Zircon SHRIMP U-Pb Dating, Geochemical Characteristics and Tectonic Significance of Granitic Gneisses in Amdo, Tibet

The Amdo microcontinent is located within the middle of Bangong-Nujiang suture（BNS） zone in the shape of lens. The basic geological research restricts geologists from understanding the histories of tectonic evolution of BNS and regional geology more deeply. This paper systematically studies the geochronology and geochemistry of granitic gneisses from Amdo basement. These data provide constraints on formation age, source characteristics and tectonic setting of their protolith. The SHRIMP zircon U-Pb dating is operated for granitic gneisses. Samples AGS-2 and AGS-3（granitic gneiss） yield average zircon U-Pb ages of 485±14 and 487±6 Ma, respectively. These ages should represent the formation age of protolith and indicate that they are formed in the Early Ordovician. Granitic gneisses are characterized by high SiO2, Na2O, K2O and Al2O3, low Fe and Mg, enrichment in light rare earth elements（LREEs） and large ion lithophile elements（LILEs）, depletion in heavy rare earth elements（HREEs） and high field strength elements（HFSEs）, with negative Eu anomaly. The Rittmann index（σ） is 1.77 to 2.60, less than 3.3. The aluminum saturation index（A/CNK） values range from 0.88 to 1.26. These features suggest that protolith of granitic gneisses from Amdo basement show characteristics of calc-alkaline and S-type granite, and they could be derived from partial melting of metamorphic greywackes in the upper crust of low maturity. The tectonic setting is syn-collision. These all suggest that the formation of protolith of granitic gneisses from Amdo are caused by the Early Paleozoic orogeny, which could be related to proto-Tethyan oceanic subduction along Gondwana continental margins, and does not result from the production of Pan-African orogenesis.