Carboniferous-Triassic tectonic and thermal evolution of the middle crust section of the Dervio-Olgiasca Zone (Southern Alps)

A well-preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio-Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre-Alpine (rift-related) history. We reconstruct the P-T-t-d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U-Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono-thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite-facies Carboniferous metamorphic basement (7.6-10 kbar and 610-660 degrees C at 318-312 Ma) was affected by pervasive extensional deformation (5.1-7.6 kbar and 580-660 degrees C) in the Middle- to Late-Permian (257.5 +/- 3.8 Ma). Pegmatite intruded at 249.8 +/- 1.1 Ma in an extensional phase that re-equilibrated rocks of the basement at 3.5-4.5 kbar and 560-600 degrees C. During the Middle- to Late-Triassic (241-235 Ma), the basement experienced static thermal recrystallization (T = 689 +/- 41 degrees C and similar to 5.0 kbar). This Late-Anisian to Early-Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large-scale extensional detachments developed in the upper crust (e.g., the Lugano-Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post-Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting.