Dyke magmatism in the Sila Massif (Calabria, southern Italy): witness of Early Permian exhumation

In the Sila Massif the end of the Hercynian orogeny is marked by widespread dyke magmatism. Unfortunately, little is kmown about serial affinity, age and tectonic framework of this magmatic event. The research proposed here aims to close this knowledge gap, with the main goal of reconstructing the tectonic context favouring magma dyke ascent and emplacement. during the waning stages of the Hercynian cycle. To reach these objectives, new structural, petrological, geochemical and geochronological data have been collected. In the Sila Massif a late-Hercynian Batholith crops out. It is mainly made up of granodiorites and tonalites, but includes also peraluminous granites and mafic bodies such as gabbros and diorites. Intrusive rocks emplaced between 300 and 304 Ma. At the southwest the Sila Batholith is bordered by medium- to high-grade metamorphic rocks mainly represented by migmatitic paragneisses. Dykes transect essentially intrusive rocks but can be also found in the metamorphic basement. The studied dykes display a wide compositional range from rhyolite to basaltic andesite. Texture is commonly porphyritic, but in some instances can be equigranular. They crosscut intrusive contacts between magmatic intrusive rocks and metamorphic wall rocks. A database of orientation and geometrical features has been prepared with new and literature field data for a considerable number of dykes (n = 350). At the outcrop scale dykes mostly show sub-planar and sub-vertical sharp walls. Map view suggests that most dykes have a nearly tabular shape that sometimes can be undulated. Thickness ranges from 1-2 m to 50 m, whereas length spans from ≈ 50 m to 2 km. Rose diagrams indicate a strong preferred orientation of dykes along a E-W strike. Other significant groups are orientated along ESE-WNW, NW-SE, NNW-SSE and ENE-WSW directions. Sills are represented by two sub-horizontal sheeted bodies of microgranite having an elliptical shape with maximum and minimum axes of 1.5 km and 0.8 km, respectively. Their maximum thickness is of ≈ 35-40 m. Petrological and geochemical analyses were performed to define magmatic affinity and level of emplacement of the dykes. Chemical features indicate that, in comparison to host calc-alkaline granitoids, dyke magmatism drifts to towards an alkali calcic serial affinity, characterized by a moderately high Na2O content, consistent with late- to post-orogenic timing. Level of emplacement is shallow since magma quenching textures are quite common. They are represented by alkali-feldspar spherulites in felsic types and by acicular hornblende nucleated around quartz, rounded xenocrysts in mafic types. A quantitative estimate of the emplacement level was obtained by Al-in-hornblende barometry for basaltic andesite dyke (z ≈ 7 km). The same method was adopted to define depth of emplacement of the granodiorite wall rock (z ≈ 12 km). Geocrhonological analysis was performed for selected dykes and granodiorite wall rock. To this purpose, euhedral and elongated zircons were dated by U-Pb method (LA-ICP-MS). Preliminary results indicate that dyke magmatism took place at ≈ 280 Ma and post-dated emplacement of late-Hercynian granodiorite of ≈ 20 Ma. Integrating data on time and level of emplacement, both for dykes and granodiorite, it is possible to evaluate an average exhumation rate of the Calabria contine...