Geochronology and geochemistry of a newly identified Permian hornblende gabbro suite in Aqishan-Yamansu Belt, eastern Tianshan, NW China: Implications on petrogenesis and tectonic setting

Permian mafic-ultramafic intrusions have great significance for understanding the geodynamic evolution of the Late Palaeozoic eastern Tianshan Orogen due to containing important information on the nature of mantle sources, crust-mantle interaction, and magmatic differentiation. Increasingly, more Permian mafic-ultramafic intrusions are discovered in the Jueluotage belt, especially in the Kangguer ductile shear zone, whereas the mafic-ultramafic intrusions in the Aqishan-Yamansu belt are ill-informed. In this study, we provide zircon U-Pb geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data of a newly identified hornblende gabbro suite at the Shaquanzi Fe-Cu deposit in the Aqishan-Yamansu Belt. Zircon U-Pb dating results indicate that the Shaquanzi hornblende gabbro was emplaced at Early Permian (ca. 274-265 Ma). The rocks are calc-alkaline and have arc-like geochemical features, including enrichments in large-ion lithophile elements (LILEs: Rb, Ba, K, Pb and Sr) and light rare-earth elements (LREEs: Nb, Ta, Zr, Hf, and Ti), and depletions in high-field-strength elements (HFSEs) with markedly negative Nb and Ta anomalies. The rocks also exhibit depleted-mantle isotopic signatures, with positive bulk-rock epsilon(Nd)(t) values of +3.34 to +4.44 and positive zircon epsilon(Hf)(t) values of +2.8 - +8.7, which are relatively more enriched than those of coeval mafic-ultramafic intrusions from the Kangguer ductile shear zone, but similar to those in the Central Tianshan Massif. We suggest that the Shaquanzi mafic intrusion suite was generated by 10-30% partial melting of a depleted-mantle source at over 85 km depth, corresponding to the garnet to garnet-spinel stability field. And the mantle source had likely been metasomatized by slab-derived fluids of previous subduction. Integrating with geochemical data of the coeval mafic-ultramafic and felsic intrusive rocks in the adjacent tectonic belts of eastern Tianshan Orogen, we speculate that the Shaquanzi mafic intrusion was formed in a post-collision extensional setting, probably triggered by slab breakoff. Metasomatism of the depleted lithospheric mantle had likely occurred during the pre-Permian subduction of the Kangguer oceanic slab.