Magma recharge dynamics in Middle Triassic volcanoes: features and timing of Ladinian feeding systems in the Dolomites (Southern Alps; Italy)

The Southalpine tectonic domain hosts numerous magmatic manifestations related to the trachybasaltic to trachyandesitic magmatic event that shaped the area during the Ladinian age (Middle Triassic). The identification of systematic compositional zoning patterns in the clinopyroxene population among volcanic products allowed us to unravel the architecture and dynamics of the feeding systems of the main magmatic centres from the Dolomites (Southern Alps; Italy) that remained unknown until now.

The recurrent zonation consists of different step zoning patterns between lower Mg# and Cr2O3 contents (Mg# 67-77; Cr₂O₃<0.1 wt%) augitic composition and high-Mg# and Cr2O3-rich diopsidic parts (Mg# 78-91; Cr₂O₃ up to 1.2 wt%). The diopsidic domain appears to a lesser extent and is more frequently present as a variable thick single or multiple bands between augitic cores and rims. It could be documented in cores as resorbed or mottled. Oscillatory zoning and sector-zoned crystals are also present in minor numbers.

Thermometric calculations reveal that the diopsidic domain delivers an equilibrium temperature that ranges from 1143 to 1204°C, remarkably higher than the temperature calculated from the augitic domain which ranges from 966 to 1150°C. Since the barometric computation show the same range of pressure for both compositional domains (120-400 MPa), our proposed model involves periodic mafic recharge pulses of primitive and hot basaltic magma into a crystal mush in the shallower portion of the plumbing system (4-14 km) that led to the formation of the high-Mg# domain within the already formed augitic crystals. These new results support the model proposed for the Cima Pape volcano-plutonic complex (Nardini et al., 2022) and extend it also to the other Dolomitic centres.

Diffusion chronometry computations based on Fe–Mg diffusion in clinopyroxene have been applied to evaluate the residence times for the crystals in each compositional zone with the NIDIS model (Petrone et al., 2016). Computations have revealed a time span from injection to eruption from short timescales (less than a year) to decades with variation between the different systems considered. Further studies are requested to better constrain the timing of each feeding system in the Dolomites, and this will enable an extremely detailed description of the dynamics that fueled the Middle Triassic magmatism in the Southalpine domain. Our final aim is to discuss this ancient magmatism as a proxy for active volcanic complexes thanks to the forthcoming analysis of the outcropping plutonic counterparts, impossible to do in an active system, making this part of the Alps a volcanological laboratory for testing the approaches/models currently adopted for active volcanoes.

References

Nardini, N., Casetta, F., Ickert, R. B., Mark, D. F., Ntaflos, T., Zanetti, A., & Coltorti, M. (2022). From the Middle Triassic Cima Pape complex (Dolomites; Southern Alps) to the feeding systems beneath active volcanoes: Clues from clinopyroxene textural and compositional zoning. Journal of Volcanology and Geothermal Research, 422, 107459.

Petrone, C. M., Bugatti, G., Braschi, E., & Tommasini, S. (2016). Pre-eruptive magmatic processes re-timed using a non-isothermal approach to magma chamber dynamics. Nature communications, 7(1), 1-11.