Zircon U–Pb and Hf isotopic study of Neoproterozoic granitic gneisses from the Alatage area,Xinjiang:constraints on the Precambrian crustal evolution in the Central Tianshan Block

The architecture and growth history of Precambrian crustal basements in the Central Tianshan Block play a key role in understanding the tectonic evolution of the Chinese Tianshan Orogenic Belt.In this study,we present precise LA-ICP-MS zircon U–Pb dating and LAMC-ICPMS zircon Hf isotopic data for two granitic gneisses from Alatage area in the Central Tianshan Block.The magmatic zircons from both samples yield similar protolith ages of 945±6 and 942±6 Ma,indicating that the early Neoproterozoic magmatism is prevailed in the Alatage area.These zircons have crustal Hf model ages of1.82–2.22 and 1.70–2.03 Ga,respectively,which are significantly older than their crystallization ages.It indicates that their parental magmas were derived from the reworking of ancient crust.However,we suggest that these Paleoproterozoic Hf model ages might result from mixing of continental materials with different ages in the Neoproterozoic crust.The inherited(detrital)zircon cores not only yield a wide age range of ca.989–1617 Ma,but also exhibit large Hf-isotope variations with Hf model ages of1.54–2.30 Ga.In particular,some 1.4–1.6 Ga zircons show high initial176Hf/177Hf ratios,consistent with those of depleted mantle,which indicates that the Mesoproterozoic event involved both reworking of older crust and generation of juvenile crust.The Central Tianshan Block has different Precambrian crustal growth history from the Tarim Craton.Therefore,it would not be a fragment of the Precambrian basement of the Tarim Craton.