Early Neoproterozoic Granitic Gneisses In The Chinese Eastern Tianshan: Petrogenesis And Implications For Crustal Evolution

The architecture, evolutionary history and tectonic affinity of Precambriancrustal basements in the Central Tianshan Block (CTB) plays a key role inunderstanding the tectonic evolution of the Chinese Tianshan Orogenic Belt. In thisstudy, we present precise LA-ICP-MS zircon U-Pb dating and LA-MC-ICPMS zirconHf isotopic data for the granitic gneisses from Alatage and Xingxingxia areas in theCTB. The magmatic zircons from five samples yield similar protolith ages of945±6Ma,942±6Ma,923±5Ma and930±4Ma, indicating that the early Neoproterozoicmagmatism is presented in the Alatage and Xingxingxia areas. Considering thepublished contemporaneous records in other areas, the paper thinks that the earlyNeoproterozoic magmatism is prevailed in the CTB. The age is between ca.970Maand ca.920Ma, and these rocks mainly occur in Alatage, Xingxingxia and Baluntaiareas. The granitic gneisses of these areas show peraluminous to stronglyperaluminous, low CaO and Na2O, and Th and Y contents. Zircon saturationthermometry and Ti-in-zircon thermometry yielded a relatively low crystallizationtemperature of approximately750–800°C and700–800°C for the magmaticprotoliths of the granitic gneisses, respectively. The five samples possess consistentHf isotopic compositions, showing εHf(t) values of6.4to4.2, with εHf(t) model agesof1.53to2.22Ga, which are significantly older than their crystallization ages. Itindicates that their parental magmas were derived from the reworking of ancient crust.However, we suggest that these Paleoproterozoic Hf model ages might resulte frommixing of continental materials with different ages in the Neoproterozoic crust. Thecharacteristics mentioned above suggest that the protoliths belong to S-type granite.Moreover, high-grade metamorphism that would indicate collisional tectonics has notbeen found in the CTB during the early Neoproterozoic and the protolith of thegranitic gneisses shows the geochemical features of volcanic arc, therefore the earlyNeoproterozoic granitic gneisses of the CTB may be formed along an activecontinental margin, and the CTB was located at the periphery of Rodinia. Theinherited zircon cores not only yield a wide age range of ca.989–1617Ma, but also exhibit large Hf-isotope variations with Hf model ages of1.54–2.30Ga. In particular,some1.4–1.6Ga zircons show high initial176Hf/177Hf ratios, consistent with those ofdepleted mantle, which indicates that the Mesoproterozoic event involved bothreworking of older crust and generation of juvenile crust, showing the CTB hasdifferent Precambrian crustal growth history from the Tarim Craton. Therefore itwould not be a fragment of the Precambrian basement of the Tarim Craton.