U-Pb zircon data of Variscan meta-igneous and igneous acidic rocks from an Alpine shear zone in Calabria (southern Italy)

The paper deals with the U-Pb data of zircon separated from three samples representative of mylonitic leucogranites, trondhjemites and pegmatites occurring along the Alpine tectonic zone between the Castagna and Sila Units in northern Calabria. These mylonites are associated to Variscan granitic-granodioritic biotite-rich augen gneisses derived from Neo-Proterozoic-Early Cambrian protoliths. Apparent ages ranging from Early Cambrian to post-Variscan have been obtained. Th, U and rare earth elements have been determined in two zircon domains of mylonitic leucogranite and trondhjemite giving different ages in order to get information relative to their geological significance. The pegmatite preserves intrusive contact with the augen gneisses and with the other mylonites; it turns out to be emplaced at 290-300 Ma, like the Variscan plutonites of the Castagna Unit. The deformation masks the original contacts of the mylonitic leucogranite and trondhjemite with the biotite-rich augen gneisses. The age-group averaging 540 Ma is interpreted as indicative of the emplacement of the protoliths and it coincides with the age previously determined for the emplacement of the protoliths of the biotite-rich augen gneisses. Zircon from the mylonitic pegmatite includes domains showing concordant and discordant ages younger than 290 Ma, thus reflecting various degrees of partial resetting and Pb-loss caused by post-Variscan events. Zircon from the mylonitic leucogranite and trondhjemite includes apparent ages between 300 and 280 Ma as well as ages younger than 250 Ma. Perturbation of U-Pb system by Alpine shearing appears evident; however, possibile effects caused by thermal input and hydrothermal fluid infiltration from the Variscan plutonites cannot be excluded.