| Summary: | A series of ultrabasic-acid igneous rocks are distributed in the belt of Urad houqi-Damaoqi-Siziwangqi, in middle Inner Mongolia. The formation age and geochemistry of these rocks can provide basic geological information for the metallogenic regularities of Cu-Ni deposits related to ultrabasic-basic rocks. The age and geochemistry can also be used to infer the tectonic setting during magma emplacement. The Huanghuatan copper-nickel deposit in Damaoqi was selected for this research. Because of less fractures, high transparency, uniform cathodoluminescence and clear oscillatory zonings, zircon U-Pb dating is prior to be tested on LA-ICP-MS. The result yielded a weighted age of 262.4±1.1 Ma. The geochemical characteristics investigated by X-ray Fluorescence Spectrometry (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) indicate that Huanghuatan gabbros may have formed in a post-collision tensional environment. Combining with previous studies, it is suggested that the central region of Inner Mongolia had entered into the post-collision stage during the late Permian (255-275 Ma), whereas the close of Paleo-Asian Ocean in this limited area occurred before late Permian (about 285 Ma). This conclusion provides the upper time limit for the close of Paleo-Asian Ocean and the tectonic setting of the central part of Inner Mongolia during middle-late Permian.
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