Unravelling the tectono-metamorphic evolution of medium-pressure rocks from collision to exhumation of the Variscan basement of NE Sardinia (Italy): a review

The basement of NE Sardinia consists of magmatic-sedimentary sequences affected by a polyphase deformation (D1, D2, D3 and D4)) and an increase in the metamorphic grade northwards, toward the inner zone of belt. D1 is mainly detected in the low-grade zone, while D2 is widespread in the basement but partitioned into domains with prevailing folding in the southernmost areas and shearing in the north-central part. D2 structures were later overprinted by two main systems of open to tight folds. On the basis of the mineral assemblages of pelitic and quartz-feldspathic rocks, six metamorphic zones have been distinguished. From south to north they are: biotite, garnet, staurolite + biotite, kyanite + biotite, sillimanite + muscovite and sillimanite + K-feldspar. The P-T path of different metamorphic zones is a typical clockwise one due to thermal relaxation after a homogeneous thickening stage. The metamorphic peak is pre- to-syn S2 in the garnet and staurolite zones, whereas in the sillimanite zones it is essentially syn-S2. In biotite and garnet zones, both the D1 and D2 stages of tectono-metamorphic evolution can be documented. In the garnet zone, petrological evidence indicates that from the S1 to the S2 mineralogical assemblage there was an increase in temperature (up to 30°C), along with a decrease in pressure (0.2-0.6 GPa). From the staurolite + biotite zone to the sillimanite + K-feldspar-zone, the D1 phase is not clearly documented, and samples recorded only the retrogressive path after the thermal climax. The field gradient is nearly concave towards the temperature axis. This is a consequence of the diachronous development of the main parageneses and fabrics in the six metamorphic zones during the exhumation stage. An age of about 340 Ma, found on syn-S1 celadonite-rich white mica, is interpreted as the time of maximum thickening stage in the garnet zone; ages of 315-320 Ma for syn-S2 muscovites are attributed to the end of chemical re-equilibration of white mica during the D2 stage. The emplacement of granitoids in the axial zone has had no influence on the early exhumation of the lower crust, but the different exhumation rate may be related to the transpressional tectonic regime which affected NE Sardinia during Variscan orogeny