Continental collision from two perspectives: a review of Variscan metamorphism and deformation in northern Sardinia

摘要

Structural, petrological and geochronological data from the Variscan chain of northern Sardinia are discussed, with the aim to improve our knowledge on the tectono-metamorphic evolution of the northern Sardinian basement and its role in the tectonic puzzle of the southern Variscan massifs. In northern Sardinia, which corresponds to the Inner zone of the chain, the metamorphic grade rapidly increases towards the NE from biotite to sillimanite+K-feldspar zone. The highest grade rocks cropping out in the Migmatite zone reached amphibolite fades conditions in the sedimentary and igneous-derived migmatite and eclogite/granulite facies in the enclosed metabasite lenses. Evidence of HP metamorphism has also recently been found more to the south in micaschist from the garnet zone. The D-1 deformation can be observed in the southern part of the Inner zone, whereas, moving progressively northwards, the D-2 deformation progressively obliterates the previous foliation. The other, less pervasive, deformation phases (D-3, D-4, D-5) are sometimes recognizable in the outcrop localities, but they are not as ubiquitous as the previous D-1 and D-2. HP metamorphism in metabasites and schists is related to the collision and/or northwards-directed subduction of the continental crust, which was part of the northern Gondwana or peri-Gondwanian terranes in Early Palaeozoic times. The HP metamorphism, which is pre- to syn-D-1, was followed by Barrovian metamorphism. The stacking of metamorphic sequences with the building up of the nappe pile was a consequence of the High Grade Metamorphic Complex exhumation. This was activated by NW-SE striking and top-to-the S and SW shear zones and faults with a major dip-slip component of movement, coeval with the late phase of the D-1 deformation. Transpressional deformation then developed until the High-Grade Metamorphic Complex continued to be exhumed and the Low- to Medium-Grade Metamorphic Complex was underthrust to the north. Finally, the extensional tectonic regime was accompanied by local occurrences of HT-LP metamorphism. Deformation and metamorphism in the Sardinian Variscides were accompanied by the intrusion of igneous suites of the Sardinia-Corsica batholith, which was emplaced during the Middle Carboniferous/Permian post-collisional phase.