Anatexis in the Hercynian basement of NE Sardinia, Italy: a case study of the migmatite of Porto Ottiolu

The migmatites of Porto Ottiolu, NE Sardinia, underwent polydeformation and partial melting during the Hercynian orogeny. The migmatites are cross-cut at the outcrop scale by leucogranite dykes and tourmaline-bearing pegmatit e dykes. Three groups of small-scale structures (D-1, D-2, D-3) and two typ es of leucosomes formed. One leucosome type is concordant with the main fol iation, while the other is discordant and mostly intruded along shear zones . Type I leucosomes consist of quartz, plagioclase, K-feldspar, biotite, and muscovite. Type II leucosomes contain additional garnet. The mesosomes show a variable texture defined by the orientation of biotite parallel to the m ost pervasive foliation D2. Based on mineralogy and textural features, two main types of mesosomes have been distinguished: MI mesosomes are texturall y similar to the Ordovician orthogneiss outcropping in NE; Sardinia, while MII mesosomes are characterised by a vague foliation due to a lower biotite content. Leucosomes are enriched in Rb, Sr, Ba, and Pb, and depleted in Li, Sc, V, C r, Co, Y, Zr, Nb, Cs, Th, and U compared to the mesosome. The total REE con tent varies from 40 to 100 ppm in the leucosomes and from 120 to 180 ppm in the mesosomes. Most leucosomes of both types show a positive Eu anomaly an d moderately fractionated REE patterns. The garnet-bearing type II leucosom es are characterised by high HREE content. Three metamorphic stages: M-1,M-2 and M-3 have been recognised in the migma tite of Porto Ottiolu area. M-1 assemblages have been only inferred on the pelite migmatite. Anatexis via the muscovite dehydration melting reaction o ccurred at T = 700-720 degreesC and P similar to 6-8 kbar during the M-2 st age. M-3 is an incipient recrystallisation defined by the formation of musc ovite. The leucogranites are peraluminous with more than 1.1% normative corundum. It is unlikely that the leucogranite, even though enclosed in the migmatite of Porto Ottiolu, should have been formed by the partial melting of the sa me protolith.