Tracking Late-Pan-African fluid composition evolution in the ductile crust of Madagascar: Insight from phase relation modelling of retrogressed gneisses (province of Fianarantsoa)

•Testing the influence of shearing and fluid transfer on phase relationships.•Using P–T–MH20–CO2–O2 phase diagrams sections to model fluid element transfer.•Investigation in a paragneiss belonging to the Ikalamavony domain of Madagascar.•Rock retrogressed in the amphibolite facies with coronitic structures development.•Highlighting the influence of fluid red-ox conditions on the mineral evolution.