Evoluzione metamorfica di rocce eclogitiche e granulitiche della Zona Assiale della Catena Varisica sarda, Sardegna NE

The High Grade Metamorphic Complex of the Axial Zone of the variscan Sardinian chain is characterized by the widespread occurrence of metabasite bodies and lenses within the migmatites and gneisses. These metabasite bodies, decametric to hectometric in size, show polyphase metamorphic evolution and preserve evidences of high temperature (granulite facies) or high pressure (eclogite facies) metamorphism. This study is focused on the metabasites from NE Sardinia (eclogites and granulites) cropping out a few kilometers north of Olbia. A geological survey was carried out and a schematic geological map of the studied area, comprising Iles, Montiggiu Nieddu, Nodu Pianu and Sos Aranzos localities, was drawn. Several samples from the various lithologies were collected and then analyzed; the petrographic, mineral chemistry and geochemical data were used for the determination of P-T conditions by thermodynamic modelling and conventional geothermobarometry. The eclogites cropping out at Iles locality are characterized by garnet and kyanite porphyroblasts surrounded by symplectitic and coronitic microstructures. Based on garnet compositional zoning and P-T pseudosection calculations, the P-T path of these rocks reveals a first prograde segment until baric peak P-T conditions of T = 620-690 °C and P = 2.0-2.3 GPa were reached in eclogite facies. Subsequently, the rocks underwent a slight temperature increase accompanied by a pressure decrease until they reached their thermal peak at HP conditions in the granulite/eclogite facies (T = 650-700 and P = 1.4-2.1 GPa). The resulting clockwise P-T path is similar to that observed for other Sardinian eclogites described in the literature. The amphibolite lens at Mt. Nieddu consists of plagioclase-banded amphibolites and ultramafic amphibolites. The plagioclase-banded amphibolites, characterized by an alternation of withish plagioclase- and dark-green amphibole-rich layers, locally show centimetric-thick layers with several garnet porphyroblasts up to 1 cm in size. The microstructural investigations and thermodynamic modelling on the plagioclase-banded amphibolites and on their garnet rich layers allow to define the following P-T evolution; these rocks firstly recorded an isothermal pressure increase up to the metamorphic peak ( T = 690-740 °C, P = 1.3-1.4 GPa), before the retrograde equilibration in amphibolite facies. The resulting P-T path is anticlockwise. The ultramafic amphibolites, which are featured by coronitic microstructures around igneous olivine and plagioclase, occur as a lens within the plagioclase-banded amphibolites. The coronitic microstructures consist of single or composite thin corona of orthopyroxene and clinopyroxene around olivine as well as by spinel + clinopyroxene symplectite and garnet coronas around plagioclase. All these minerals were partially replaced by amphibole. The geothermobarometric results obtained from the ultramafic amphibolites indicate P-T conditions of P = 0.2-0.5 GPa and T = 780-850 °C for the igneous crystallization and of P = 1.3-1.7 GPa and T = 680-730 °C for garnet growth. These results are in agreement with the anticlockwise P-T path reconstructed for the adjacent plagioclase-banded amphibolites. The geochemical data an the P-T paths reconstructed for the studied metabasites are discussed in the geodynamic scenario of the variscan Sardinian chain. A new geodynamic model for the evolution of the studied rocks is also proposed.