Reinterpretation of metamorphic age of the Hengshan Complex,North China Craton

摘要

Hengshan granulite facies terrane consists of tonalite-trondhjemite-granodiorite(TTG)gneisses and minor high-pressure mafic granulite blocks.LA-ICP-MS zircon dating for two TTG gneiss samples indicates metamorphic ages of 1916.7±9.9 Ma and1850–1930 Ma,and magmatic protolith age of ca.2.5 Ga,with similar HREE-rich patterns.Phase equilibria modelling in ZrO2-bearing system reveals that zircon abundance varies inversely with melt abundance in suprasolidus metamorphism,zircon growth occurs with melt crystallization during cooling,and thus,the newly grown zircon can only record the age of retrograde metamorphism.Consequently,we suggest that the Hengshan granulite facies terrane experienced slowly uplifting and cooling during 1.85–1.93 Ga in an extensional setting under middle-crust,while the collision orogeny corresponding to peak stage of high-pressure granulite may have happened much earlier.