Modelling of the phase relations in high-pressure and ultrahigh-pressure eclogites

摘要: Phase relations of basic rocks under high pressure (HP) and ultrahigh (UHP) metamorphic conditions are modelled on the basis a MORB composition. The calculated pseudosections predict that will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc HP-lawsonite eclogite subfacies (1.8–2.8 GPa, 500–600°C). In these assemblages, pyrope content (Xpy) in garnet mainly increases temperature rising, grossular (Xgr) chiefly decreases silica (Si-) phengite linearly increasing pressure; their contents subtly affected by variations bulk-rock Thus, isopleths compositions P–T potentially present robust geothermobarometric method for natural glaucophane-bearing HP eclogites. Under low-T UHP (>2.8 GPa, 550–650°C), common assemblage is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. this assemblage, Xpy steadily as rises Si-phengite whereas Xgr very sensitive changes. peak eclogites can determined using maximum pseudosections. medium-T UHP (>2.8 GPa >650°C), commonly garnet + omphacite + lawsonite + phengite + coesite. mostly depends compositions, regularly increase, respectively, rises, thus, provide good thermobarometric constraints decompression UHP assemblages dominated lawsonite dehydration reactions, which result disappearance formation epidote and/or kyanite releasing large amount bound fluid.