Mylonite development in the Hercynian basement of Sardinia (Italy)

Plastic deformation predominates over large areas in the Hercynian basement of Sardinia during Lower Carboniferous continental shortening. Mylonitization associated with ubiquitous top-to-south-southwest thrusting decreases from internal (central Sardinia) to foreland areas (southern Sardinia). In the Barbagia unit, mylonites completely obliterate all the previous structures; in the Meana Sardo unit, plastic deformation strongly overprints early folds; and in the Gerrei unit, mylonites are found only below the Meana Sardo thrust. A thick shear zone, the Baccu Locci mylonite zone, develops between the Gerrei and Riu Gruppa units. Inferences from deformation mechanisms in quartz mylonites suggest a temperature during deformation below 400°C along thrusts throughout the study area. Overprinting relationships between foliations, deformation of annealed microstructures and porphyroblasts with inclusion patterns indicate that crystal growth occurred after early thrusting events and before the main nappe emplacement phase. Microstructural investigations in the Baccu Locci mylonites record two changes in the main deformation mechanism operating during progressive mylonitization. In the early stages of deformation, most of the strain is accommodated by dislocation creep in a fine-grained quartz matrix. At higher strain, dynamic recrystallization fully affects larger quartz crystals, producing pure quartz layers where plastic deformation and strain localize. Ongoing deformation together with syntectonic breakdown of feldspar producing mica leads to mineral changes, grain size reduction and reaction softening with strain localization in the fine matrix again. Dynamically deformed quartz is boudinaged in the later stages of mylonitization. Significant fluid infiltration during deformation can account for some of the large ore bodies hosted in the Baccu Locci mylonites. © 1998 Elsevier Science Ltd.