From orthogneiss to migmatite: Geochemical assessment of the melt infiltration model in the Gföhl Unit (Moldanubian Zone, Bohemian Massif)
Abstract
The Gföhl Unit is the largest migmatite terrain of the Variscan orogenic root in Europe. In its Rokytná Complex there is a well-preserved sequence documenting the entire migmatitization process on both outcrop and regional scales.
The sequence starts with (i) banded orthogneiss with separated monomineralic layers, through (ii) migmatitic mylonitic gneiss, (iii) schlieren migmatite characterised by disappearance of monomineralic layering and finally to (iv) felsic nebulitic migmatite with no relics of the original banding. While each type of migmatite shows a distinct whole-rock geochemical and Sr-Nd isotopic fingerprint, the whole sequence evolves along regular, more or less smooth trends for most of the elements.
The individual migmatites can be explained by different degrees of equilibration between the bulk rock and the passing granitic melt. The modelling of the mass balance resulted in identification of a component added by a heterogeneous nucleation of Fsp, Qz and Ap from the melt.