Shedding light on the volcano-plutonic link: evidence of magma mixing dynamics in the shallow crustal plutonic rocks from the Dolomites (Southern Alps; Italy)

The preservation of volcanic and plutonic records holds the potential to improve our comprehension of plumbing systems. It is widely accepted that numerous magmatic complexes are fed by crystal-rich magma ponding at mid to shallow crustal depth, which is periodically remobilized by the intrusion of hotter melts over extended periods. However, these mechanisms are rarely recorded in plutons. The Dolomites (Southern Alps; Italy) host large volumes of Middle Triassic volcanic and plutonic outcrops, characterized by an excellent state of preservation. This condition enables us to deeply investigate the plumbing system dynamics and connect the effusive and intrusive components of the Middle-Triassic magmatism in the Southalpine domain. In this study, a detailed textural and compositional investigation of clinopyroxene crystals in shallow crustal pyroxenites and gabbros has been done to link plutonic features to the already investigated volcanic counterparts (Nardini et al. 2022). Augitic clinopyroxene is the dominant mineral phase, with euhedral to subhedral crystals up to cm-sized dimensions. Its compositions show Mg# [MgO/(MgO+FeOtot) mol%] from 62 to 75 coupled with low Cr₂O₃ concentration (< 0.1 wt.%) and 0.5-1 wt.% of TiO₂. The crystals occasionally show step zoning patterns towards diopsidic domains having high-Mg# (80-90), higher Cr₂O₃ and lower TiO₂ (both from 0.1 to 0.5 wt.%) organised as outer rims, intermediate bands or as resorbed cores. These preliminary data confirm what recorded by the volcanic counterparts, suggesting a periodical mixing between trachyandesitic and trachybasaltic melts (see Nardini et al. 2022). The remarkable preservation of zoning patterns in the plutonic rocks, coupled with the consistency of clinopyroxene composition and zoning in alignment with the lavas and dykes, provides a powerful volcano-plutonic link. The analyses of exposed plutonic rocks, chemically associated with the erupted materials, provide concrete evidence of the magmatic processes that are inferred through petrogenetic modelling in volcanic rocks, with the potential to deepen our understanding of the plumbing system dynamics.

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.