Petrogenesis of Archaean Trondhjemites, Tonalites, and Granodiorites from Eastern Finland: Major and Trace Element Geochemistry |
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| Authors: | MARTIN HERV? |
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| Institution: | Centre Armoricain d'Etude Structural des Socles, Universit? de Rennes I, Campus de Beaulieu Avenue du G?n?eral Leclerc, 35042 Rennes C?dex, France |
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| Abstract: | The grey gneisses of eastern Finland form the basement on whichthe Archaean greenstone belts were developed. They are composedof orthogneisses emplaced during two distinct magmatic episodes:2.86 Ga (Kivij?rvi gneisses) and 2.65 (Naavala gneisses). Theirmodal and chemical compositions are those of trondhjemites,tonalites and granodiorites (TTG). Both suites show low-K2Ocalc-alkaline differentiation trends (trondhjemitic). The aim of this study is to qualify and quantify the successionof different mechanisms by which the TTG series evolved. Theyoungest process was studied first, and the arguments then appliedin order to go back in time to the older ones. For each one,quantification was arrived at with the major elements, and theseresults provided a basis for calculation with the rare earthelements (REE). Finally the whole model was tested with othertrace elements. The petrogenetic model may be summarized as follows: meltingof the upper mantle to form a tholeiitic crust; melting of thesetholeiites transformed into garnet-bearing amphibolites to yieldthe parental magma of the TTG. The residue of the melt consistedof hornblende, plagioclase, clinopyroxene, and garnet with minoramounts of ilmenite and magnetite (10 < F < 30); and fractionalcrystallization of hornblende, plagioclase, and ilmenite withoccasional allanite and/or zircon in small amounts ((1-F) <40). No matter when they were emplaced during the Archaean, all theTTG of this part of the Baltic Shield arose from similar parentalmagmas. The petrogenetic study has shown that garnet and hornblendewere necessarily residual phases during the melting of the Archaeantholeiites. This constraint is very important, as it impliesthat the Archaean geothermal gradients occurring in subduction-zoneswere much higher than in modern times, thus allowing the partialmelting of the subducted oceanic crust. |
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