Hercynian subduction‐related processes within the metamorphic continental crust in Calabria (southern Italy)

Abstract

Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a Dn‐1 under‐thrusting event. The following exhumation during the Dn mylonitic event, and contact metamorphism during Dn+1 and Dn+2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to Tmax of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during Dn‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.

The Curinga–Girifalco Line in the framework of the tectonic evolution of the remnant Alpine chain in Calabria (southern Italy)

“…In general, the foliation wrapping around the different porphyroblasts (garnet, K-feldspar, sillimanite aggregates) follow an anastomosing pattern (Langone et al 2006). Towards the top, the Hercynian continental crust consists of a thick batholith of granitoids, intruded at mid-crustal levels, and with amphibolite to greenschist facies in the upper crust, represented by paragneisses and phyllites (Schenk 1980;Caggianelli et al 2000;Langone et al 2014;Festa et al 2018;Tursi et al 2020b). The Sila and Serre Unit ends with upwards of hundreds of metres of a Mesozoic carbonate sedimentary succession lying unconformably on the phyllites (e.g.…”

Building an Orogen: Review of U-Pb Zircon Ages from the Calabria–Peloritani Terrane to Constrain the Timing of the Southern Variscan Belt

“…The phyllite units and the underlying paragneiss ones were already tectonically juxtaposed, when the Late Carboniferous-Early Permian granitoids were intruding between the deep and the middle-upper crust [38][39][40]. The emplacement of the late Variscan granitoids determined a contact aureole in the middle-upper crustal levels, which was characterized by fluid-saturated or -undersaturated metamorphism near the batholith [41][42][43][44][45].…”

“…The paragneisses of the Mandatoriccio and Mammola units record lower amphibolite facies peak-pressure conditions at about 0.50 GPa [39,41] and about 0.85 GPa [45,46], respectively, which occurred during Upper Mississippian (i.e., 326 ± 6 Ma) regional metamorphism [41,45,47].…”

“…According to [45], the CPT took part in the growth of the southern Variscan belt, due to the subduction of the Palaeotethys-Gondwana beneath Laurussia [91][92][93][94][95][96]. The Variscan zircon U-Pb ages from the CPT indicate that the subduction occurred up to the Upper Mississippian-Lower Pennsylvanian, as also recorded in metamorphic terranes from other sectors of the southern Variscan belt like Sardinia [97,98].…”

“…part of the Paleozoic covers) were under-thrusted at depth, and subjected to greenschists-, amphibolite-, and granulite-facies metamorphic conditions (Figure 10c). [96]; cross section, modified from [45]); the area of the belt subjected to Variscan metamorphic imprint is indicated with transparency. In the plan view geodynamic reconstructions the possible location of the CPT, as well as of the Sardina, is indicated; the thick black segment represents the trace of the cross section; meridians and parallel spacing is of 10° (roughly adapted after [96]).…”