Gochemical fractionation in migmatitic rocks from Serre granulitic terrane (Calabria, southern Italy

The Hercynian crustal section in the Serre (southem Italy) is about 20 km thick and includes sub-greenschist to granulite facies metamorphic rocks which have been used to study chemical modifications during metamorphism and migmatization. The rocks studied include: (l) Stilo phyllites and Mammola paragneisses from the upper crustal metasediments; (2) migmatitic paragneisses from the lower crustal segment that contain evidence for multi-stage melting and different degrees of partial melting and extraction of melts. The upper crustal rocks and the uppermost migmatites retain the composition of common shales. Descending to the lower levels the migmatitic metapelites show modified bulk compositions relative to common shales. The degree of chemical modification increases towards the base of the investigated section. The bulk-migmatites and the melanosomes show an enrichment in MgO, FeO*, Ti02, A1203, MnO, Sc, Y, V, Cr and REE and a depletion in Si02, K20, Na20, CaO, Rb, P, U. In the lowermost migmatitic metapelites enrichment of Th and Zr is also evident. Bulk migmatites have negative Eu anomaly which is larger in the residua. The migmatites reflect: i) diverse protolith bulk compositions, ii) the effects of hydrous melting and mica dehydration melting producing both melts crystallizing in situ and melts which were extracted, iii) a different degree of melting and of separation melt-restite, iv) different effects of rehydration on cooling. The overall geochemical features suggest that the lower migmatites were derived from a source similar to the Stilo phyllites, and that the upper migmatites from protoliths similar to the uppermost unmodified Stefanaconi paragneisses. With these assumptions the major element geochemical fractionation between restites, bulk migmatites and melts formed during the multi-stage partial melting has been modelled.