Source characteristics and tectonic setting of mafic–ultramafic intrusions in North Xinjiang, NW China: Insights from the petrology and geochemistry of the Lubei mafic–ultramafic intrusion

Abstract

The newly discovered Lubei sulfide-bearing mafic–ultramafic intrusion forms the western extension of the Huangshan–Jin'erquan mafic–ultramafic intrusion belt in East Tianshan, NW China. The Lubei intrusion comprises hornblende peridotite, lherzolite, and harzburgite in its southern portion, gabbro in its middle portion, and hornblende gabbro in its northern portion. Intrusive relationships indicate that three magma pulses were involved in the formation of the intrusion, and that they were likely evolved from a common primitive magma. Estimated compositions of the Lubei primitive magma are similar to those of island arc calc-alkaline basalt except for the low Na2O and CaO contents of the Lubei primitive magma. This paper reports on the mineral compositions, whole-rock major and trace element contents, and Rb–Sr and Sm–Nd isotopic compositions of the Lubei intrusion, and a zircon LA–MC–ICP–MS U–Pb age for hornblende gabbro. The Lubei intrusion is characterized by enrichment in large-ion lithophile elements, depletion in high-field-strength elements, and marked negative Nb and Ta anomalies, with enrichment in chondrite-normalized light rare earth elements. It exhibits low (87Sr/86Sr)i ratios of 0.70333–0.70636 and low (143Nd/144Nd)i ratios of 0.51214–0.51260, with positive εNd values of +4.01 to +6.33. LA–ICP–MS U–Pb zircon ages yielded a weighted-mean age of 287.9 ± 1.6 Ma for the Lubei intrusion. Contemporaneous mafic–ultramafic intrusions in different tectonic domains in North Xinjiang show similar geological and geochemical signatures to the Lubei intrusion, suggesting a source region of metasomatized mantle previously modified by hydrous fluids from the slab subducted beneath the North Xinjiang region in the early Permian. Metasomatism of the mantle was dominated by hydrous fluids and was related to subduction of the Paleo-Asian oceanic lithosphere during the Paleozoic. Sr–Nd–Pb isotopic compositions suggest that the mantle source was a mixture of depleted mid-ocean-ridge-basalt mantle and enriched-mantle I components. The Permian mafic–ultramafic intrusions in North Xinjiang were formed from tholeiitic basaltic magmas derived from decompression partial melting of the metasomatized mantle in a post-collision extensional tectonic setting. The tholeiitic basaltic magmas are equivalent to the voluminous underplated basaltic magmas that formed during vertical crustal growth of the Central Asian Orogenic Belt in the later Paleozoic.