On and Off the North China Craton: Where is the Archaean Keel?

Abstract

Geophysical data indicate that the lithosphere beneath the North China Craton (NCC) is ∼80 km thick with high heat flow within the craton. This is somewhat in disagreement with the presence of Archaean crustal rocks and kimberlite-hosted xenoliths that point to the existence of garnet–diamond-facies mantle beneath the craton, as recently as the Ordovician. Basalt-hosted mantle xenoliths entrained during the Cenozoic may provide a clue to Phanerozoic changes. Of particular note is the predominance of spinel-facies peridotites (75–80 km) and the paucity of garnet-facies peridotites. The modal mineralogy of the spinel peridotites is similar to that observed in peridotite 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 the peridotites have ‘depleted’ Sr and Nd isotopic ratios similar to those of oceanic basalts. The basalt-hosted xenolith data from eastern China support geophysical data in revealing the presence of thin, hot lithosphere with a similarity, over distances of several thousand kilometres, 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 (∼200 km) in the last 400 my. What is clear is that the pre-existent, presumably heterogeneous, Archaean lithosphere has been very effectively replaced by ‘oceanic’-like mantle. The extent to which it was totally replaced is open to debate.