Genetic link between pegmatites, granites and migmatites from NE Sardinia, Italy

The migmatized orthogneiss of Porto Ottiolu (NE Sardinia) contains several tourmaline pegmatites and leucogranite dykes. Field data suggest a simultaneous emplacement of leucogranites and pegmatites in an extensional environment with the formation of sub-parallel fractures. Leucogranites consist of quartz, plagioclase, K-feldspar, biotite, muscovite and garnet. Pegmatites consist of quartz, plagioclase, K-feldspar, muscovite and tourmaline. Leucogranites are enriched in MgO, TiO2, Fe2O3, P2O5, and depleted in K2O, Rb, Sr, Be in comparison with pegmatites. Modelling of REE in the leucogranites supports the idea that some Hercynian granites were generated through a palingenetic partial melting of the Early Paleozoic granitoids, The association between Ab-rich leucogranite and Or-enriched pegmatite of Porto Ottiolu may be explained by the behaviour of boron. When abundant B is dissolved in the melt, the minimum melting point is shifted towards the Ab apex, when tourmaline precipitates the minimum melting point is shifted towards the Qtz-Or side.