Petrogenesis of Archaean Trondhjemites, Tonalites, and Granodiorites from Eastern Finland: Major and Trace Element Geochemistry

Abstract

The grey gneisses of eastern Finland form the basement on which the Archaean greenstone belts were developed. They are composed of orthogneisses emplaced during two distinct magmatic episodes: 2.86 Ga (Kivijärvi gneisses) and 2.65 (Naavala gneisses). Their modal and chemical compositions are those of trondhjemites, tonalites and granodiorites (TTG). Both suites show low-K2O calc-alkaline differentiation trends (trondhjemitic). The aim of this study is to qualify and quantify the succession of different mechanisms by which the TTG series evolved. The youngest process was studied first, and the arguments then applied in order to go back in time to the older ones. For each one, quantification was arrived at with the major elements, and these results provided a basis for calculation with the rare earth elements (REE). Finally the whole model was tested with other trace elements. The petrogenetic model may be summarized as follows: melting of the upper mantle to form a tholeiitic crust; melting of these tholeiites transformed into garnet-bearing amphibolites to yield the parental magma of the TTG. The residue of the melt consisted of hornblende, plagioclase, clinopyroxene, and garnet with minor amounts of ilmenite and magnetite (10 < F < 30); and fractional crystallization of hornblende, plagioclase, and ilmenite with occasional allanite and/or zircon in small amounts ((1-F) < 40). No matter when they were emplaced during the Archaean, all the TTG of this part of the Baltic Shield arose from similar parental magmas. The petrogenetic study has shown that garnet and hornblende were necessarily residual phases during the melting of the Archaean tholeiites. This constraint is very important, as it implies that the Archaean geothermal gradients occurring in subduction-zones were much higher than in modern times, thus allowing the partial melting of the subducted oceanic crust.