Geological evolution of the south European Variscan fragments from Calabria inferred from P-T computations and U-Pb zircon ages

In Calabria (southern Italy) fragments of south European Variscan chain consisting of ortho- and paraderivates affected by low- medium- high- grade metamorphism crop out. Ortho-derivates involved by green schist to granulite facies metamorphism characterize middle and deep continental crust units. Metagabbros and metabasites affected by granulite facies metamorphism are associated with felsic granulites and migmatitic metapelites of the lower continental crust unit of the Serre. U-Pb spot analyses on zircon separates (performed by SIMS and LA-ICP-MS) have provided constraints on the ages of protoliths and on chronology of Variscan metamorphism. The age of magmatic protoliths of the orthogneisses and of the metabasic rocks was fixed at ~ 540 and ~ 580 Ma, respectively, representing a bimodal magmatic contribution to Pan-African crust in Early Cambrian-Neoproterozoic times. Mafic and felsic magmas were emplaced in the Pan-African/Cadomian basement preserving memory of older terrains belonging to West African Craton. These peri-Gondwana terrains were reworked by Variscan orogenesis and look like the blocks occurring in the east European chain from the Alps to Turkey. The effects of Variscan metamorphism depend on P-T conditions and rock chemical compositions. A time memory, step by step, of the Variscan evolution was preserved in rock-types where the re-growth or recrystallization of zircon happened. So the U-Pb zircon ages together with the petrological data in granuliteamphibolite facies metamorphic rocks of the Serre allow to depict the Variscan evolution. Ages around 450 Ma were recorded in the metaigneous rocks which coincide with the age derived from Rb-Sr isochron relative to metasedimentary rocks; this age can be related to an Ordovician tectonothermal activity or to Eo-Hercynian events. Cluster ages of 347-340 Ma, 323-318 Ma, 300-294 Ma and 279 Ma were measured in metagabbros and metabasites from the lower portions of the section. Age peaks around 323, 300 and 270 Ma are recorded in overlying migmatitic metapelites. Undated thin luminescent rims of zircon are the only evidence of post- Ordovician events in orthogneisses interleaved with migmatitic metapelites. Conventional thermobarometer, Thermocal calculations and pseudosections indicate clockwise trajectories Ky-Sil-And-Ky and Ky-Sil-Ky for the migmatitic metapelites and metabasites, respectively, from the deep continental crust unit of the Serre. The P-T conditions in the metabasic portion indicate a possible P peak (P ≥ 1.08 GPa and T < 850°C) predating the T-peak (T = 850-900°C and P ~ 1.08 GPa) at 347-340 Ma, probably followed by quite isothermal decompression (P = 0.8 GPa) at 323-318 Ma and successive decrease of T (~ 750°C); a new decompression at P about 0.65 GPa around 300 and 280 Ma ago, occurs. During the decompression stages pervasive partial melting episodes, producing leucogranitic and trondhjemitic melts, interested these rocks in which luminescent overgrowths mantle primary zircon cores. The re-constructed P-T path shows even for the upper migmatitic metapelites, P-peak at 0.9 GPa and T = 650°C pre-dating T-peak (750°C) under quite isobaric conditions 323 Ma ago and a multistage decompression at 300, 280 Ma up to 270 Ma with incremental partial melting episodes. An integrate approach involving U-Pb analyses on zircon and REE distribution in garnet orthopyroxene and zircon on thin section, relatively to the mafic granulites better constrain the Variscan evolution