Geochronologic-petrochemical studies of the Hongshishan mafic-ultramafic intrusion, Beishan area, Xinjiang (NW China): Petrogenesis and tectonic implications

The Hongshishan mafic-ultramafic intrusion (SIMS zircon U-Pb age 286.4 ± 2.8Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni-Cu mineralization. Mass balance estimates and Sr-Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting. Southward subduction of the Palaeo-Tianshan-Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic-ultramafic intrusions distributed in the eastern Tianshan-Beishan area. Integrating the regional positive εNd(t) granites, high-Mg and low-Ti basaltic magmas (mafic-ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.