Geochronology and geochemistry of Early Permian mafic–ultramafic complexes in the Beishan area, Xinjiang, NW China: Implications for late Paleozoic tectonic evolution of the southern Altaids

Abstract

The Hongshishan and Pobei mafic–ultramafic complexes outcrop in the Beishan area in the southernmost Altaids. They consist of olivine gabbros, dunites, and pyroxenites. Zircons from olivine gabbros from the Hongshishan complex yield a U–Pb age for emplacement of 281.8 ± 2.6 Ma. The olivine gabbros have low SiO 2 (47.08–48.66%), TiO 2 (0.14–0.29%), MnO (0.07–0.09%), and K 2O (0.06–0.09%), but high Mg # values (81 < Mg # < 81.4) together with high MgO (8.95–12.54%) and TFe 2O 3 (4.93–6.29%). The Hongshishan and Pobei mafic–ultramafic complexes are characterized by negative anomalies in high field strength elements (Zr, Hf, Nb, Ta), which we interpret as arc-related signatures. They also have spikes in large ion lithophile fluid mobile elements (Ba, U, Pb, Sr) and high mobile/immobile element ratios (i.e. primitive mantle-normalized Sr/Nd, Ba/La >> 1), which are attributed to an aqueous fluid component in the source. Zircon Hf isotopes indicate that the source magma was derived either from a depleted mantle with some crustal contamination or from an enriched mantle. These data are interpreted to indicate that the Hongshishan and Pobei mafic–ultramafic complexes were emplaced in the Early Permian subduction-related environment, which suggests that the Paleo-Asian Ocean in the southernmost part of Altaids did not close until after Early Permian.