MELT PRESENT DEFORMATION OF THE MAFIC LOWER CRUST EXPOSED IN CALABRIA

Introduction In the Serre Massif of Calabria an about 8 km thick sequence of granulite facies rocks constitutes the lower crust of the former Variscan orogen. The base of this lower crustal section is made up of layered garnet-bearing meta-gabbros which are interspersed with lenses of metaperidotites and layers of metapyroxenites and meta-hornblendites (Moresi et al., 1978; Schenk, 1984). Single zircon conventional and spot U-Pb isotopic analyses place the age of the gabbroic protoliths into the Precambrian (Schenk, 1984, 1990; Micheletti et al. 2008). The main metamorphic overprint under granulite facies conditions of the gabbros has occurred during the Variscan orogeny. Zircon dating constrained the metamorphic peak close to 300 Ma in coincidence with the emplacement of huge masses of granitoid magmas in the middle crust (Schenk, 1984; Caggianelli et al., 2000; Micheletti et al., 2008). Reaction microstructures have induced Schenk (1984) to infer that the lower crust of the Variscan orogen in Calabria was subjected to isothermal decompression immediately after the thermal maximum. He has deduced that the lower crust became decompressed from about 750 to 550 MPa at 800° C and then affected by slow isobaric cooling down to 200° C over the next 250 million years. Acquafredda et al. (2008) have shown that decompression occurred during two different stages and involved a much thicker continental crust than previously assumed. Internally consistent thermodynamic modelling of the pre-decompression mineral assemblage results in 900° C and 1.1 GPa for the metamorphic peak conditions. New data We can now show that metagabbros became deformed under melt present conditions. This is evidenced by the occurrence of cm-thin shear zones wherein the partial melt was drained. Shear-zone development occurred in a general deformation regime, with shortening partitioned next to the shear zone resulting in an intense crenulation of the earlier Variscan foliation. Ti-rich pargasitic amphibole layers were involved in this crenulation deformation and recrystallized. Recrystallized amphiboles have the same chemical composition as those tracing the older layering. Application of the semiquantitative "Ti-in-amphibole" geothermometer of Ernst and Liu (1998) to the recrystallised pargasites results in a deformation temperature of 870° C, whereas application of amphibole-plagioclase thermometry (Holland & Blundy, 1994) results in temperatures of 818 to 857°( in the pressure range 0.5 to 1 GPa). Minimum pressure conditions of 500 MPa can be assessed using the semiquantitative Al-in-amphibole geobarometer of Ernst and Liu (1998). However, application of the crystal structure modelling geobarometer of Nimis & Ulmer (1998) to clinopyroxene in equilibrium with the plagioclase-rich melt of the shear zones shows that pressures during deformation have been rather close to 1 GPa. Melt present deformation of the mafic complex was followed by cooling starting from temperature conditions closely identical to that registered by the recrystallized amphiboles. Reaction seams of quartz+phlogopite around Opx suggest that a dehydration reaction of the type phlogopite (Phl) + quartz (Qtz) = orthopyroxene (Opx) + melt has been crossed backwards during cooling. Peterson & Newton (1989) place this reaction above 800° C. Conclusion The recognition that plagioclase/Cpx melt-bearing shear zones developed at about 1GPa pressure and 870°C temperature suggests that incipient melting of the mafic complex of the lower crust may have triggered the extensional tectonics in Calabria.