The North Qilian orogenic belt in North China has been defined as a subduction–collision zone between the Alxa Block and the Qilian Block. We present petrography, zircon U–Pb geochronology, major- and trace-element geochemistry, and Sr–Nd–Pb–Hf isotope analysis for the Yushigou diabase from the Longshoushan area, which is located SW of the Alxa Block, aiming to understand its petrogenetic link to subduction processes. The Yushigou diabase belongs to the tholeiite series, and shows enrichment in light rare earth and large-ion lithophile elements, and a depletion in heavy rare earth and high-field-strength elements. Laser ablation – inductively coupled plasma – mass spectrometry U–Pb zircon dating yielded an emplacement age of 414 ± 9 Ma, with an ϵHf(t) value in the range of −10.3 to 1.8. The whole-rock initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the diabase range over 16.811–17.157, 15.331–15.422 and 37.768–37.895, respectively. The (87Sr/86Sr)i ratios vary between 0.7086 and 0.7106, and ϵNd(t) values vary between −14.4 and −13.4, which are significantly higher than the ϵHf(t) value (Nd–Hf decoupling). An interpretation of the elemental and isotopic data suggests that the Yushigou diabase was derived from partial melting of an enriched mantle I (EM-I) -type lithospheric mantle in the spinel–garnet transitional zone. Based on the geochemical features and previous regional geological data, we propose that the Silurian magmatism was most likely triggered by slab break-off after the closure of the North Qilian Ocean, and ancient continental materials from the subduction slab metasomatized the overlying lithospheric mantle during exhumation.

The North Qilian Orogenic Belt is on the northeastern margin of the Tibetan Plateau. It is connected with the Alxa Block in the north and the Tethyan orogenic assemblage in the south. The Lower Silurian Angzanggou Formation, deposited in the northern area of the North Qilian Orogenic Belt, is the most important for revealing the architecture and orogenic processes of the North Qilian Belt. Provenance analysis of the Angzanggou Formation can reveal not only the tectonic evolution of the central orogenic belt of China but also Palaeozoic Asia plate reconstructions. Petrographic analysis indicated that the compositional and textural maturity of the sandstones was low. The detrital composition of the Angzanggou Formation samples consists of quartz (8–14 %), feldspar (6–29 %) and lithic fragments (56–86 %). The sandstones could be classified as litharenites or feldspathic litharenites. The detrital modal composition suggests that these sandstones were probably deposited in a fore-arc basin. The element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks. The SiO2/Al2O3 ratio, the index of chemical variability and the Th/Sc versus Zr/Sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low. The index of alteration (CIA) and the A–CN–K diagram indicate the intensity of weathering was moderate. The discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the Angzanggou Formation rocks were derived from a continental island arc, and a sedimentary cover probably overlaid the volcanic arc. Therefore, we infer that during Early Silurian time the North Qilian Belt sediments accumulated in a fore-arc basin.