REMOVAL OF LITHOSPHERIC MANTLE IN THE NORTH CHINA CRATON: Re-Os ISOTOPIC EVIDENCE FOR COUPLED CRUST-MANTLE GROWTH

Mantle xenoliths from Ordovician kimberlite at Fuxian, Liaoning and from Neogene alkaline basalts at Hannuoba and Qixia of the North China craton were measured for Re-Os isotopic compositions. Xenoliths from the kimberlite yield Re depleted Os model ages (T_(RD)) of 2.5～2.8 Ga. This confirms the presence of Archean lithospheric mantle beneath the North China Craton. The Hannuoba xenoliths give a Re-Os isotopic isochron of (1.9±0.18) Ga, which demonstrates that the present-day lithospheric mantle preserved there is Paleoproterozoic in age. The surface of the Hannuoba area is dominated by Archean crustal rocks, suggesting former presence of Archean mantle. However, lack of Archean ages in the Hannuoba peridotite xenoliths show that the Archean mantle had been replaced by the newly formed Paleoproterozoic mantle. This event coincides with the widespread Paleoproterozoic granulite-facies metamorphism resulting from continental collision. This in turn implies that the Paleoproterozoic removal of the lithospheric mantle were associated with lithospheric delamination. The Qixia xenoliths have Os isotopic compositions similar to the present-day convecting mantle and show no correlations between Os isotopic and Re/Os ratios, which suggest that they are very young in age. Together with other lines of geological and geochemical evidence, these results point to Mesozoic removal of the lithospheric mantle in the eastern North China Craton, which is speculated to be associated with delamination of the lower crust and lithospheric mantle induced by the Triassic collision between the North China and Yangtze cratons. The present study documents that timing and location of removal of the lithospheric mantle vary from the western to the eastern parts of the North China craton. The 2.5～2.8 Ga and 1.9 Ga lithospheric mantle forming events agree with the two most important crustal growth episodes in the North China craton. Therefore, formation of the continental crust and lithospheric mantle is coupled in time in the North China craton.