Impact of magmatic intrusions on metamict zircon annealing as constrained by Raman spectroscopy in the eastern Adamello batholith (Central Alps)

The impact of a magmatic polyphasic intrusion on the empirical relationship between zircon metamict state as revealed by Raman spectroscopy (Zhang et al., 2000) and α-damage accumulation of the same grains is investigated in the eastern Adamello batholith (Central Italian Alps). Eighteen samples, spanning the contact between the Val di Genova pluton, the Sostino-Corno Alto pluton, and their host rocks – including Permian intrusives, were examined.

Zircons were analysed with a 100mW Nd:YAG solid state laser emitting at 532 nm Raman spectrometer to assess their metamict state based on the position and full width at half maximum of different Raman bands. For each zircon, on the same spot, LA-ICP-MS analyses were performed to determine the crystallization age and U, Th, and Pb concentrations. Crystallization ages and actinides content were used to calculate α-damage since zircon crystallization.

Magmatic zircon ages display a south to north younging trend from the Sostino pluton (44.79±0.35 Ma) to the Val di Genova pluton (34.18±0.20 Ma), consistent with literature data (Schaltegger et al, 2009 and 2019; Ji et al., 2019 and references therein). Moreover, magmatic zircons from the Adamello batholith exhibit a metamict state roughly consistent with calculated α-doses, indicating nearly full retention of decay-related damage. In contrast, zircons from the Paleozoic country rocks and Permian plutons show a nearly crystalline state conflicting with calculated high α-doses, thus suggesting a thermal overprint that annealed their crystalline structure, here identified with the thermal aureole of the Adamello magmatic intrusions. In fact, the measured metamict state of zircons in the host rocks is in overall agreement with α-damage accumulation calculated since intrusion rather than crystallization ages.

Raman analysis of radiation damage in zircon is being currently investigated as a new thermochronological method (Pidgeon, 2014; Nasdala et al., 2001; Nasdala et al., 2002; Hartel, 2021) with particular emphasis on unannealed zircons to test its feasibility. With this study, we present an application of Raman analysis of radiation damage in annealed zircons to trace their thermal history, thus providing independent validation of contact-metamorphic overprints.