SmNd evidence for late Archaean crust formation in the Lapland-Kola Mobile Belt, Kola Peninsula, Russia and Norway

Abstract

A SmNd model age traverse across the Kola Peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early Archaean “Saamian” crust in the region. Granitoid gneisses and subordinate metasediments have been sampled from the main Archaean geological units, i.e. from northeast to southwest: the Murmansk domain (model ages of 2.68–2.96 Ga); Central Kola domain including samples from the Superdeep Borehole (2.74–2.92 Ga); Belomorian domain, including the eastern part of the Tersk area (2.70–2.93 Ga). The narrow range of depleted mantle model ages from 2.68 to 2.96 Ga for Archaean granitoid orthogneisses, close to the oldest crystallization ages from the region, indicate a major episode of late Archaean crustal growth and reveal no indication of ancient “Saamian” material in keeping with other geochronological evidence. Similar results (2.86 to 2.93 Ga) for Archaean metasediments support this conclusion. Proterozoic model ages for supracrustals from the western part of the Tersk area (2.16 to 2.23 Ga) indicate that the crust in this area is much younger than previously recognized. Initial ϵ Nd values for late Archaean samples with reasonable age constraints fall within a range of +2.3 to −2.5. Two well-dated samples from the Belomorian domain have the highest initial ϵ Nd values [ ϵ Nd(2.81 Ga) = + 2.0 and ϵ Nd(2.78 Ga) = + 2.3] and suggest that a depleted mantle source was present in the Kola region in late Archaean times. Dated samples from the Central Kola domain have lower initial ϵ Nd (between − 2.5 and + 1.2) suggesting a possible regional difference in mantle character. In contrast to the depleted signature of the Archaean orthogneisses between 2.7 Ga and ∼ 2.8 Ga, previous studies of earliest Palaeoproterozoic rift-related majic intrusives in the Kola and Karelia regions report negative ϵ Nd values at 2.4–2.5 Ga. Possibly these data reflect a change to more enriched-mantle values.