Multi-stage rodingitization of ophiolitic bodies from Northern Apennines (Italy): constraints from petrography, geochemistry and thermodynamic modelling

The investigated mantle bodies from the External Ligurians (Groppo di Gorro and Mt. Rocchetta) show evidences of a very complex evolution determined by an early high temperature metasomatism,, due to percolating melts of asthenospheric origin, and a later metasomatism at relatively high temperature by hydrothermal fluids, with formation of rodingites. At Groppo di Gorro, the serpentinization and chloritization processes obliterated totally the pyroxenite protolith, whereas at Mt. Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization. The rodingite parageneses consist of diopside+vesuvianite+garnet+calcite+chlorite at Groppo di Gorro and garnet+diopside+serpentine±vesuvianite±prehnite±chlorite±pumpellyite at Mt. Rocchetta. Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures (264-334°C at 500 bar and 300-380°C at 1 kbar). Garnet, the first phase of rodingite to form, consists of abundant hydrogarnet component at Groppo di Gorro, differently from Mt. Rocchetta where it is mainly composed of grossular and andradite. The last stage of rodingitization is characterized by the vesuvianite formation. Through the activity diagrams inferred by reaction path modelling, the features of the involved fluids have been obtained. Hydrogarnet nucleation requires high Ca and low silica fluids, whereas the formation of vesuvianite does not necessarily involve a fluid phase with very low CO2 activity. The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids, whereas the presence of andradite associated with serpentine and magnetite at Mt. Rocchetta suggest Fe3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer. We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal activity mainly occurring in an ocean-continent transitional setting and, locally, in a accretionary prism associated with intra-oceanic subduction.