Geochemistry and petrogenesis of the Baishiquan Cu-Ni sulfide-bearing mafic-ultramafic intrusion in the central Tianshan, Xinjiang, NW China

Located in the Central Tianshan block, northern Xinjiang, NW China, the Baishiquan Cu-Ni sulfide-bearing mafic ultramafic intrusion is composed of harzburgite, olivine pyroxenite, troctolite, gabbro and hornblende gabbro. Many of them have a cumulate texture. The major element, trace element and REE geochemistry of the intrusion indicates that: the intrusion is mafic-ultramafic and shows a trend of tholeiitic magmatic differentiation; it is relatively enriched in large ion lithophile elements (Rb, Ba, Sr etc.) and depleted in high field strength elements (Nb, Ta, Zr, Hf) and exhibits LREE-enriched ((La/Yb)(n) = 1.27 to 9.95)), right-inclined REE distribution patterns. According to these geochemical characteristics, combined with their Ph isotope characteristics, it may be inferred that the primary magma may have been derived from the asthenospheric mantle and EM 11 mantle materials. The integrated geochemical characteristics suggest that the parent magma originated from the high-Ma tholeiitic magma in the asthenospheric mantle that had undergone contamination with early-subducted oceanic crustal materials. During the rise of the magma, the upper continental crust and country rocks had no contribution to it. After its formation, the primary magma underwent fractional crystallization of olivine, pyroxenes, plagioclase and spinel (or chromite) and liquation of sulfides. The intrusion exposed now is the product of the evolution of olivine- and sulfide-laden crystal mush in the high-level magma chamber and its in-situ differentiation.