Analisi strutturale, microstrutturale e petrologica del basamento di Mammola e Stilo (Serre Meridionali - Calabria): nuovi vincoli per il metamorfismo Varisico regionale e di contatto

Reconstruction of the tectono – metamorphic evolution of the crystalline basement in the Serre Massif (Calabria, Southern Italy) is crucial for a thorough understanding of the crustal dynamics during the Variscan orogeny. The present thesis focuses on the P – T evolution of three different crustal levels juxtaposed along two extensional shear zones within the southern Serre Massif. To this aim a structural, microstructural and petrological study was carried out, in order to assess the relationships among these different crustal levels. Field observations enabled to recognize two main extensional, synthetic, shear zones at the base and the top of an intermediate crustal level. Shearing temperatures were obtained through the assessment of the quartz recrystallization mechanisms and the quartz c-axis pattern established by EBSD analysis. SEM imaging was used to understand the reaction microstructures, while the mineral chemistry was established by microprobe analysis. Constructions of pseudosections for the lower crust using THERMOCALC, in the NCKFMASHTO system, allowed to establish the P – T regional baric and thermal metamorphic peak conditions at P = 11-12 Kbar and T = 530 °C and P = 10,8 – 9,5 Kbar and T = 570 °C, respectively. Subsequent exhumation lifted this crustal level to a pressure of 3 – 2,7 Kbar and a temperature of 500 °C, where it was thermally overprinted to 650 °C due to the emplacement of the huge Serre batholith. A comparison between the P-T-path of this lower crustal level with the path of the intermediate crustal level (Angi et al. 2010), suggests that these two different crustal levels were uplifted in a time frame between 320 and 306 Ma. Therefore, they came in contact to each other along the lower extensional shear zone, sometimes after the thermal peak within the lower crustal level and before the emplacement of the Serre batholith. This is confirmed by the contact metamorphic overprinting in both crustal levels at upper crustal pressure conditions. Hence, the reconstruction of the P-T-path in this thesis has significant new implications for the whole tectono – metamorphic evolution of the Serre Massif. It results that exhumation of deep crustal levels occurred at different times within the Serre Massif, and precisely before the emplacement of the huge Serre batholith in the South and during its growth in the North. This timely different exhumation of deep crustal levels provides further credit to the hypothesis forwarded by Caggianelli et al. (2007) and Langone et al. (2014). They hypothesize that the principal heat source for the granulite facies metamorphism in the northern Serre Massif is the emplacement of the huge amounts of mantle melts during the general extensional regime that involved the Serre Massive at about 300 Ma. This heat budget triggered granulite facies metamorphism and up to 60 % partial melting of the metapelites in the northern Serre Massif. Mixing of mantle melts with crustal melts resulted in the hybrid geochemical signature of the Serre batholithic melts. In the south, instead, the emplacement of the Serre batholith triggered, within the exhumed lower crust, only contact metamorphism at upper crustal level conditions.