Microstructural and petrological investigations of a poly-orogenic shear zone: the example of the Castagna Unit - (Sila Piccola Massif)

the Calabria crystalline basement, known in the literature as "Castagna Unit” (Dubois and Glangeaud, 1965). Thisunit represents a pervasively mylonitized horizon located within the Calabride continental crust involving amphiboliteto greenschist facies metamorphics intruded by late-Hercynian granitoids. Structural analyses highlighted a pervasivemylonitic fabric (Sm), defining the field foliation, as well as a pervasive stretching lineation (Lm). The Sm, averagelystriking NW-SE to SW-NE, has obliterated previous metamorphic surfaces locally preserved as relicts in the low straindomains of the metapelite horizons. The Lm, marked by elongate quartz and feldspar crystals alignment, roughly runsESE-WNW to ENE-WSW, while, at the meso-scale, several kinematic indicators indicate a consistent top to the ESEsense of shear in the present-day geographic coordinates. Micro-scale analysis confirmed the pervasive mylonitic fabricdepicted by S/C texture, core-mantle and strain shadow structures in feldspar porphyroclasts, oblique foliation in quartzmicrodomains, boudinaged feldspar and mica-fish textures, confirming the top-to-the ESE sense of shear observed inthe field. Nevertheless, locally preserved microstructural elements seems to define a contrasting top-to-the WNW andW sense of shear, also supported by quartz c-axis pattern analyses. Such an analysis allowed also to constrain theshearing temperatures (i.e. 400-450 °C) consistent with lower greenschist facies conditions with subordinate relicevidences of prism c-slip system activation consistent with higher shearing temperature. Finally, mineral-chemicalanalyses highlighted a bimodal distribution of the syn-shearing white mica phengite content, suggesting thedevelopment of two different baric regimes. All of the abovementioned evidences suggest the occurrence of twomylonitic events, probably ascribable: the first one to the latest extensional stages of the Hercynian orogeny; the secondone to the Alpine reactivation operating during the stacking of the present-day nappe-pile edifice. This is also confirmedby the occurrence of microboudin and pinch and swell structures, which can justify the exhumation of the deepest partsof the continental crust along a main binary leading to the continental rifting following the final stages of the Hercyniantectonics. This extensional regime is also supported by the occurrence of late-Hercynian undeformed leucocratic dykescrosscutting the mylonitic foliation. Overall, the “Castagna Unit” can be interpreted as an extensional late-to-postHercynian shear zone, locally re-activated during the building of the Alpine edifice.