Contrasting tectonic records in pre-Alpine metabasites of the Southern Alps (lake Como, Italy)

The microstructural and petrological evolution of pre-Alpine metabasites from the Southern Alps allow to compare the P-T-d-t paths in the three adjacent basement zones of Domaso-Cortaf? (DCZ), Dervio-Olgiasca (DOZ) and Monte Muggio (MMZ) for delineating the Variscan and post-Variscan evolutions of the deep and middle crust in the Lake Como region. The DCZ rocks show a P-T evolution characterized by a pre-D2 almost isobaric T-increase up to T = 560-650°C and P = 0.7 -1.1 GPa during D2 deformation. A decompressional cooling characterizes the exhumation during which D3 deformation took place. The DOZ amphibolites show a P-T-d-t path in which the physical metamorphic conditions predating D2 were: T = 530-590°C and P = 0.8-1.2 GPa. D2 deformation developed during a decompression accompanied by a T-increase (T = 630 -710°C and P = 0.4-0.7 GPa). The final exhumation stage is characterized by cooling occurring during a slight decompression (T<400°C and P > 0.2 GPa). This P-T evolution of the amphibolites well matches the P-T-d-t path of the surrounding metapelites. The DOZ and DCZ tectono-thermal evolutions, compared with that of the MMZ, evidentiate that three different tectono-metamorphic units constitute thin interleaved crustal slices. The DCZ and MMZ P-T evolutions may represent the thermal record of the Variscan subductioncollision and subsequent uplift, whereas the DOZ P-T evolution occurred under a higher thermal regime, consequent to a Permo-Triassic crustal thinning. The complex stacking geometry of such thin portions of deep crust, exhumed in different times and under different thermal conditions, requires a polyphase juxtaposition history.