite–plagioclase gneiss mesosomes of the paragneisses is nearly identical with the global average for Neoarchaean greywackes. The paragneiss mesosomes typically contain graphite, which exhibits δ13C values from −36.0‰ to −14.2‰, around a mean of –22.6 ± 5.6‰. These low δ13C values and the stratiform occurrence of graphite within discrete layers containing iron sulphides, are indicative of that it had a biogenic origin. The intercalated amphibolites correspond to oceanic tholeiites with flat to mildly LREE enriched chondrite-normalised distributions, and have ɛNd (2.7 Ga) values around +1.6 pointing to derivation from depleted mantle.After allowing for analytical uncertainties (2σ), U–Pb age data (SHRIMP, TIMS) on zircon grains from the paragneiss mesosomes and crosscutting granitoid plutons constrain deposition of the protolith wackes to between 2.71 Ga and 2.69 Ga. This estimate for the timing of sedimentation is also supported by a six-sample Sm–Nd isochron of 2756 ± 89 Ma obtained for the associated amphibolites. A relatively short average crustal prehistory for the sediment source terrains is implied by the clustering of ɛNd (2.7 Ga) values of the mesosomes around zero. Trace element and U–Pb data suggest that the source terrains comprised mainly 2.75–2.70 Ga TTG and/or sanukitoid-type plutonic and mafic volcanic rocks, for which the adjacent Ilomantsi terrain is the most likely source candidate.The data presented here indicate that the Nurmes paragneisses closely resemble wackes in the linear metasedimentary belts of the Superior Province of Canada, in terms of depositional age and detrital zircon provenance ages, as well as chemical composition.The postdepositional metamorphic–plutonic histories of the Nurmes and Canadian belts also seem to have been strikingly similar. These similarities lend further support to proposals advocating that the Karelia and Superior provinces were juxtaposed during the Neoarchaean. However, in contrast to the wackes in the Canadian belts, which are usually interpreted as deposits in accretionary prisms, the presence of MORB-type volcanic intercalations in Nurmes wackes suggests they were deposited in a back arc or intra-arc setting.