Late Archean to early proterozoic lithospheric mantle beneath the western North China craton: Sr-Nd-Os isotopes of peridotite xenoliths from Yangyuan and Fansi

Sr-Nd-Os isotopic analyses are presented for peridotite xenoliths from Tertiary alkali basalts in Yangyuan and Fansi with the aim of identifying and characterizing the relics of ancient lithospheric mantle that survived lithospheric removal in the western North China Craton (NCC). The analyzed samples are residual lherzolites and harzburgites, ranging from fertile to highly depleted (0.36-4.0 wt% Al2O3) composition. Some LREE-enriched samples are characterized by moderate Sr-87/Sr-86 (0.7044 to 0.7047) and low epsilon(Nd) (-6.9 to -10.6), pointing to an EMI-type signature. This is distinct from the predominant depleted isotopic composition in mantle xenoliths from eastern China. Os isotopic ratios range from 0.1106 to 0.1325. The lower limit is the most unradiogenic value measured so far for Cenozoic basalt-borne xenoliths from eastern China. Two samples show radiogenic Os ratios higher than that of the primitive upper mantle, one sample has an anomalously high Os concentration (> 9 ppb). These samples also show high La/Yb, consistent with the addition of radiogenic components during the infiltration of asthenosphere-derived and/or subduction-related melts in the lithospheric mantle. The remaining samples define positive correlations between Os-187/Os-188 and indices of melt extraction, which yield a model age of similar to 2.6 Ga. This age of melt extraction overlaps with the Nd model age of the overlying crust, indicating a coupled crust-mantle system in the western NCC. This contrasts with the decoupled nature in the eastern NCC, suggesting distinct mantle domains underneath the NCC. Such a heterogeneous age structure of the upper mantle is compatible with the view that the lithospheric removal was largely limited to the eastern NCC. (c) 2007 Elsevier B.V. All rights reserved.