In situ U-Pb and Hf isotopes analyses on zircon from the Giglio Island pluton

The monzogranitic pluton of Giglio Island in the Tuscan Archipelago (Italy) is a relatively poorly studied intrusion belonging to the Tuscan Magmatic Province (TMP), an association of intrusive and effusive magmatic rocks formed between 14 and 0.3 Ma. New LA­ICP­MS geochronological U­Pb and Hf isotopic data were obtained from zircon grains separated from the two main tex­ turally distinct units forming the intrusion: the outer, foliated Pietrabona facies (PBF), and the inner porphyritic Arenella facies (ARF). We defined from U­Pb dating an emplacement age for the Giglio intrusion of 5.448 ± 0.030 (2SE) Ma (weighted mean on 80 analyses, 206 Pb/ 238 U ages, Th disequilibrium corrected), improving the previously published age. Although individual spot ages are indistinguishable within error, the obtained weighted mean ages for the two facies are different, 5.493 ± 0.034 (49 analyses) for PBF, and 5.309 ± 0.060 (31 analyses) for ARF, thus suggesting that the pluton formed incrementally. We developed a robust method for the deter­ mination of Hf isotopic composition in zircon by LA­MC­ICP­MS obtaining crustal values of ini­ tial εHf comprised between ­3.5 and ­11.9 (2SE≈1). No differences between the two facies are apparent, however each sample exhibits a large Hf isotopic heterogeneity that is statistically significant compared to the repeatability obtained for reference zircons. Such variability suggests that different crustally­derived magmas contributed to the magmatic evolution of the pluton, advocating for a complex mechanism of melt production, ascent and final emplacement.