Durbachity Trebicskeho masivu ve vrtu pozdatky V-5 – geneticke implikace

Melanocratic K, Mg-enriched granitoids (durbachites) represent one of the most typical feature of European Variscan orogeny. Although studied many times, their origin remains enigmatic. Borehole VP-5 drilled in 2006 at the village Požďátky near the town of Třebíč penetrated two contrasting intrusive units of the Třebíč durbachite pluton. The amphibole-biotite meladiorite (55-56 wt. % SiO2) is crossed here by a dyke of biotite (+amphibole) granite (66-69 wt. % SiO2). Chemical composition of amphibole and biotite in both rock types is identical. These two rocks are well comparable with the most mafic and the most acid rock types within the whole Třebíč pluton. From mathematic point of view, composition of all rock samples from the Třebíč pluton (Fig. 5) can be modeled by mixing of the mafic (5 % Amphibole + 45 % Biotite) with the acid (0 % Amphibole + 13 % Biotite) "endmembers" (depicted as points No. 1 and 3 in Fig. 5). But the overall evolution of the pluton, e.g. increase of the Si-content inwards (compare Fig. 1), is in contradiction with the hypothesis about contamination of primary mafic mantle magma with an acid crustal melt from its surroundings. The acid magma should have been transported from depth to the axial part of the pluton and from there penetrated horizontally to the contacts. Such process is from geological point of view hardly imaginable.