Phase Equilibria Modeling of Coesite Eclogite from the Sulu Belt, Eastern China

Modeling of phase equilibria and tectonic processes are essential components to understand controls on P-T paths of UHPM rocks. However, diffusion at higher temperatures (> 700 °C), and issues with determination of Fe3+ in minerals and estimating H2O contents limit our ability to determine prograde, peak P and retrograde P-T data. Also, the lack of an appropriate activity-composition model for melt in basic rocks has limited the application of phase equilibria modeling to understand partial melting associated with exhumation. Here we apply phase equilibria modeling to coesite eclogite from Yangkou to assess the influence of Fe3+ and fluid during metamorphism, monitor reactions and phase relations in eclogite during deep subduction and exhumation and investigate partial melting at HP conditions. The modeling used the THERMOCALC software and the new internally consistent thermodynamic dataset for basic rocks (http://www.metamorph.geo.uni-mainz.de/thermocalc/dataset6/index.html). Here we investigate bimineralic (gt+omp+coe/qz+ru/ilm), phengite-bearing (gt+omp+phen (2 samples, <5 vol% and >5 vol%) +coe/qz+ru/ilm) and kyanite-bearing (gt+omp+phen+ky+coe/qz+ru/ilm) eclogites. Coesite in the matrix is the hallmark of the Yangkou eclogite. For each sample, we use an iterative process to estimate the H2O and O content in the bulk composition, and then calculate a P-T pseudosection. The results suggest that some prograde information (670-770 °C, > 3.0 GPa) is retained in large garnet cores in bimineralic and phengite-bearing eclogite. The peak P-T conditions are a challenge because in the field of gt+omp+coe/qz±phen+H2O at T > 750 °C and P > 3.5 GPa mode and compositional changes are small. However, isopleths of Si in phengite suggest that the peak P could have been > 5-6 GPa. Re-equilibration of garnet and omphacite compositions occurred during exhumation, yielding P-T conditions of 700-790 °C at 3.1-2.0 GPa. Amphibolite facies metamorphism occurred at 630-710 °C, 1.3-1.2 GPa. The retrograde P-T path passes through the suprasolidus field; however, the melt produced is very low (< 5 mol%). Our work provides new quantitative P-T data for UHP eclogite in the Sulu belt and contributes to further understanding of the processes that affect deeply subducted continental crust.