Trondhjemitic leucosomes in paragneisses from NE Sardinia: geochemistry and P–T conditions of melting and crystallization

Abstract The sedimentary-derived migmatites from Porto Ottiolu consist of mesosomes and tonalitic to trondhjemitic leucosomes. These rocks contain quartz + plagioclase + muscovite + biotite in different proportions and locally fibrolite, K-feldspar and kyanite relicts. The contact between leucosomes and mesosomes is commonly marked by mm- to cm-thick melanosomes mainly formed by oriented biotite and fibrolite, muscovite, and variable amounts of quartz and plagioclase. Pressure-temperature (P-T) modeling using pseudosections and their contouring by isopleths for modal abundances of minerals and melt and Si/Al and Na/K ratios of the melt suggests that anatexis occurred at ~700 °C and 1.1 GPa with the contribution of ~2 wt% of free water. The complete crystallization of the leucosome melt was finalized at P-T conditions of 640–690 °C and 0.40–0.55 GPa. The leucosomes are folded or parallel to the S 2 schistosity indicating their pre to syn-D 2 formation. The leucosomes are enriched in SiO 2 , CaO, Na 2 O, Sr and depleted in Fe 2 O 3 , MgO, TiO 2 , K 2 O, Rb, Ba, Cr, V, Zr, Nb, and REE, except Eu, compared to the surrounding mesosomes. The high CaO contents up to 3.3 wt%, the low K 2 O contents (mean 0.57 wt%) and Rb/Sr ratios (