Geochemistry of late mesozoic mafic magmatism in west Shandong Province, eastern China: Characterizing the lost lithospheric mantle beneath the North China Block

The mantle lithosphere beneath the North China Block (NCB) was significantly thinned during the last 400 million years. Comparative studies of early Paleozoic and Cenozoic lithospheric mantle have shown that the lost lithospheric mantle should be of similar character to an Archean enriched mantle reservoir, but this hypothesis has been difficult to test due to the absence of basalt-born mantle xenoliths from the late Paleozoic to Mesozoic. Late Mesozoic (early Cretaceous) mafic intrusive and volcanic rocks occurring in west Shandong Province within the NCB are characterized by large ion lithophile element (LILE), light rare earth element (LREE) enrichment and high filed strength element (HFSE) depletion (Nb, Zr, Hf and Ti) as well as EM1-like Sr-Nd isotopic ratios (initial Sr-87/Sr-16(i) = 0.7040 to 0.7055 and epsilon(Nd)(i)= -18.9 to -9.2), that originated from an aged and enriched lithospheric mantle. The inferred Mesozoic subcontinental lithospheric mantle (SCLM) mainly comprised chemically refractory peridotites with disseminated phlogopite, completely different from that present in Cenozoic time. The Mesozoic SCLM was geochemically and isotopically different compared with the early Paleozoic kimberlite-born peridotite xenoliths that contain high modal proportions of phlogoopite. These older xenoliths have no or insignificant HFSE anomalies but EM2-like isotopic features that might represent the lowermost part of Archean lithosphere. The widespread occurrence of EM1-like Sr-Nd isotopic characteristics in basaltic magmas of west Shandong Province during the late Mesozoic indicates that the most of the Archean lithospheric mantle was progressively removed through thermal-mechanical erosion by a hot convective mantle, and further modified by subsequent partial melting processes in response to extensional tectonics.