Zircons from the gneisses mostly preserved their primary diverse signatures (¦ÅHf(t) = + 15.8 to ? 17.9; crustal model ages = 2.54-0.80 Ga), but some ~ 440 Ma metamorphic grains show lower trace-element concentrations than the Proterozoic zircons. U-Pb and Lu-Hf isotopic data in this study and from the literature reveal the existence of Neoarchean (ca. 2.8-2.5 Ga) components in the Qaidam block as the initial protoliths of the gneisses, which were intruded by depleted-mantle (DM) magmas at ~ 2.1-2.0 Ga and 1.1-0.8 Ga. The igneous protoliths of the eclogites were emplaced into the protoliths of gneisses at ~ 850-750 Ma, and are interpreted here as having formed by magmatic mixing between juvenile components from the DM and evolved material from the Archean Qaidam crust. This 1.1-0.8 Ga magmatism has largely ¡°rejuvenated¡± the Qaidam bock, at least its northern margin, and added more mafic material into its deep crust and upper mantle, as also indicated by geophysical observations. This lithosphere, and perhaps lower crust, of this continental block would become denser during subduction because of the extensive generation of garnet in the deep lithosphere, causing it to act as a ¡°sinker.¡± The intrinsically basic nature of the deep levels of the Qaidam block pre-determined its deep subduction into the mantle depths at ~ 440 Ma, and then the less dense upper crust was detached from the deeper levels and exhumed later.