Disequilibrium melting in granulite-facies metasedimentary rocks of the Northern Serre (Calabria-Southern Italy)

Leucosomes, mesosomes and melanosomes, including 12 leucosome-host rock pai rs have been studied for Sr and Nd isotopic systematics in migmatitic metap elites of the Serre (southern Calabria) which experienced granulite facies conditions in Hercynian times. Most samples came from the deep to middle pa rt of a metapelitic unit, which is 5-6 km thick. The leucosomes presently occurring in the migmatites are mainly K-poor pera luminous leucotonalites; the host rocks (metapelites, metagreywackes and op x-bearing rocks) are more or less depleted in "granitic" component and have molar Al2O3/(CaO + Na2O + K2O) (A/CNK) ratios up to 15; the highest values relate to garnet + sillimanite-rich metapelites. Sm/Nd ratios are generall y lower in the leucosomes and Rb/Sr ratios are very low (much less than 1) and lower than in related mesosomes and melanosomes. The isotopic Sr and Nd values vary in a wide range within the various kinds of host-rocks and wit hin the leucosomes. A correlation of Nd- and Sr-isotopic compositions betwe en coexisting leucosomes and host-rocks has not been found. The isotopic Sr and Nd values of the leucosomes are less variable than the host-rocks; mel anosomes having A/CNK ratios > 3 concentrate at the lowest epsilon Nd-290 M a values, owing to retention of garnet and accessories and depletion of fel dspars. The leucosomes define a hyperbolic trend where most samples concent rate in the middle part. These features are interpreted as reflecting melti ng of several rock types generating melts having a weighted mean compositio n from the contributing sources. The prevailing leucotonalites and their ve ry low Rb/Sr ratios indicate that plagioclase played a more important role than mica. in the melting processes which produced the studied leucosomes. Since assemblages lacking or very poor in biotite and K-feldspar are common in metapelitic restites, potassic melts, as one should expect from melting of micas, must have been extracted before the leucosomes under study forme d.