Exploring the roots of a plumbing system: insights from ultramafic xenoliths ejected during the Middle Triassic magmatic event in the Dolomites (Southern Alps; Italy).

Knowledge of plumbing systems architecture and dynamics has increased in recent years. However, while the mid- to shallow-crustal regions are well-explored, the deepest parts of plumbing systems remain poorly understood. The Middle Triassic magmatic event in the Dolomites (Southern Alps; Italy) provides an exceptional opportunity to study all sections of ancient plumbing systems, owing to the excellent exposure and preservation of different magmatic lithologies representing various magma storage levels. Here, we present detailed textural and compositional analyses of ultramafic xenoliths embedded in mafic volcanic breccia from a diatreme outcropping in the Triassic Latemar carbonate platform (Zan de Montagna locality; 2576 m.a.s.l.). Ultramafic nodules have cumulate equigranular to inequigranular texture and are mainly clinopyroxenites, with subordinated wehrlites and websterites. Clinopyroxene goes up to 3 mm in size in all samples, while olivine in the wehrlite samples attains sizes of up to 1.5 mm. Clinopyroxene is diopsidic in composition (Wo_45-49 En_42-48 Fs_4-10) with Mg# [MgO/(MgO+FeOtot) mol%] of 82-93 and CaO, TiO₂, Cr₂O₃ and Al₂O₃ contents in the range of 22-24 wt%, 0.1-1.2 wt%, 0-0.7 wt% and 0.9-5.5 wt% respectively. Olivine has Fo contents between 84 and 89 and NiO concentration from 0.10 to 0.15 wt%. Notably, more primitive olivine can be found in the host lava, where crystals reach Fo₉₂ and NiO content of 0.4 wt%. Orthopyroxene in the websterite is <1 mm in size and has enstatite (Wo_1-4 En_76-79 Fs_17-23) composition, with Mg# values ranging from 77 to 82 and Al₂O₃ contents between 1.1 wt% and 1.8 wt%. Spinel is ubiquitous, occurring as chromite, magnetite and Ti-magnetite (Cr₂O₃=0.1-45.5 wt%; TiO₂=0.8-16.7 wt%; FeO_tot=29.0-81.0 wt%).

Overall, these xenoliths show compositional similarities with clinopyroxenitic nodules already reported in other localities of the Latemar platform (Nardini et al., 2024) and differ only for the wehrlite presence.

These new data represent an advancement in tracking back to the early stages of the liquid line of descent of the Middle Triassic magmas and help to reconstruct the deepest portion of the plumbing system of these ancient volcanoes. Moreover, the composition of clinopyroxene hosted by these nodules brings another piece of evidence about the source of the high-Mg# and high-Cr diopsidic antecrystic cores in the trachy-basaltic effusive rocks associated with this magmatism (Nardini et al., 2024).

Reference

Nardini, N., Casetta, F., Petrone, C.M., Buret, Y., Ntaflos, T., Coltorti, M., 2024. Modelling ancient magma plumbing systems through clinopyroxene populations: a case study from Middle Triassic volcanics (Dolomites, Italy). Contrib. Mineral. Petrol. 179, 22.