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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

【摘要】:正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 (81Mg~#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.

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