The characteristics, origin, and significance of mafic microgranular enclaves in the granitoids from the Bailingshan complex, Eastern Tianshan, NW China

Abstract

The Bailingshan complex is located to the south of the Jueluotage tectonic belt of Eastern Tianshan, consisting of quartz monzodiorite, granodiorite, monzogranite, and granite. Mafic microgranular enclaves (MME) are commonly found in granodiorites and monzogranites and have sharp contacts with the host rocks. Sometimes, the fine veins of the host rocks cut through the enclaves. The MMEs are mainly fine‐grained dioritic rocks, including diorite and pyroxene diorite with igneous textures, and having condensing edges with rich fine dark minerals. Among the enclaves, the orthoclase xenocryst with hornblende aggregation rim at its edge, acicular apatites in plagioclase, sometimes late‐crystallized biotites between the oscillatory zonings of plagioclase, and hornblende metasomatic rims occur around the pyroxene phenocrysts can be observed. Both the host rocks and the enclaves are metaluminous and calc‐alkaline to high‐K calc‐alkaline rocks. The enclaves have lower SiO2 and higher Al2O3, FeOT, MgO, CaO, and TiO2 concentrations, as well as higher amounts of the trace elements Cr, Sc, and Th than the host rocks. The host rocks and the enclaves exhibit similar rare earth elements patterns and linear relation of oxides. The sulphur isotopes (δ34S(V‐CDT) = −1.7‰ to +0.7‰) from the iron ores in the Bailingshan area indicate that the mineralization materials originated from the upper mantle. All of these lithology and geochemistry characteristics demonstrate that the enclaves were formed by the mixture of mantle‐derived mafic magma and crust‐derived felsic magma. The iron ore mineralization in the Bailingshan area may be associated with the magma mixing.