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Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

QIUGEN LI^*,, SHUWEN LIU^*,, ZONGQI WANG, QUANREN YAN, ZHAOJIE GUO^*, ZHICHENG ZHANG^*, HAIFEI ZHENG^*, CHUNFA JIANG, TAO WANG and ZHUYIN CHU

^* The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, People’s Republic of China
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, People’s Republic of China
Geological and Geophysical Institute, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China

Author for correspondence: swliu{at}pku.edu.cn

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (< 5.3) and Sr/Y (< 28), relatively high Mg no. (51.0–57.0), K₂O (2.65–4.04 wt %) contents and _Nd(t) values (–2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K₂O contents, low Al₂O₃/(FeO^* + MgO + TiO₂) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low _Nd(t) values and older T_DM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Key Words: Proterozoic • granitoid gneisses • geochemistry • petrogenesis • tectonic setting

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