Late Paleozoic magmatism and metallogenesis in the Agishan-Yamansu belt, Eastern Tianshan: Constraints from the Bailingshan intrusive complex

The Aqishan-Yamansu belt in the Eastern Tianshan (NW China) contains many intermediate to felsic intrusive rocks and spatially and temporally associated Fe (-Cu) deposits. Zircon U-Pb dating of the Bailingshan granitoids, including diorite enclaves (in granodiorite). diorite, monzogranite and granodiorite, and andesitic tuff from the Shuanglong F-Cu deposit area yielded ages of 329.3 +/- 2.1 Ma, 323.4 +/- 2.6 Ma, 313.0 +/- 2.0 Ma. 307.5 +/- 1.7 Ma and 318.0 +/- 2.0 Ma, respectively. These new ages, in combination with published data can be used to sub-divide magmatism of the Bailingshan intrusive complex into three phases at ca. 329-323 Ma, ca. 318-313 Ma and ca. 308-297 Ma. Of the analyzed rocks of this study, the Shuanglong diorite enclave, diorite and andesitic tuff show calc-alkaline affinities, exhibiting LILE enrichment and HFSE depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg-# values, with depleted epsilon(Hf)(t) and positive epsilon(Nd)(t) values, similar crustal-derived Nb/Ta and Y/Nb ratios, low Th/Yb and Th/Nb, and high Ba/La ratios. which are consistent with them being sourced from a depleted mantle wedge metasomatized by slab-derived fluids and crustal contamination. However, the monzogranite and granodiorite are metaluminous with characteristics of low- to high-K calc-alkaline I-type granites. The granitic rocks are enriched in LILE, depleted in HFSE and have significant Eu anomalies, with high Y contents and low Sr/Y ratios, resembling typical of normal arc magmas. Depleted epsilon(Hf)(t) and positive epsilon(Nd)(t) values with corresponding young T-DM(C) ages of zircons, as well as Nb/Ta, Y/Nb, Th/U and La/Yb ratios suggest that the granitic rocks were probably formed by re-melting of juvenile lower crust or pre-existing mantle-derived mafic-intermediate igneous rocks. Integrating published data, we conclude that the Bailingshan granitoids (excluding the Shuanglong diorite and diorite enclave) were derived from re-melting of juvenile lower crust and mantle-derived mafic-intermediate igneous rocks, with mantle components playing a more prominent role in the formation of the younger and more felsic rocks. A comprehensive review, including our new data, suggests that the Aqishan-Yamansu belt formed as a fore-arc basin during the Carboniferous (ca. 350-300 Ma) when the Kangguer oceanic slab subducted beneath the Yili-Central Tianshan block. The ongoing southward subduction of the slab resulted in the closure of the Aqishan-Yamansu fore-arc basin (ca. 320-300 Ma), due to slab steepening and rollback followed by slab breakoff and rebound. During the Aqishan-Yamansu fore-arc basin inversion, the main phase of the Bailingshan granitoids emplaced in the Aqishan-Yamansu belt, accompanied by contemporary Fe and Fe -Cu mineralization. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.