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

Abstract:

The Tojottamanselkä 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 of T=3.06±0.12 (2 σ) Ga, with corresponding INd=0.50848±9 (2 σ), or εNd(T)=-3.7±1.8. This age is in good agreement with the zircon U-Pb discordia age (3.1 Ga) reported by Kröner et al. (1981) and is interpreted as the time of magmatic emplacement. The distinctly negative εNd(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, possess εNd(T) values close to zero, hence they are thought to be derived by partial melting of basaltic sources with nearchondritic REE distribution patterns. Common Pb isotopic data yield an isochron age of 2.64±0.24 (2 σ) Ga which is in agreement, within error limit, with the published Rb-Sr isochron age of 2.73±0.24 Ga (Kröner et al. 1981). The age of ca. 2.7 Ga is interpreted as the time of regional metamorphism during which both Pb and Sr isotopes were rehomogenised. The tonalitic gneisses have highly fractionated REE patterns with (La/Yb)N ratios varying from 9 to 43. Like most Archean gneisses of TTG composition (tonalite-trondhjemite-granodiorite), they could be derived by partial melting of crustal sources of basaltic to granodioritic compositions. Direct derivation by melting of mantle peridotites is excluded. The present geochemical study indicates that the Tojottamanselkä gneisses have had a very complex history that involved multi-stage development. Together with the published age data for the basement gneisses and greenstone belts of eastern central Finland (Vidal et al. 1980; Martin et al. 1983a), we conclude that the Archean crustal development in Finland started at least 3.5 Ga ago and passed through a series of magmatic and metamorphic events at 3.1, 2.85, 2.65 and 2.5 Ga before the final intrusions of K-rich granites about 2.4 Ga ago. © 1984 Springer-Verlag.

Año de publicación:

1984

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Fuente:

scopus