On and off the North China Craton: Where is the Archaean keel?

Geophysical data indicate that the lithosphere beneath the North China Crat on (NCC) is similar to 80 km thick with high heat flow within the craton. T his is somewhat in disagreement with the presence of Archaean crustal rocks and kimberlite-hosted xenoliths that point to the existence of garnet-diam ond-facies mantle beneath the craton, as recently as the Ordovician. Basalt -hosted mantle xenoliths entrained during the Cenozoic may provide a clue t o Phanerozoic changes. Of particular note is the predominance of spinal-fac ies peridotites (75-80 km) and the paucity of garnet-facies peridotites. Th e model mineralogy of the spinal peridotites is similar to that observed in peridotitic xenoliths from the lower oceanic lithosphere but distinct from that of abyssal peridotites. The orthopyroxene/olivine ratio is like that of peridotites from ocean basins and tectonically active continents, and th e peridotites have 'depleted' Sr and Nd isotopic ratios similar to those of oceanic basalts. The basalt-hosted xenolith data from eastern China suppor t geophysical data in revealing the presence of thin, hot lithosphere with a similarity, over distances of several thousand kilometers, to that found beneath tectonically active continents or ocean basins. These data do not, however, allow us to constrain which of the thermo-tectonic processes (i.e. plume, extension , delamination) was responsible for the loss of the cold, thick Archaean lithospheric root (similar to 200 km) in the last 400 my. W hat is clear is that the pre-existent, presumably heterogeneous, Archaean l ithosphere has been very effectively replaced by 'oceanic' -like mantle. Th e extent to which it was totally replaced is open to debate.