New insights for the tectono-metamorphic evolution of the upper-intermediate crustal section exposed in the southern Serre Massif (Calabria, Southern Italy): phase equilibria modelling of a garnet-hornblende bearing metandesitic lense

The reconstruction of the tectono-metamorphic evolution of the Variscan intermediate and upper crustal section exposed in the southern Serre Massif (Calabria, southern Italy) is crucial for a broader understanding of the crustal dynamics during the Variscan orogeny. In the southern Serre Massif, Variscan tectonics juxtaposed the Mammola Paragneissic unit (hereafter MPu), representative of the intermediate crust, and the Stilo-Pazzano Phyllite unit (SPu), representative of the upper crust. Subsequently, both units were affected by a contact metamorphic event related to the emplacement of the Upper-Carboniferous Serre batholith. Within the MPu, in the Levadio Stream area, garnet-hornblende bearing metandesitic lenses are locally interlayered with the paragneisses. The juxtaposition of the MPu and SPu is marked by a mylonitic shear zone affecting also the metandesite. This study examines via phase equilibria modelling the metamorphic evolution of a sheared garnet-hornblende bearing metandesite. To this purpose, the rock was modelled in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system using the software THERMOCALC v.3.45, with the thermodynamic dataset ds63, including a – x models for both metapelitic and metabasic minerals. The pre-peak metamorphic mineral assemblage (ep-q-pl-mu-chl-bi-rieb) is hosted as primary inclusions in garnets having the following composition: almandine (50-53%) - grossular (29-31%) - spessartine (17-20%). The constructed P-T pseudosection shows that the peak mineral assemblage (g-ep-mu-bi-chl-rieb-ab-sph-q-H2O) was stable in a wide, low-variance P-T field of 6 – 13 kbar and 350 – 525 °C. Garnet isopleth modelling suggests P-T conditions for the regional metamorphic peak of 7.9 – 9.2 kbar and 495 – 510 °C. The subsequent, near isothermal exhumation from about 32 to 10 km depth at 450 – 500 °C brought the metandesite close to the emplacement level of the Serre batolith (at 2.7 kbar). The constructed T-MH2O pseudosection points to T values of 570 – 593 °C for the peak pl-bi-q-g-hb-sph mineral assemblage of the contact metamorphism, under H2O-saturated conditions. The derived P-T path for the garnet-hornblende bearing metandesite is consistent with the one derived by Angì et al. (2010) for the MPu paragneiss, and highlights that comparable peak pressures were reached at the regional metamorphic acme in the southern Serre Massif within the MPu, as well as within the granulite facies rocks cropping out in the north. This suggests that the thermal gradient significantly changed across the Variscan chain exposed in Calabria in response to distinct tectono-magmatic contexts.