Tectono-metamorphic evolution of the Calabria continental lower crust: the case of the Sila Piccola Massif

Abstract

A combined structural and petrological study focuses on the Sila Piccola Massif to obtain new constraints on the exhumation history of a relic fragment of the intermediate to deep portion of the Variscan basement, which now belongs to the northern Calabria Peloritani Orogen. The timescale of the early (i.e. late-Variscan) shear-assisted exhumation stage is determined by diffusion-modelling and the strain rate of the latest (i.e. Alpine) shear event is determined by microstructurally derived paleopiezometry. The intermediate to deep portion of the Sila basement is characterised by a pervasive mylonitic horizon involving orthogneiss and, to a lesser extent, garnet paragneiss. Such a pervasive mylonitic foliation widely obliterated an older metamorphic fabric, which is preserved as relics in low-strain domains. The pre-mylonitic relics consist of plagioclase, biotite, white mica, sillimanite, quartz and the first generation of chemically homogeneous garnet. Our results show that the later mylonitization can be ascribed to two metamorphic stages. The first stage is associated with a late-Variscan extensional shearing, which shows a syn-kinematic growth of a second-generation garnet with plagioclase, biotite and quartz developed in the pressure shadows of garnet porphyroclasts likely during an early retrograde metamorphic stage. The second stage, characterised by a syn-shearing growth of chlorite, white mica, plagioclase and quartz, observed along the C-planes, is interpreted as a late Alpine mylonitic overprint in compressional regime.