Metamorphic evolution of preserved Hercynian crustal section in the Serre Massif (Calabria-Peloritani Orogen, southern Italy)

This paper presents and discusses the results of an integrated structural and petrological study, in order to delineate the entire tectono metamorphic history of a still little-known crystalline fragment of the southern Hercynian European Belt, currently framed within the central Mediterranean region after the superposition of the Alpine tectonics. These results were obtained by correlating P-T constraints yielded step by step with the sequence of the identified blasto-deformational relationships in an intermediate continental crustal level outcropping in the southern Serre Massif (Calabria). This allowed a detailed P-T evolution characterised by a multi-stage metamorphic history to be reconstructed. Structural investigations showed the presence of a pervasive mylonitic foliation, that obliterated most of the previous metamorphic textures. This fabric contains kinematic indicators consistent with an average top-to-ENE-NE sense of shear in the present-day geographic coordinates. In addition, the occurrence of late-tectonic leucogranite rocks partly affected by sub-solidus deformation, cut in turn by later undeformed ones, allowed the final stages of the shearing event to be bracketed at the same time as the Late Hercynian magmatic activity in the area. Microstructural investigation by quartz c-axis orientation pattern analysis allowed the temperature of shearing to be constrained as occurring under greenschist to amphibolite facies conditions. The latter are set in relation with the influence of the heat deriving from the intrusion of the Late Hercynian granitoids. Lastly, pressure temperature (P-T) pseudosection computations in the MnNaCaKFMASH system allowed a detailed P-T path to be reconstructed, consisting of an initial orogenic cycle characterised by a prograde lower amphibolite facies evolution, developing from P of 590 MPa at T of 500 degrees C to peak P-T conditions of 900 MPa at 530 degrees C. This stage was followed by retrograde quasi-adiabatic decompressional (P=400 MPa; T = 500 degrees C), evolving towards an extensional deep-seated shearing, with P of 300 MPa at T of 470 degrees C. This last orogenic stage played a role in favouring the intrusion of granitoid bodies, which were indeed found to be partly affected by sub-solidus non-coaxial deformation. Progressive emplacement of large volumes of granitoid bodies gave rise to a gradually distributed thermal metamorphic overprint with thermal peak conditions at P of 300 MPa and T of 685 degrees C. This episode was finally followed by a low-pressure cooling path (P = 150 MPa; T = 500 degrees C), consistent with the final unroofing stage of the former crystalline basement complex. A detailed reconstruction of the tectono-metamorphic evolution of this Hercynian continental crustal portion allowed a Late Palaeozoic geodynamic scenario to be envisaged, in which tectonic and magmatic processes mutually interacted to define, in a feedback-type evolution, the tectono-thermal regime operating during the gravitational collapse of a previously thickened Hercynian crust. (C) 2009 Elsevier By. All rights reserved.