Phase relations in garnet-bearing metabasites of prehnite-pumpellyite facies from the Darbut-Sartuohai ophiolite, Western Junggar of Xinjiang, China

Abstract

Previous petrogenetic grids for very low grade metamorphism do not apply to garnet-bearing prehnite-pumpellyite facies rocks. Garnet-bearing metabasites with Act - absent of prehnite-pumpellyite facies have been found in the Darbut-Sartuohai ophiolite, Xinjiang, China. Results of thermodynamic calculation in the CMASH system using the internally consistent mineral thermodynamic database of Berman (1988) show that equilibria reactions among the end-member species in mineral paragenesis of Pmp-Prh-Grs-Zo-Chl-Qtz intersect at a pressure of 4.75 kbar and temperature of 350°C. The association of grossular-rich garnet and pumpellyite occurs at pressures and temperatures similar to the Pmp-Act facies. Based on the petrogenetic grid derived in this study, peak metamorphic conditions for metabasites from the Darbut-Sartuohai ophiolite are 310 to 330°C, and 3.0 to 4.0 kbar, which are affected by the substitution of Fe 3+Al -1. This study shows that grossular-rich garnet can appear in metabasites of the prehnite-pumpellyite facies, but it depends on the substitution between Fe 3+ and Al in the CMASH+Fe 3+ system.