Hybrid phase equilibria modelling with conventional and trace element thermobarometry to assess the P–T evolution of UHT granulites : An example from the Highland Complex, Sri Lanka

Abstract

Here we attempt to constrain the P–T evolution of ultrahigh-temperature granulites using textures coupled with multiple thermobarometric approaches. Sapphirine-bearing granulites were collected from a quarry in the central part of the Highland Complex of Sri Lanka. Three sapphirine-bearing domains were selected and petrographically studied. Homogeneous sample domains were thermodynamically modelled using their bulk compositions (forward phase equilibria modelling). One heterogeneous sample from a single domain, composed of irregularly distributed residuum and melt, was also used. The bulk composition of its residual part was calculated using mineral compositions and their respective modes. Equilibrium T–X(Fe 2O 3) phase diagrams were constructed in the chemical system NCKFMASHTO to estimate the bulk ferric/ferrous iron ratio, and conventional geothermometers (garnet–orthopyroxene and Al in orthopyroxene) were applied. The Ti in zircon trace element thermometer was also applied to calculate peak metamorphic conditions. Modal abundance isopleths of each mineral in equilibrium phase diagrams and textural observations were combined to constrain the retrograde P–T path. Our hybrid approach of forward and inverse phase equilibria modelling and conventional thermobarometric calculations indicate that the sapphirine-bearing granulites have reached their peak T of 920–940°C at P ~10 kbar under relatively highly oxidizing conditions. Subsequently, the rocks followed a near-isobaric cooling path down to 890–860°C, prior to near-isothermal decompression up to 6 kbar. The results highlight the importance of dealing with Fe 3+. Multiple thermobarometric approaches on carefully observed mineral textures are required to retrieve the most reliable P–T conditions of HT/UHT mineral assemblages.