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RESEARCH PRODUCT

Multi-chronometric ages and origin of Archaean tonalitic gneisses in Finnish Lapland: A case for long crustal residence time

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The Tojottamanselka gneisses of the Koitelainen region, northern Finland, have been dated by the Sm-Nd and the common Pb methods. The Sm-Nd data of seven samples from a small area (100 m × 100 m) define an isochron ofT=3.06±0.12 (2σ) Ga, with correspondingINd=0.50848±9 (2σ), oreNd(T)=−3.7±1.8. This age is in good agreement with the zircon U-Pb discordia age (3.1 Ga) reported by Kroner et al. (1981) and is interpreted as the time of magmatic emplacement. The distinctly negativeeNd(T) value is found for the first time for Archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long (200–500 Ma) crustal residence time. A few samples, on the other hand, possesseNd(T) values close to zero, hence they are thought to be derived by partial melting of basaltic sources with nearchondritic REE distribution patterns.