Post-magmatic hygrochronology of plutonic micas

The Eo-Oligocene Adamello plutonic complex (Southern Alps) has been mapped and studied for over a century. The NW unit, Avio granodiorite, was recently reexamined by [1], who observed that magmatic biotite gave constant ³⁹Ar-⁴⁰Ar ages for four grainsizes between 71 and 1000 µm. We dated zircon [2] and micas from two plutonic masses in the E of the massif, near the crustal-scale Giudicarie Fault. The smallest unit, Sòstino, had been least well studied. It is the oldest pluton of the entire complex (c. 45 Ma zircon ages, [2]), intruded at about 10 km depth. Its biotite (38-39 Ma by Rb–Sr and K–Ar [3]) was sampled anew and studied in detail. We obtained electron microprobe element maps and analyzed two size fractions by ³⁹Ar-⁴⁰Ar stepheating. The "isochemical" [4] age is 38.81±0.03 Ma. In contrast to the size-independent ages of Avio biotites, the 125-250 µm sieve fraction is 0.5 Ma younger than the 250-500 µm fraction, and has a different Ca-Cl-K signature; both K contents are sub-stoichiometric. Two mica generations are inferred, with the smaller fraction lying on the high-Cl-low-age alteration trend of the large fraction. The microprobe maps confirm chloritization, Ti unmixing, local Ba enrichment, and patchy Fe/Mg heterogeneity. The neighbouring Corno Alto pluton has a higher age gap (c. 43 Ma zircon ages, [2]; 33-34 Ma Rb–Sr and K–Ar biotite ages [3]). We followed the same redundant petrochronological approach as for Sòstino. The biotite "isochemical" [4] age is 35.88±0.04 Ma. The fine grainsize is 2 Ma younger than the coarse one, its Ca-Cl-K signature is distinct, both K contents being substoichiometric. The two biotite generations are texturally very clearly identified by microprobe, with correspondingly evident Ti-Ba-Fe-Mg-K compositional differences. The larger age difference between size fractions, and the larger zircon-biotite age gap, are explained by the higher mass fraction of the secondary biotite generation in Corno Alto relative to Sostino. The fluid-controlled formation of secondary biotite may be related to the hydrothermal circulation surrounding the large, younger intrusions in the N and NW. The plutonic biotite samples are c. 20 Ma younger than the basement biotite ages near and far from the plutons.

[1] Mittempergher et al, J Geol Soc 179 (2022) 2021-101 – [2] Favaro S., Resentini A., Tiepolo M., Malusà M.G., Zanchetta S. (2024) Abstract EGU24-14975 – [3] Del Moro et al, Mem Soc Geol It 26 (1983) 285 – [4] Müller et al, Contrib Miner Petrol 144 (2002) 57-77