Re-Os evidence for replacement of ancient mantle lithosphere beneath the North China craton

摘要

Re-Os data for peridotite xenoliths carried in Paleozoic kimberlites and Tertiary alkali basalts confirm previous suggestions that the refractory and chemically buoyant lithospheric keel present beneath the eastern block of the North China craton (and sampled by Paleozoic kimberlites) is indeed Archean in age and was replaced by more fertile lithospheric mantle sometime after the Paleozoic. Moreover, lithospheric mantle beneath the central portion of the craton (west of the major gravity lineament) formed during the last major Precambrian orogeny, around 1900 Ma ago. This age is significantly younger than the overlying crust (2700 Ma), suggesting that the original Archean lithosphere was replaced in the Proterozoic. The timing of lithospheric replacement in the eastern block of the North China Craton is constrained only to the Phanerozoic by the Re-Os results. Circumstantial geologic evidence suggests this new lithosphere is Jurassic or Cretaceous in age and formed after collision of the Yangtze and North China cratons in the Triassic, an event that was also responsible for the subduction and uplift of ultrahigh-pressure metamorphic rocks. Collectively, these data suggest that lithosphere replacement occurred in response to two continent collisional events widely separated in time (～1900 and ～220 Ma). Coupled with observations from other Archean cratons we suggest that wholesale replacement of lithospheric mantle (±lower crust) may require large-scale continental collision.