COMPOSITION AND EVOLUTION OF LITHOSPHERIC MANTLE BENEATH THE NORTHEASTERN NORTH CHINA CRATON

Decratonization of the North China Craton is widely accepted. However, the mechanism of the formation of newly accreted lithospheric mantle is still controversial. The petrography and the geochemical data for a series of spinel-facies peridotite xenoliths from the Huinan Cenozoic basalts within the northeastern margin of the NCC are reported, providing insights into the nature and evolution of the lithospheric mantle. These peridotite xenoliths display coarse-grained and porphyroclastic textures, and can be subdivided into two groups. Group 1 peridotites have high Ol modal contents (79–96%), whole-rock MgO (44.76–47.86 wt %), and low Al₂O₃ (0.53–1.06 wt %) levels, but relatively uniform Mg# values (90.1-90.7). In contrast, Group 2 peridotites have low Ol modal contents (64–83%), whole-rock MgO (38.57–44.46 wt %) and high Al₂O₃ (1.48–3.23 wt %) levels. The Mg# value of Group 2 is low and varies from 87.6-90.1. Group 1 xenoliths experienced higher degree of partial melting than that of Group 2 (>20% vs. <15%). Both groups contain orthopyroxene–spinel clusters, which represent the products of breakdown of previous garnet. The studied peridotites deviate from the partial melting trend because of low SiO₂ (39.74–45.41 wt %) and high FeO^T (8.22–10 wt %). Meanwhile, embayment is resulted from replacement of Opx by Ol. All these suggest the studied peridotites are not simple residues of partial melting, but possibly have been subjected to melt–rock interaction during which orthopyroxenes were dissolved and accompanied by the precipitation of olivine. The clinopyroxenes show four types of REE patterns, including LREE-depleted, spoon-shaped, LREE-enriched and convex-upward. They can be explained by the chromatographic effects through melt percolation. We conclude that the lithospheric mantle beneath the northeastern margin of the North China Craton experienced decompression melting owing to lithospheric extension and melt–rock interaction ascribed to reactive porous flow, which probably plays an important role in the formation of lithospheric mantle and the destruction of the northeastern NCC.