Geochronological and geochemical features of the Xiaowulangou complex plutons, Xilinhot, Inner Mongolia, and their geological significance

Abstract

The southern margin of the Central Asian Orogenic Belt (CAOB) is pivotal to understanding the ultimate closure process of the Palaeo‐Asian Ocean; however, controversial issues regarding the petrogenesis and tectonic setting of its Late Palaeozoic igneous rocks have caused uncertainty regarding the Late Palaeozoic oceanic and continental evolution of the area. Herein, the Xiaowulangou complex plutons near Xilinhot, Inner Mongolia, were considered as a case study for the southern margin of the CAOB, and they were analysed via multiple geological methods, including field geology, petrology, zircon U–Pb isotopic geochronology, and geochemistry to achieve greater clarity on these issues. Lithologically, the plutons consist of diorite, quartz diorite, granodiorite, and monzonitic granite. Zircon U–Pb dating gives weighted mean ages of the diorite, quartz diorite, and granodiorite plutons as 329 ± 1 (mean squared weighted deviation [MSWD] = 0.22) and 306 ± 1 (MSWD = 0.83), 310 ± 1 (MSWD = 0.23), and 328 ± 1 (MSWD = 0.40) and 300 ± 1 Ma (MSWD = 1.4), respectively, corresponding to the late Early Carboniferous to the Late Carboniferous. The diorites and the quartz diorites constitute a suite of low‐K tholeiitic calc‐alkaline I‐type granitoids. They have low rare earth element (REE) content with a flat distribution pattern lacking pronounced negative Eu anomalies (δEu = 0.74–1.06). Geochemically, they are more enriched in large‐ion lithophile elements (LILEs) such as Rb, Th, K, and Sr than in high‐field‐strength elements (HFSEs) such as Nb, Ta, Ti, and P, indicating that these rocks were formed in a subduction setting as a result of partial melting of a mantle wedge that had been metasomatized by subduction fluids. The granodiorites and monzonitic granites belong to the peraluminous and calc‐alkaline to high‐K calc‐alkaline series and feature high silicon and alkali content; low P2O5 and CaO content; enrichment in Rb, Th, K, Zr, and Hf; depletion in Nb, Ta, Sr, P, and Ti; and significant negative Eu anomalies (δEu = 0.66–0.95 and 0.06–0.46, respectively). The granodiorites and monzonitic granites show the characteristics of I‐type granite that were mainly derived from partial melting of the crust. The petrological and geochemical characteristics of the rocks indicate that the plutonic complex is a result of superposed injection of a mixture of mantle‐ and crust‐derived magmas with different degrees of evolution. This was probably triggered by underplating of mantle magmas that originated from the partial melting of a wedge mantle metasomatized by hydrous fluids from the subducted slab. The diorites and quartz diorites were produced by fractional crystallization of mantle‐derived magma. The granodiorites and monzonitic granites crystallized from magma formed by mixing of the intermediate to basic magma and crust‐derived acidic magma. Considering their petrological and geological features, the Xiaowulangou complex plutons were formed in an active continental margin setting related to the northward subduction of the Palaeo‐Asian Ocean from the Early Carboniferous to the Late Carboniferous. Furthermore, consequently, a tectonic evolution model was established from the Late Palaeozoic to the Early Mesozoic in the Xilinhot–Xi Ujimqin Banner regions based on these new data and the regional geology.