Time and duration of Variscan high-temperature metamorphic processes in the south European Variscides: constraints from U-Pb chronology and trace element chemistry of zircon

Abstract

The lower crust of the Serre massif (Calabria, southern Italy) provides a window into the mid- to lower crust of the south European Variscan orogenic belt. Previously, zircon U-Pb ages were employed to date high-temperature processes affecting this portion of the Variscan crust. The present paper reports new LA-ICP-MS U-Pb data on the zircon of a deformed quartz-monzodiorite dike and of three mafic granulites sampled at the base of the lower crust section. Determination of trace elements on zircon, including rare earth elements (REE), has been also performed. The end of the Variscan exhumation, dated by anatectic zircon from migmatitic metapelites, and the growth-modification of zircon with respect to the growth of Variscan metamorphic garnet have been assumed as “time markers”. The concordant zircon ages of the metamorphic basic rocks cover a range from 744 ± 20 Ma to 231 ± 10 Ma with high age density from 357 ± 11 Ma to 279 ± 10 Ma, a few ages comprised between 418 ± 14 Ma and 483 ± 12 Ma and between 505 ± 11 Ma and 593 ± 14 Ma. Zircon from the deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323 ± 5 Ma. Most of the analysed zircon domains dated between 357 ± 11 Ma to 279 ± 10 Ma from garnet-bearing metabasic rocks show flat patterns of heavy rare earth elements (HREE), as expected in the case of their simultaneous growth with garnet. This allows to consider (1) zircon domains giving Variscan ages as “metamorphic” with specific geological significance, and (2) zircon domains with ages ranging from 564 ± 17 Ma to 593 ± 14 Ma as dating the emplacement of the magmatic protoliths as shown by internal microtextures, fractionated patterns of HREE and Th/U ratios (0.16–0.19). The Variscan zircon ages (357–279 Ma) reflect effects of crustal thickening, peak metamorphism and subsequent multistage Variscan decompression documented by the statistically significant clusters of ages around 347–340 Ma, 323–318 Ma, 300–294 Ma and 279 Ma. The U-Pb zircon ages of the metabasic rocks suggest a period of about 60–70 Ma for granulite facies metamorphism and anatectic conditions. Literature data indicate that the migmatitic metapelites of the upper part of the Serre crust section also underwent a long period, about 40 Ma, of granulite facies metamorphism and anatectic conditions. A diachronism emerges through the time comparison of the Variscan evolution between the upper and the lower portions of the Serre deep crust. The duration of the Variscan processes defined in Calabria is comparable to that of other south European Variscan blocks.