~(207)Pb/~(206)Pb Zircon Dating of the Huangtuling Hypersthene-Garnet-Biotite Gneiss from the Dabie Mountains, Luotian County, Hubei Province, China: New Evidence for Early Precambrian Evolution

Abstract The Huangtuling hypersthene-garnet-biotite gneiss at Luotian County, Hubei Province, is a typical granulite-facies rock of the Dabie Group Complex in the Dabie orogenic belt. Investigations on the morphology and occurrence of zircons and their internal structures shown in the thin sections lead to the recognition of three types of zircons, which are in good agreement with the types identified on the basis of morphology, colour and external features from the related zircon concentrates. The observation of zircons in the rock reveals that part of type 1 zircons show signs of a double-layered structure. The interval part existed in the protolith prior to the granulite-facies metamorphism. Type 2, the prismatic zircons which mainly occur in garnet and hypersthene are metamorphic minerals of the granulite-facies metamorphism. Type 3, the round multifaceted zircons in felsic minerals and biotite, are probably attributed to a later geological event related to migmatization. The ²⁰⁷Pb/²⁰⁶Pb zircon dating by direct evaporation on (thermal evaporation ion mass spectrometer) yields ages ranging from 2814 Ma to 1992 Ma. The age discrepancy among these different zircon types is conspicuous. The yellow-brown (type 1) zircons give ages of 2814±29 Ma to 2527±6 Ma, the prismatic euhedral zircons (type 2), 2456±7 Ma to 2254±4 Ma, and the round multifaceted zircons (type 3), 1992±10 Ma. The results are geologically interpreted in consideration of the complicated behaviours of zircons during Precambrian geological evolution of the Dabie area. (1) If the protolith of the gneiss is a sedimentary rock, then type 1 zircons are clastic ones and the ages 2814±29 Ma and 2811±27 Ma may reflect the minimum age of the rocks of its source region, also the first geological event in the area. Sedimentation of the protolith occurred between 2814 Ma and 2527 Ma, probably close to 2814 Ma. If the protolith is a volcanic rock, then the formation age of the supracrustal rocks of the Dabie Group Complex is around 2814 Ma. The age 2456±7 Ma reflects the time when the granulite-facies metamorphism took place. The later migmatization event is dated at about 1992±10 Ma, and is probably the latest early Precambrian event in the area. The present work provides geochronological evidence for the existence of the Dabie Archaean craton, which had probably experienced 3 or 4 geological events during its early Precambrian evolution.