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

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