Major and trace element geochemistry and crustal evolution of Archaean granodioritic rocks from eastern Finland

Abstract

In the eastern part of central Finland vast areas of granodioritic gneisses surround the ca. 2.65 Ga greenstone belts. These granodioritic gneisses can be divided into two chronological and lithological units: (1) grey gneisses, 2.86 Ga old, with a typical Archaean TTG (tonalite-trondhjemite-granodiorite) composition; (2) augen gneisses, 2.51 Ga old, with a granodioritic to granitic composition. The Sr and Nd isotopic data preclude derivation of the grey gneisses from very much older continental material. The highly fractionated REE distribution patterns [(La/Yb) N = 37 − 103)] with few or no Eu anomalies militate against a direct mantle origin. The data suggest that the grey gneisses have most likely been derived by partial melting of short-lived basaltic crustal material (garnet-bearing amphibolite). It is estimated that the basaltic protoliths were separated from a mantle source no more than 150 Ma before the melting. Such an early basaltic crust might have been completely destroyed, or might have left some remnants which are now represented by mafic inclusions in the grey gneisses. The augen gneisses have a relatively high I Sr value (0.705) which suggests an origin by recycling of pre-existing crustal material (grey gneisses?). Their REE patterns are less fractionated than those of the grey gneisses and sometimes have negative Eu anomalies. Using the Sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting (< 50%) of mixed protoliths, with TTG/greenstone belt volcanics in a proportion of 70/30 by volume, followed by fractional crystallisation.