Multi-stage melting in the lower crust of the Serre (southern Italy)

The lower-crustal section exposed in the Serre, southern Italy, consists mainly of Al-rich metasediments, which underwent granulite-facies metamorphism, partial melting and melt extraction. The paper considers the formation of melts in metapelites and metagreywackes. Leucosomes and host rocks have been studied to investigate the melting process. Biotite-rich and biotite-free melanosomes with scarce felsic components are present; the biotite-rich types are widespread in the upper part of the section and the two types may occur side by side in the lower part. Na-rich and K-rich leucosomes including residual phases are interspersed within the metasediments; on the whole they do not show geochemical signatures suggestive of magmatic fractionation. Leucotonalitic types prevail among the sampled leucosomes, which generally are rare earth element (REE) depleted with positive Eu anomalies whereas the host rocks are REE enriched with overall negative Eu anomalies. Melanosomes and migmatites show restitic chemistries. The precursor metagreywackes underwent depletion in Na2O and enrichment in K2O. The precursor metapelites document general depletion in Na2O and they may be enriched or depleted in K2O. All the characteristics of the migmatites and of their components reflect a two-stage melting: (1) H2O-present melting, involving mainly plagioclase, and (2) dehydration melting of micas. All the metasediments underwent H2O-present melting, forming mostly sodic melts which, owing to their removal from the source as fast as they formed, did not accumulate in such proportions as to allow migration and mostly remained within the lower-crustal metasediments; metapelites also underwent variable dehydration melting, depending on chemical features and physical conditions, forming larger volumes of mobile granitic melts, most of which migrated far from the source. Extractions of 57-66 vol. % of total melts (sodic + potassic) from the most residual metapelitic melanosomes and of about 27-44 vol. % of potassic melts from metapelitic migmatites have been calculated. Higher volumes of the extracted melts have been calculated for the metapelites of the lower part of the section; the most depleted metagreywackes underwent melt extraction of about 9-13 vol. %. The two-stage melting occurred during the prograde metamorphism and continued during the isothermal decompression.