Long-lasting mantle-derived activity and evolution in the late-Variscan Sarrabus igneous complex (SE Sardinia, Italy).

The Sàrrabus igneous massif (400 km2), belongs to the Sardinia-Corsica batholith and is a late-Variscan post-collisional intrusive complex occurring in the frontal part of the orogenic wedge. It is mostly made up of Bt-granodiorites and Bt-monzogranites predating late leucogranites. This complex experienced the contribution of several mafic mantle-derived pulses for about 20 Ma. Mafic intrusions are dominated by Hb-gabbroic rocks cropping out for 10 km along the southern coastline, as stretched and dismembered dikes arranged on a general WNW/ESE trend. They were emplaced as forced intrusions -mostly hosted into Bt-granodiorite- surrounded by discontinuous shells of foliated quartz-diorite/tonalite grading to Hb-granodiorite. Mechanical interaction between different terms of a gabbro/tonalitic association is represented by lobate plagioclase xenocrysts and quartz-diorite fragments in gabbroic rocks. Variations in plastic rheological interaction are testified by gabbroic/quartz-diorite interdigitations or by large (up to 4 m) cuspate fragments of mafic dikes into Hbgranodiorites lobes. Mafic magmas experienced (polybaric?) low-pressure crystal/liquid fractionation at depth, as documented by decametric angular fragments of stratified olivine-two pyroxene-bearing gabbroic rocks. Progressive water saturation of mafic magmas is commonly documented by ortho- and clinopyroxene relic cores rimmed by diffuse amphibole growth. A possible plagioclase + amphibole fractionation path (<2 kb) may account for production of quartz-diorite as residual melt. Overall, contrasting characters (e.g., sharp contacts, dark enclaves, color index, plagioclase textures and accessory phases) between Hb-granodiorite associated to mafic rocks and hosting Bt-granodiorite, support a resident -previously mixed- granodioritic magma reservoir, later replenished by mantle-derived magmas. Low-rate of mafic injection favored (1) the stratification at depth and (2) the production of hybrid granodiorites at the upper mafic/felsic magma boundaries. Magma pulses were dismembered during the injection at shallower crustal levels along narrow shear zones. The mafic activity evolved towards diffuse spessartite diking, which records petrographical and geochemical features similar to synplutonic mafic dikes. They are commonly associated to meta- and peraluminous felsic dikes, which represent different crustal end-members. The mafic activity in southern Sàrrabus is related to decompression melting of lithospheric mantle. Sr-Nd isotope data of the late-Variscan whole Sardinia-Corsica mafic activity point to a progressive mantle enrichment southward, possibly inherited from a Pre-Variscan subduction. This magmatic evolution is part of the post-collisional scenario of the Sardinia-Corsica block, marked by its Permian rotation accompanied by a variable stress field. In southern Sardinia this variability is represented by diffuse NNW mafic/felsic dikes switching to NE in central and north-Sardinia.