Geochronology, geochemistry and Sr-Nd isotope compositions of the Niubiziliang mafic-ultramafic complex in the South Altyn orogen, NW China: Implications for Ni-Cu mineralization and tectonic setting

The Niubiziliang mafic-ultramafic complex located in the South Altyn orogen consists of lherzolite, plagioclase-bearing lherzolite, olivine websterite, and gabbro. The intrusion contains a newly discovered zone of magmatic Ni-Cu sulfide mineralization, which is categorized into three units (Nos. 1, 2, and 3) based on discernible morphological features, with drilling investigations revealing sulfide mineralization in complexes Nos. 2 and 3 in both mafic and ultramafic rocks. The rocks of the Nuibiziliang complex have olivines with Fo 76.9-82.4 and Ni contents between 79 and 1957 ppm. The clinopyroxenes have variable Al2O3 contents between 0.52 and 5.05 wt %, TiO2 contents between 0.02 and 1.30 wt%, and NaO contents from 0.08 to 1.27 wt%. The geochemical variations are consistent with the intrusion belonging to a subalkaline magma series and the major element variations are consistent with igneous differentiation from a single magma type. The rocks display enrichment in light rare-earth elements and large-ion lithophile elements (K, Ba, and Sr) and depletion in high-field-strength elements (Ti, Zr, Hf, Ta, and Nb) and heavy rare-earth elements, which are typical features of enriched mid-ocean ridge basalt (E-MORB). These rocks exhibit enriched Sr-Nd isotope compositions with low (87Sr/86Sr)i ratios and mostly low positive epsilon Nd(t) values, indicating that the Niubiziliang complex originated from the partial melting of the asthenospheric mantle with up to 10% contamination by continental crust. Fractional crystalli-zation enriched sulfur in the magma during the initial stage of magmatic evolution, but crustal contamination was the main factor causing sulfide saturation in the magma. Laser ablation inductively coupled plasma-mass spectrometry U-Pb dating of baddeleyites in the ultramafic rocks of the Niubiziliang complex yielded an age of 452 +/- 12 Ma for the crystallization of the magma. This age is similar to the age of eclogites formed during exhumation for the South Altyn orogen and indicates that the mafic-ultramafic magmatism formed in a rift setting by a process of break-up of an oceanic crustal slab at depth in the mantle.