Geochronology and geochemistry of igneous rocks in the Bailingshan area: Implications for the tectonic setting of late Paleozoic magmatism and iron skarn mineralization in the eastern Tianshan, NW China

The late Paleozoic Bailingshan intrusions and volcanic rocks are located in the Aqishan-Yamansu arc belt in the southern part of the eastern Tianshan and are associatedwith an important group of iron skarn deposits. The exposed intrusive rocks are mainly granodiorite, monzonitic granite, and granite. Zircon U-Pb dating of the Tugutublak Formation tuffaceous dacitic lava yields an age of 324 Ma, whereas dates of the Bailingshan granodiorite, monzonitic granite, and granite yields ages of 317 Ma, 313 Ma, and 307 Ma, respectively. The results indicate that the Bailingshan granitoids were emplaced soon after the eruption of the Tugutublak dacite. All these rocks studied show calc-alkaline to high-K calc-alkaline andmetaluminous affinities, with A/CNK values ranging 0.83-1.10. They are enriched in Rb, K, and Pb, depleted in Nb, Ta, Ti, and P, and contain low Sr/Y (4.16-23.7) and Sr (109.0-347.0 ppm) values, displaying typical arc geochemical affinities. The tuffaceous dacitic lava has low Nb/Ta (10.3-14.1) values, a wide range of Mg-# (6-64), positive zircon epsilon(Hf)(t) (3.2-7.5) values, and elevated wholerock epsilon(Hf)(t) (2.03-4.41), but low ISr values (0.70427-0.70530), indicating that the source magma may have been derived from the juvenile lower crust with minor mantle input. The Bailingshan I-type intrusions also exhibit a mixed source signal, as constrained by Nb/Ta ratios, Mg-#, and isotopes characteristics. Because the granodiorite, monzonitic granite, and granite intrusions have higher zircon eHf(t) (3.3-7.5, 11.8-13.5, and 10.2-14.4, respectively) and epsilon(Hf)(t) (3.90, 5.78, and 5.94, respectively) values than those of the tuffaceous dacitic lava, it is suggested thatmantle-derivedmaterials may have played a more prominent rolewith their petrogenetic evolution. Integrating our new geological, age, geochemical and isotopic data we propose that the Aqishan-Yamansu iron skarn belt may have formed in a back-arc position or within an intra-arc basin generated by the southward subduction of the Kanggur oceanic plate beneath the Yili-Central Tianshan block during the late Paleozoic, with felsic-intermediate magmatism occurring during the basin inversion. (C) 2015 International Association for Gondwana Research. Published by Elsevier B. V. All rights reserved.