Metamorphic evolution, mineral chemistry and thermobarometry of orthogneiss hosting ultrahigh-pressure eclogites in the North Qaidam metamorphic belt, Western China

Many UHP terranes contain abundant felsic gneiss with volumetrically minor mafic and ultramafic blocks. The mafic rocks typically preserve the high-pressure assemblages, whereas felsic gneisses preserve low pressure mineral assemblages. In the absence coesite it can be difficult to confirm that felsic lithologies experienced high-pressure conditions. This has lead to uncertainty over the regional extent of UHP terranes. We conducted a detailed study of textures, mineral parageneses and compositions of a suite of eclogite-bearing orthogneisses in the Luliang Shan from the North Qaidam UHP metamorphic belt. Two distinctive gneisses were observed: volumetrically dominant, uniform leucocratic orthogneiss and mica-rich gneiss which occurs only in proximity to eclogite blocks. Two pseudosections have been constructed to constrain the P–T conditions recorded by the mineral assemblages and mineral zoning observed in the orthogneiss. Relic high-pressure inclusions and predicted conditions of garnet growth suggest that the orthogneiss followed a metamorphic P–T path comparable to the eclogite it encloses, with peak pressure at ∼26 kbar very close to the quartz–coesite phase boundary.