Role of asthenosphere and lithosphere in the genesis of the Early Permian Huangshan mafic–ultramafic intrusion in the Northern Tianshan, NW China

Abstract

The Huangshan mafic–ultramafic intrusion hosts a large Ni–Cu sulfide deposit and is situated in the Northern Tianshan at the southern margin of the Central Asian Orogenic Belt (CAOB). The Early Permian intrusion consists of lherzolite, websterite, gabbronorite, gabbro and diorite. The Huangshan deposit contains ~80.4Mt of ore grading 0.54wt.% Ni and 0.3wt.% Cu and is the second largest magmatic sulfide deposit in Northern Xinjiang, China. The Huangshan intrusive rocks are enriched in large ion lithophile elements and depleted in high field strength elements relative to N-MORB, with low Nb/U (1.53–5.27) and high Ba/Nb (1.68–121) ratios, indicating that the primary magma was derived from partial melting of a metasomatized mantle source. The mafic–ultramafic rocks in the Northern Tianshan are characterized by lower Ca contents (<1000ppm) in olivine, more depleted Nb and Ta, lower Nb/U ratios, and higher εΝd(t) than those of the Tarim mafic–ultramafic intrusive rocks and ocean island basalts (OIB). The range of εΝd(t) values of the mafic–ultramafic rocks in the Northern Tianshan over time suggests a greater role for upwelling asthenospheric mantle in the younger rocks. This implies that they were produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts rather than a mantle plume. The linear distribution of Permian mafic–ultramafic intrusions along the Kangguer fault in the Northern Tianshan suggests that slab breakoff played a key role in the genesis of the mantle-derived magma in a syn-collisional setting.