Architecture, emplacement mode of late-Variscan plutons and their relationships with post-collisional phases: examples from Sarrabus igneous massif (SE Sardinia, Italy).

Sardinia is a southern transect of the Variscan Belt intruded by many post-collisional coalescent igneous complexes forming the Sardinia-Corsica batholith. The Sardinian portion of this batholith grew during two main magmatic peaks, clustered at about 305 Ma (Old Magmatic Peak, OMP) and at 285 Ma (Young Magmatic Peak, YMP); plutons intruding different parts of the Variscan basement show different geological styles in terms of emplacement style and mantle/crust contribution. The Sàrrabus igneous massif (400 km2) is a multi-pulse, composite intrusive complex, occurring in the frontal part of the orogenic wedge in SE Sardinia. It records a complex evolutive history, consisting of several intrusive sequences. The coalescing intrusion are roughly elongated WNW, with sharp sub-vertical contacts. Schematically, the Sàrrabus igneous massif consists of a southern part dominated by granodioritic up to monzogranitic rocks related to the OMP, and a northern part made up of leucogranitic rocks referred to the YMP. A continuous contribution of mantle pulses is documented mostly in the OMP. Early mantle-derived products are gabbrotonalites exposed close to the northern contacts with the host basement; these mafic intrusives occur as a homogeneous sills or masses dismembered by YMP leucogranites. A further mafic pulse consists of disrupted sub-vertical, syn-plutonic, gabbroic dikes associated with quartz-diorites, which are well exposed along the southern Sàrrabus coastline. These dismembered dykes, scattered within the granodiorite with a general WNW trend. The mafic activity evolved towards diffuse diking (mainly spessartites), which resulted in several NNW trending dike swarms that crosscut the OMP intrusives with cool, sharp contact. Spessartites are the latest episodes referable to the OMP; they are frequently associated to metaluminous and garnet-bearing peraluminous felsic dikes and stocks. A subsequent generation of olivine plagioclase -phyric mafic dikes with tholeiitic signature crosscut the YMP. The resulting scenario suggests a northward growth of Sàrrabus igneous massif, accompanied by (1) bimodal character of this magmatism; (2) increasing contribution of crustal melts and a progressive decrease of mantle/ crust interactions; (3) progressive shallower emplacement conditions. The late NS trending dyke swarm, which emplaced in dilational jogs accounts for a NS σ1 stress field affecting the entire southern Sardinia lithosphere during Permian time. Conversely, the emplacement of the OMS intrusions and the related dismembered mafic bodies and dykes were controlled by a possible WNW transtensional kinematics. On a larger scale, this variability of stress field, joined to crustal inhomogeneity, could account for different mechanisms of magma ascent and decompression melting that result in different rock associations and petrological signatures.