U–Pb ages and Hf isotope composition of zircons in a metasomatic xenolith (Hyblean Plateau, SE Sicily)

Morphological and trace element studies of a hundred and twenty-one zircon grains separated from a chlorite–alkali feldspar xenolith of metasomatic origin collected from Valle Guffari (Hyblean Plateau, south-eastern Sicily, Italy) indicated the coexistence of igneous and hydrothermal zircons within the same sample. The subdivision into igneous and hydrothermal zircons may be consistent with in situ LAM-ICPMS U-Pb and Lu-Hf data. Hf isotope analyses were carried out on fifty-nine out of sixty-six zircon grains already analysed for U-Pb isotopes. U-Pb ages for fifty-eight (hydrothermal) zircons scatter from 184 to 283 Ma, with ~ 72% data clustering around 245 ± 9 Ma and lying close to Concordia. Their Hf isotope signature ranges from Hf +8.5 to –1.2. This trend may indicate the mixing of low- Hf (old crustal) components and a high- Hf (juvenile) source. The Hf isotope composition is considered to be that of hydrothermal fluid, whose F-rich composition is highly probable. The relatively narrow range of U-Pb ages indicates that the age is geologically meaningful, and corresponds to the timing of the hydrothermal event generating these zircons. U-Pb ages for eight igneous-like zircons are scattered (622 to 2687 Ma, Proterozoic to Archean age), but Lu-Hf isotope signatures allow us to distinguish the grains that can be genetically linked. Five out of eight igneous-like zircons show scattered U-Pb ages (from 1939 to 2687 Ma) but they have very similar Lu-Hf signature (176Hf/177Hfi ~ 0.2811), and hence they should be genetically related. Their position on Concordia diagram confirms this observation: the Archean age for three zircons is geologically meaningful since they are concordant, while the other two ages are unreliable (discordant values on Concordia), and reflect non-zero-age Pb loss. Hence, these five zircons are all interpreted as igneous, but some of them were affected by incomplete U-Pb resetting, and the oldest (Archean) age represents the crystallization age. The Lu-Hf system seems to have retained the original signature, given its better resistance to thermal disturbance. For remaining igneous zircon grains a scrupulous comparison between the Concordia plot and a plot of 176Hf/177Hfi vs U-Pb age distinguishes between geologically meaningful (two samples: Proterozoic) and unreliable (one sample) ages. This study reveals the presence of distinct zircon populations, characterised by clearly distinguishable U-Pb and Lu-Hf isotope signatures, coexisting in the same sample. The isotopic results testify to (1) the presence of a few Archean zircon grains of igneous origin in rare metasomatic xenoliths from Hyblean area and (2) the occurrence of a hydrothermal event during Early Triassic time.