Garnet-bearing clinopyroxenites in the lower crustal units of Ivrea Verbano Zone, Italian Alps

The crust-mantle transition zone in continental settings is a key and still unexplored part of our planet. It can help to understand the pathways of magmas from their source to the surface, the formation of continental crust and its past and present architecture. The Ivrea-Verbano Zone (IVZ, Southern Alps) is a well known exposed section of lower continental crust and includes units of mantle peridotites that could testify the presence of a crust-mantle transition zone at or near the surface. This is well exposed along the Sesia river, where a km-sized mantle peridotite body (i.e., Balmuccia peridotite body) is in contact with a sequence of pegmatoid clinopyroxenites, websterites, cumulus peridotites and gabbros. This study focuses on a pegmatoid clinopyroxenite outcrop showing a possible magmatic contact with the Balmuccia peridotite to the west and a transition to a gabbro-pyroxenite sequence to the east. The objectives are to determine the origin of this clinopyroxenite and decrypt its metamorphic history through petrographic analyses and thermodynamic modelling.

Field mapping has shown that the pegmatoid clinopyroxenite is a relatively heterogeneous body, being variably rich in mm- to cm-sized patches rich in garnet, plagioclase and spinel. In some parts of the outcrop these patches are larger and can be defined as a mineralogical banding. The pegmatoid clinopyroxenes show evidence of ductile deformation and exhibit exsolution lamellae of orthopyroxene ± garnet ± spinel ± plagioclase. The texture suggests a cumulus origin for the pegmatoid clinopyroxenes, with the garnet-bearing assemblages possibly representing a recrystallized intercumulus assemblage. In particular, the texture of the garnets suggests a metamorphic origin and may be related to high bulk aluminium content (clinopyroxene Al₂O₃ is up to 8.1 wt%) or high equilibration pressure, or a combination of both. Further work is being devoted to the garnet-bearing exsolution lamellae in the pegmatoid clinopyroxenes, as these might provide insights into the P-T path undergone by the original assemblage. We are investigating these aspects through phase equilibrium calculations using a set of bulk compositions assumed to represent the original pegmatoid clinopyroxenite and the garnet-bearing assemblage. Altogether, the results are will shed light on the petrological history of this pegmatoid clinopyroxenite and provide new insights into the magmatic and metamorphic evolution of this sector of the IVZ.