Structural and petrological features of a ductile shear zone in late-hercynian granitoids (Sila massif, Calabria)

Near Mesoraca (southern Sila, Calabria) a ductile shear zone transects syn-tectonic Late Hercynian granitoids and wall rocks. Granitoids are chiefly represented by foliated granodiorites and, to a minor extent, by foliated tonalites. Wall rocks are mainly represented by migmatitic paragneisses, which underwent peak metamorphism at 300-304 Ma. In the core of the shear zone mylonitic granodiorites show a clear grain size reduction and microstructures related to deformation at medium- to high-temperature conditions. Migmatitic paragneisses, foliated and mylonitic granodiorites mostly show concordant fabric elements. In addition, the preferred orientation of euhedral feldspars in granitoids indicates that fabric anisotropy started to develop in the magmatic state. These features strongly suggest that shear deformation was active during granitoids emplacement and protracted in subsolidus conditions. In mylonitic granodiorites, kinematic indicators and quartz axis analyses coherently suggest a top-to-W sense of shear, in the present geographic coordinates. Geobarometry indicates that deformation took place at middle crustal levels (P = 445 ± 138 MPa). Overprinting and crosscutting relations allowed to bracket timing of deformation. This outlasted peak metamorphism in wall rocks and was active during emplacement of the granitoids. On the other hand, late pegmatitic dykes were unaffected by significant deformation. Muscovite in pegmatite, dated by Rb-Sr method, provided an age of 265± 3 Ma. Therefore, shear deformation took place between 304 and 265 Ma.