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

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 SiO2 (47.08-48.66%), TiO2 (0.14-0.29%), MnO (0.07-0.09%), and K2O (0.06-0.09%), but high Mg-# values (81 <Mg-# <81.4) together with high MgO (8.95-12.54%) and TFe2O3 (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. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.