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

The basement of Madagascar has been submitted to a strong reworking during a late Pan-African (D2) shearing event. The influence of the relevant fluid transfer on phase relationships is investigated on a calc-silicate gneiss belonging to the Ikalamavony domain, deformed and retrogressed, from low-temperature amphibolite to upper-greenschist metamorphic conditions. The rock is characterized by different generations of pyroxene growing with epidote as coronitic structures around relics of garnet and magnetite. Phase diagram sections are commonly used to study phase relationships as a function of pressure and temperature in closed-systems. Here, the thermodynamic system has been considered as only open for the fluid components (i.e. H 2 O, CO 2 and O 2 ) in order to discuss the interaction between fluid flow and mineral phase evolution occurring during D2 deformation. Prediction in terms of phase composition and modal proportion of P – T – M H2O–CO2–O2 phase diagrams (using Perple-X calculations) are in good agreement with natural observations. They suggest that a large input of aqueous-carbonic fluid throughout the “paragneiss” during the exhumation of the rocks led to the destabilisation of the early Fe 3+ -rich garnet (andradite) and magnetite. Thermodynamic modelling illustrates how the replacement of such mineral assemblage by Fe 2+ -rich pyroxene (diopside), sphene, epidote and plagioclase (anorthite-rich) then amphibole (actinolite) and K-feldspar has preferentially been controlled by the amount of H 2 O and CO 2 brought through minor shear zones during the D2 reworking, rather than by change of the effective bulk rock composition.