Syntectonic melt pathways in granitic gneisses, and melt-induced transitions in deformation mechanisms

Abstract

Partial melting of granodioritic gneisses in the contact aureole of the Bergell Pluton (Central Alps) occurred during regional deformation. Melting occurred in the presence of water, which was released from the pluton. The presence of melt is evidenced by local segregations of granite in shear zones, veins and dykes, and by grain-scale interstitial films of K-feldspar and quartz that do not occur in the unmelted protolith. These films are oriented parallel, as well as perpendicular to the foliation plane. In contrast, on the outcrop scale, cm-wide leucosome veins are oriented almost exclusively parallel to the foliation plane, indicating foliation-parallel flow. The partially molten granitic rocks contain dm-long clasts of restitic, well-foliated gneiss, showing a higher competence than their granitic matrix. K-feldspar is lacking in these clasts, the microstructure of which is characterized by elongate aggregates of quartz and feldspar, both dynamically recrystallized. In contrast, the granitic matrix is characterised by a random distribution of minerals, whith a shape preferred orientation defining a weak foliation. These microstructures are indicative of granular flow, whereas the microstructures of the clasts indicate dislocation creep involving dynamic recrystallization. The presence of K-feldspar controls the onset of melting and thus the transition from dislocation creep to granular flow. The weakening resulting from this transition is indicated by the formation of strong clasts in a weaker matrix.