Osmium isotopic constraints on the age of lithospheric mantle beneath northeastern China

Northeastern China is composed of the Paleozoic Xing'an Mongolian Orogenic Belt (XMOB) in the north, and the Archean North China Craton (NCC) in the south. Mantle xenoliths from the subcontinental lithospheric mantle (SCLM) underlying these crustal blocks were entrained in Cenozoic basalts and brought to the surface. The Os isotopic compositions of these xenoliths are quite variable. The most refractory xenoliths (Al2O3 < 1.2 wt.%) from both blocks yield model Os T-RD ages between 1.2 and 0.4 Ga. For the XMOB, this indicates that some portions of the underlying SCLM are approximately the same age as the overlying continental crust. This is typical of the age relations between SCLM and the overlying crust for most cratons. The refractory xenoliths with minimally depleted Os-187/Os-188, however, suggest the addition of much younger, Phanerozoic lithospheric mantle. The lack of any xenoliths with Archean model ages in the SCLM underlying the NCC, sampled by this study, suggests that the original SCLM underlying the NCC block has been largely removed. This may have occurred during the Proterozoic, the Phanerozoic, or during both periods. These results highlight the complex age structure in the current SCLM beneath northeastern China, with lithosphere ranging in age from Proterozoic to additions within the past several 100 Ma. (C) 2002 Elsevier Science B.V. All rights reserved.