Timing of high grade south European Variscan evolution: constraints from trace element partitioning in garnet, zircon and orthopyroxene on U-Pb zircon ages

U-Pb zircon ages and chemistries of zircon, garnet and orthopyroxene from different structural sites of a thin section of a granulitic rock from the Serre (Calabria, southern Italy) have been considered in order to reconstruct the evolution of a fragment of south European Variscides. The rock consists of Pl+Grt+Bt+Opx+Qtz+K-feld and accessories as zircon, apatite and opaques. The matrix shows layers containing different abundances of biotite and orthopyroxene. One porphyroblastic garnet (~ 3 cm in diameter) rimmed by a Opx+Pl+Bt+opaques+Zrn+Ap symplectitic corona occurs. Ten zircon grains are exposed in the thin section: eight in the Bt-rich matrix, one within the corona and one in the rim of the garnet. Zircon shows generally core-rim structures. Frequently, zircon cores appear chaotic, consistently with modification induced by high-grade metamorphism and some are rimmed by unstructurless lobate rims, invading cores, commonly related to interaction with fluids and/or melts. U-Pb analyses have been performed in situ by LA-ICP-MS (IGG-CNR, Pavia). Two core domains of zircons from the matrix give the oldest concordant ages: 357±11 and 334±12Ma; five concordant data ranging from 324±12 to 320±11Ma give a mean concordia age of 323±2Ma. Two younger concordant ages (305±9 and 300±9Ma) are relative to low luminescent inner rims and similar apparent ages have been determined on luminescent zircon rims from matrix, symplectitic corona and garnet rim. Trace element compositions were collected on specific domains of zircon, garnet and orthopyroxene in different textural sites by LA-ICP-MS. Two trace element profiles relative to porhyroblastic garnet were performed. Garnet is very rich in middle-REE and shows nearly flat profile in the core and fractionated pattern in the rim. The transition zone from core to rim (outer core) is poorer in HREE. Zircon is characterized by a steep pattern with low LREE and high HREE contents both in cores and rims in which, however, the MREE and HREE are lower than in garnet, apart from its outer core region. The orthopyroxenes show fractionated patterns of MREE and HREE. The apparent DHREE between zircon-garnet and orthopyroxene-garnet pairs have been calculated and compared with literature data considered as suggestive of equilibrium relatively to natural samples and experimental results. The calculated DREEzrn/grt and DREEopx/grt define linear and positive trends for zrn or opx/core and outer core combinations of grt evidencing regular, but variable partitioning and reaching even values higher than unity. So the calculated DREE values seem to be indicative of steps of approaching equilibrium. The combination zrn rim/grt rim, instead, indicates disequilibrium suggesting that the zircon rim formed earlier than garnet rim, which subsequently broke down producing corona. The combination with the outer core of garnet gives higher D than the combination with the inner core suggesting, according to experimental results, lower temperature for the former; this is in agreement with the petrology. The calculated DREEopx/grt approach those suggestive of equilibrium from Er to Lu in the combination opx matrix-grt core. The combination opx corona-grt rim is far from the equilibrium. Probably, orthopyroxene from corona is a residue of the matrix. Accordingly, a well constrained contribution to PTt path relatively to fragments of south European Variscides emerges: the Variscan metamorphism of the lower part of the Serre crust section peaked much earlier than previously assumed (at least ~ 340 Ma ago against 300 Ma) and the multistage Variscan decompression after the metamorphic peak lasted tens millions of years, at least locally. The decompression probably protracted after 300 Ma.