A possible syn-tectonic Variscan mylonitic contact between the Low- and Medium-Grade Complexes of central-northern Sardinia: new insights from the CARG Sheet “482” Bitti

In Sardinia, the Variscan basement is well-exposed across the whole island, wherein rocks are characterized by an increase in the metamorphic grade from southwestern (the External Zone) to northeastern (the Inner Zone) Sardinia. In central-northern Sardinia, within the area of the CARG Sheet “482” Bitti, strongly condensed Barrovian isograds are encountered. There, the transition from the low-grade chlorite and biotite zones to the staurolite+kyanite zone occurs in less than 40km. This relatively thin Barrovian domain tectonically underlies the thicker tectonic pile of the Nappe Zone. So far, this fragment of the Sardinia Variscan basement has been referred either as i) a single tectono-metamorphic domain belonging to the Inner Zone (i.e., the Low- to Medium-Grade Metamorphic Complex, Cruciani et al., 2015), or ii) two distinct domains, both belonging to the internal Nappe Zone (i.e., a Low-Grade and a Medium-Grade Complex, Carosi et al., 2020). In addition, in the geological map of Sardinia (Carmignani et al., 2016), this transept was divided into a Bt-phyllites sequence of the internal nappes and an amphibolite-facies complex to the north. Despite of the several studies carried out in the region throughout the years, a detailed geological and structural mapping is still lacking: the recent start of the Sheet “482” Bitti within the framework of the Italy “CARG” Project represents the most favorable opportunity to unravel the geological outline of the area. In the southernmost area of the under-mapping Sheet, a monotonous pile of muscovite- and chlorite-rich phyllites with subordinate metasandstones, quartzites and metavolcanic rocks (the “Lula Phyllites”) can be observed, showing a main NE-striking pervasive S2 foliation which folded a relict S1 metamorphic layering. Northeast of the Sos Enattos mine, the Lula Phyllites are apparently abruptly overthrusted onto a thick sequence of medium- to coarse-grained biotite and, subordinately, garnet-rich micaschists, plagioclase-rich metagreywackes and metavolcanic rocks, called “Onaní Tectono-metamorphic Unit”. The transition marks the onset of the biotite and, in some cases, of the garnet zones. The transition can be observed as a ~100/150m thick mylonitic, NE-striking, SE-dipping shear zone cropping out south of the Lula village and east of the Sos Enattos mine. In the Sos Enattos area, the phyllites and metasandstones of the Lula Phyllites are directly in contact with (underlying) garnet-rich rocks. The similar geometry of the S2 foliation in the Lula Phyllites, in the Onaní Unit and within the mylonitic zone might be due to the possible over thrusting occurred during the D2 phase. Noteworthy, the geometry of the S2 and the mylonitic thrust are opposite to the typical geometry observed in the Nappe Zone. Although preliminary, the so far collected data may lead to a new interpretation of the tectonic organization of the Nappe-Inner Zones transition of the Sardinia Variscan chain.