Estimate of Hercynian metamorphic peak in upper crustal metapelites of the mandanici unit via garnet isopleth thermobarometry (Peloritani Mountains, North Eastern Sicily)

The Peloritani Mountain belt is a segment of the European Alpine orogen that preserves pre-Alpine basement relics. Very low to high grade metamorphic rocks are exposed in the present-day tectonic stack of the chain. Up to now very few PT paths are available for the basement tectonic slices of the Peloritani Mountains, due to the common absence of suitable parageneses in the low and very low grade metamorphic rocks, as well as to the pervasive late Hercynian thermal re-equlibration in the medium-high grade metamorphic rocks. In this view, to estimate the relic peak PT conditions of the Hercynian evolution of this southern Alpine chain sector, phase diagram calculation method was applied, modelling the upper greenschist facies garnet-muscovite-chlorite schists outcropping in the uppermost levels of the Mandanici Unit. The investigated fine-grained metapelite are characterized by a dominant lepidoblastic texture, with ilmenite and poikilitic garnet porphyroblasts surrounded by alternating muscovite + paragonite + chlorite and minor quartz + plagioclase layers, with apatite, monazite and zircon as accessory phases. Three schistosity surfaces were recognized: S1 consists in a partially obliterated isoclinalic folding schistosity developed during the first identifiable deformational stage (D1); S2 is related to a penetrative microfolding (D2) producing pervasive crenulation schistosity; S3 is consequent to an extensional-shearing event (D3). Porphyroblast-matrix relationships, investigated via optical and electronic microscopy allowed the blasto-deformation sequence to be reconstructed. The attention was focused especially on garnet zoning pattern analysis. Garnet porphyroblasts have cores characterized by quartz and rare ilmenite inclusion trails with folding pattern linked presumably to the D2 event. Idioblastic rims grow over the crenulated garnet, thus representing a late to post-D2 growth stage. Fe-Mn-Mg-Ca X-ray maps and compositional profiles outlined a weak compositional growth zoning, consisting in a bell-shaped profile, with increasing almandine and decreasing spessartine contents toward the rims. P-T pseudosection was calculated in the TiMnNCKFMASH system, using the XRF bulk-rock composition of metapelite samples mainly exhibiting assemblages consistent with peak or near-peak conditions. Retrograde evolution is locally represented by garnet breakdown to chlorite. Garnet inner core isopleths intersections indicate T of 510°C at P of 0.483 GPa, just nearby the garnet-in curve, in agreement with pyrophanite isopleths composition of porphyroblastic ilmenite. Garnet outer core isopleths intersections define similar P-T conditions of T = 525°C and P = 0.485 GPa. These P-T estimates are linked to the first identifiable metamorphic stage in which isoclinalic folding event (D1) and garnet core developed. Garnet rim isopleths intersections gave values of the peak metamorphism at T of 535°C with slightly lower P of 0.430 GPa, attained during the crenulation deformational event which obliterated most of the previous foliation. All the obtained intersections belong to the same pseudosection PT field characterized by chl+ms+pl+ilm+grt+pg assemblage, in agreement with the observed mineralogical assemblage. Subsequent retrograde evolution is depicted in the pseudosection PT space by garnet breakdown reaction into chlorite with T of 500±20°C at P of 0.32±0.03 GPa, consistent with the observed partial to complete pseudomorphic replacements. PT estimates obtained via garnet isopleths thermobarometry can be interpreted as the peak PT climax of the Mandanici Unit reached during the Hercynian crustal thickening at upper-middle crust conditions, while the following retrograde evolution depicts a typical retrograde clockwise PT trajectory consistent with the initial exhumation stage of the Hercynian orogen.