Magmatic response to arc-arc amalgamation: Insights from latest Paleozoic igneous rocks from the Gangou section of the Eastern Tianshan

• Published in: Gondwana research 2022-09, Vol.109, p.134-149
• Main Authors: Wang, Xiangsong, Sun, Min, Li, Pengfei, Cai, Keda, Zhao, Guochun, Xia, Xiaoping, Liu, Xijun, Li, Zhenglin, Luo, Yan, Graham Pearson, D.
• Format: Article
• Language: English
• Subjects: A-type granite; Arc-arc amalgamation; Eastern Tianshan; Tectonic transition
• ISSN: 1342-937X; 1878-0571
• DOI: 10.1016/j.gr.2022.04.018

[Display omitted] •Two stages of magmatism are identified in the Gangou section of the Chinese Tianshan.•The Late Carboniferous and Permian magmatism recorded tectonic transition.•The Carboniferous intermediate rocks were derived from melting of mantle wedge.•The Permian magmatic rocks show a genetic link with upwelling of asthenosphere mantle. Multiple arc systems were developed in response to Neoproterozoic to Mesozoic consumption of the Paleo-Asian Ocean and their magmatic evolution is crucial for understanding the arc-arc amalgamation in the Central Asian Orogenic Belt. Here, we report whole-rock geochemical data as well as simultaneous in situ zircon U-Pb and Lu-Hf isotope results for the late Paleozoic magmatic rocks associated with the shear zones in the Gangou section of the Eastern Tianshan to constrain their petrogenesis and arc-arc amalgamation processes. We obtained ∼307 Ma and ∼270 Ma zircon U-Pb ages for the Late Carboniferous diorites and Middle Permian andesites, respectively. These rocks have high Mg# (∼65 and ∼47, respectively) values, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with depleted zircon εHf(t) values (+12.1 to +18.1), suggesting an origin from partial melting of the depleted mantle with various degree of differentiation. Furthermore, our results also provided ∼276 Ma zircon U-Pb ages for the Middle Permian granitic porphyries and ∼256 Ma zircon U-Pb ages for the Late Permian diabase rocks. The granitic porphyries are geochemically similar to A2-type granites, i.e., high K2O + Na2O, FeOT/MgO, Ga/Al and low CaO, Sr and Ba. They have high zircon εHf(t) (+9.0 to +13.7) values, indicating a possible derivation from juvenile lower crust. The diabase rocks show depletion of light rare earth elements (LREEs), resembling normal mid-ocean ridge basalt. These rocks have depleted zircon Hf isotopic compositions (εHf(t) = +8.9 to +15.8), demonstrating a possible derivation from partial melting of the depleted mantle. The available geochemical data from the Eastern Tianshan show that the Permian mafic rocks along the shear zones possess higher Nb/La and lower Ba/La ratios than their counterparts in the Carboniferous. These contrasting features imply that the Carboniferous mafic rocks originated from a metasomatic mantle wedge, while the Permian mafic rocks were likely derived from the depleted mantle with addition of enriched asthenosphere components. The high zircon satur