Mafic rocks from the southern Alxa block of Northwest China and its geodynamic evolution in the Paleozoic

The southern margin of the Alxa Terrane is located between the North Qilian Orogenic Belt and the inner Alxa block, and Paleozoic mafic magmatism records the geodynamic evolution at different stages in the orogenic cycle. New zircon U−Pb ages and whole-rock geochemistry reveal three groups of mafic rocks in the southern margin of the Alxa block: (i) c. 480 Ma Xijing gabbros, characterized by high Ba/Th and low Th/La ratios with low eNd(t) (−5.7 to −4.9) values, implying their derivation from enriched mantle metasomatized by slab-derived fluids; (ii) c. 440 Ma Xijing and Jiling hornblende gabbros which have high Th/La and low Ba/Th ratios, suggesting that they were produced by partial melting of mantle modified by previously subducted sediments; (iii) c. 250 Ma Xijing pyroxenites, which are cumulates with relatively high eNd(t) (+5.2 to + 5.9), indicating that their host magmas were derived from relatively depleted mantle. Together with published results, these new data highlight that the c. 480 and c. 440 Ma mafic magmas were related to the northward subduction of the North Qilian oceanic slab and later post-collisional lithospheric delamination, respectively. The c. 250 Ma Xijing pyroxenites were related to younger lithospheric extension in the development of the Central Asian Orogenic Belt in the Alxa block. Supplementary material: Supplementary tables A1–A7 are available at https://doi.org/10.6084/m9.figshare.c.5287657