A Cambrian normal MORB tholeiite to boninite succession caused by ridge subduction in the Western Kunlun Orogen, Northwest China

The magmatic chemostratigraphic progression in Izu−Bonin−Mariana (IBM) forearc and supra-subduction zone ophiolites display magmatic conversions from mid-ocean-ridge basalt (MORB)-like/forearc basalt (FAB) tholeiites to boninites, which are always interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism. Here, we present new zircon U-Pb ages, major and trace elemental, and isotopic data of MORB-like tholeiites and boninitic rocks in Western Kunlun, Northwest China. The Early−Middle Cambrian (517−500 Ma) tholeiites exhibit compositions similar to those of the most depleted MORBs, but with lower Ti/V ratios (20−26 versus ∼33), suggesting slightly higher concentrations of fluid-soluble elements than normal MORBs. They have high εNd(t) values (6.1−7.4), low (87Sr/86Sr)i ratios (0.7044−0.7049), low zircon δ18O (4.50‰−5.69‰) values, and low initial Pb isotope ratios [(206Pb/204Pb)t: 17.174−17.842; (207Pb/204Pb)t: 15.437−15.553; (208Pb/204Pb)t: 37.052−37.815]. The Middle Cambrian (509−506 Ma) boninitic dikes are characterized by depletions in Ti, heavy rare earth elements (HREEs), and high field strength elements (HFSEs), with enrichment in large ion lithophile elements (LILEs). They have low εNd(t) values (0.6−4.9), low (87Sr/86Sr)i ratios (0.7035−0.7038), and high initial Pb isotope ratios [(206Pb/204Pb)t: 17.849−18.179; (207Pb/204Pb)t: 15.554−15.625; (208Pb/204Pb)t: 37.735−38.058]. Our results show that the tholeiites were generated from decompression-dominated melting of sub-slab asthenosphere mantle, and later the boninitic dikes were derived by high-temperature melting of refractory peridotitic mantle re-fertilized by minor hydrous melt from subducted oceanic lithosphere. Taking into account geological evidence and the occurrence of contemporaneous ophiolites and volcanic lavas in Western Kunlun, we propose that the tholeiites and boninitic rocks were formed above a subducted ridge-related slab window, distinct from strictly subduction initiation magmatism. Consequently, we suggest that caution is required before assuming such a normal MORB tholeiite to boninite succession was strictly indicative of subduction initiation without considering alternative tectonic settings.