P-T evolution of Alpine metamorphism in the southern Aspromonte Massif (Calabria - Italy)

摘要

The crystalline basement of the southern Calabrian-Peloritani nappe pile edifice (Aspromonte Massif and Peloritani Mountains) consists of several tectonic slices, which were stacked during the Miocene Apennine thrust sheet emplacement (Fig. 1). The two lowermost tectonic slices of this nappe-edifice, here called respectively Aspromonte-Peloritani Unit and Samo-Africo Complex, tectonically overlap along a thick Late Oligocene mylonitic shear zone, which involves both remnants of the Hercynian chain and Alpine metamorphic rocks (Figs. 1-2). A structural and petrological study was employed to reconstruct both the entire P-T evolution of the two units and the kinematics of orogenic transport, which characterise the Late Oligocene mylonitic stage. P-T estimates were made using an integrated approach derived from conventional thermobarometry, computation of metamorphic equilibria in the NaCaKFMASH system, and an analysis of the deformation behaviour of quartz, feldspar and garnet. The shear event marks the beginning of the joint structural and metamorphic history. Prior to this event, the two units had undergone very different tectonometamorphic evolutions. The upper unit (Aspromonte-Peloritani Unit) underwent pre-mylonitic HT-LP Variscan polyphase retrograde metamorphism (peak conditions 0.39-0.5 GPa; 650°-675℃). By contrast, the deeper Samo-Africo Complex underwent pre-mylonitic LT-HP metamorphism (peak conditions 0.98-1.2 GPa; 480°-560℃), linked to crustal thickening of an Early Alpine age. Syn-mylonitic P-T estimates confirm the beginning of the joint tectonometamorphic history, evolving from 500° to 350℃ with P ranging from 0.775 to 0.325 GPa. This event, linked to Late Oligocene Alpine shearing, developed during the uplift and exhumation of the crystalline basement rocks along a deep-seated compressional shear zone. The above results lead us to suggest a new geodynamic scenario for the southern sector of the Calabrian Peloritani Orogen: Following subduction of the Aspromonte-Peloritani Unit and the Samo-Africo Complex these two units were extruded along the collisional suture. In this tectonic reconstruction, the Samo-Africo Complex is interpreted as a post-Variscan sedimentary succession involved in a complete Alpine orogenic cycle, consisting of an Early Alpine crustal thickening stage and a Late Alpine Africa-verging compressional shear stage.