In this paper new whole rock data on Ordovician ortho- and augen-gneisses cropping out in North-Eastern Sardinia are presented. It is evidenced that the core of the outcrop (ortho-gneisses) has a less evolved geochemical composition than the peripheral portions (augen-gneisses). Inter-digitate contacts between the different portions are observed. Within ortho-gneisses are recognised Mafic Microgranular Enclaves, indicating the possible role played by magmatic interaction processes in their genesis, whereas magmatic enclaves are lacking in augen-gneisses. Discriminant diagrams are used in order to understand the type and extent of metasomatic/alteration processes suffered by these rocks. Analysis reveals that the effect of these processes can be considered negligible and that the geochemical characteristics of studied samples are the result of magmatic evolution. On the basis of field and geochemical data, a two-stage petrogenetic evolution is suggested to explain the compositional spectrum of studied rocks. Ortho-gneisses are considered as produced by Mixing plus Fractional Crystallization processes whose evolution in time produced, in the core region of the magma chamber, magma batches with different degrees of hybridisation bearing Mafic Microgranular Enclaves. Augen-gneisses are interpreted as the result of Fractional Crystallization processes acting in the peripheral portions of the magma chamber.
The Ordovician Gneisses of North-Eastern Sardinia (Italy): Hypotheses for the Petrological Evolution of their Protoliths
PERUGINI, Diego
2003
Abstract
In this paper new whole rock data on Ordovician ortho- and augen-gneisses cropping out in North-Eastern Sardinia are presented. It is evidenced that the core of the outcrop (ortho-gneisses) has a less evolved geochemical composition than the peripheral portions (augen-gneisses). Inter-digitate contacts between the different portions are observed. Within ortho-gneisses are recognised Mafic Microgranular Enclaves, indicating the possible role played by magmatic interaction processes in their genesis, whereas magmatic enclaves are lacking in augen-gneisses. Discriminant diagrams are used in order to understand the type and extent of metasomatic/alteration processes suffered by these rocks. Analysis reveals that the effect of these processes can be considered negligible and that the geochemical characteristics of studied samples are the result of magmatic evolution. On the basis of field and geochemical data, a two-stage petrogenetic evolution is suggested to explain the compositional spectrum of studied rocks. Ortho-gneisses are considered as produced by Mixing plus Fractional Crystallization processes whose evolution in time produced, in the core region of the magma chamber, magma batches with different degrees of hybridisation bearing Mafic Microgranular Enclaves. Augen-gneisses are interpreted as the result of Fractional Crystallization processes acting in the peripheral portions of the magma chamber.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
