| Abstract: | Forty-two Cenozoic(mostly Miocene) basalt samples from Jining, Chifeng, Fansi, Xiyang, and Zuoquan areas of the North China Craton(the NCC basalts hereafter) were analyzed for platinum-group elements(PGE, including Os, Ir, Ru, Rh, Pt, and Pd). Most of them are alkaline basalts and tholeiites and all of them display little crustal contamination. The total PGE contents of the NCC basalts vary from 0.1 to 0.9 ppb, much lower than those of the primitive mantle values of 23.5 ppb. Primitive mantle-normalized PGE patterns of these basalts define positive slopes and Pd/Ir ratios vary from 1.2 to 25. In terms of both PGE contents and Pd/Ir ratios, they are quite similar to the mid-ocean ridge basalts. There are no obvious negative correlations between PGE vs. Mg O, Ni, and Cu in the NCC basalts, indicating that fractional crystallization of olivine, pyroxene, and/or sulfides during magmatic process cannot be the controlling factor for the observed PGE variation. The observed Pd/Ir variations of the NCC basalts require involvement of non-chondritic heterogeneous mantle sources. Based on Sr-Nd-Pb-Hf isotopic systematics and incompatible-element signatures, a mixing of partial melts from both asthenospheric peridotites and enclosed mantle eclogites at the top of asthenosphere was proposed for the origin of these NCC basalts. The lenses of eclogites are derived from upwelling of recycled continental crust during the westward subduction of the Pacific plate from the ~600 km discontinuity zone. The PGE geochemistry of these basalts provides independent evidence to support this conclusion and the observed Pd/Ir variations may reflect variations in proportions of tapped peridotitic and eclogitic melts. |
