Abstract
The Tuscan Magmatic Province was mostly built-up by anatectic acidic magmas, along with minor mantle-derived basic and intermediate potassic magmas, commonly occurring with mingling-mixing relationships. In this work, the ages of the rocks of anatectic origin have been determined by the U-Pb in-situ method on zircon grains. The investigated igneous complexes are those closer to the crustal anatectic end-member compositions and include intrusive bodies (Monte Capanne and Porto Azzurro monzogranites in Elba Island and Castel di Pietra monzogranite in southern Tuscany), hypoabissal rocks (San Martino and Portoferraio porphyries Elba Island) and volcanic centres (San Vincenzo and Roccastrada rhyolites in Southern Tuscany). A number of 773 crystals were mounted in epoxy resin and their morphologies, both external (crystal faces) and internal (zoning shapes), were observed and photographed by electron microscope cathodoluminescence. The outer morphologies have been indexed according to the zircon typology method, allowing to formulate hypotheses on the petrological history of the magmas, including the interaction between mantle and crustal end- members. The internal morphologies have been used as a guide in the selection of the analytical spots for in-situ determination of the isotopic ratios of the U-Pb systematics by means of LA-ICP-MS, that is a plasma source mass spectrometer Element 2 coupled with an excimer laser (University of Bergen, Norway). The obtained data, after careful analytical screening, have been grouped into two sets of Miocene-Pliocene and Paleozoic-Proterozoic ages. Most of “young” data provide an age that can be referred to the time of magma emplacement. These dates have been critically compared with values obtained different systematics on different minerals, analysing the observed differences in terms of accuracy and precision of the various methods have been analysed, and considering also the possibility of the existence of more or less extended periods of crystallization recorded by the different isotope systematics.The “old” age set confirms the occurrence of inherited cores and allows to speculate about the nature/age of the continental crust involved in the anatectic process.
