Nature of the lithospheric mantle beneath NE China: Evidence from potassic volcanic rocks and mantle xenoliths

Geochemical data on Cenozoic (16.5 Ma-1721 A.D.) potassic rocks from five volcanic provinces - Wudalianchi, Erkeshan, Keluo, Nuominhe, and Xiaogulihe - in NE China (48 degrees 00'-51 degrees 00' N, 122 degrees 30'-126 degrees 45' E) indicate that the magmas were generated predominantly from a phlogopite-bearing garnet peridotite source located at the lower part of the subcontinental lithospheric mantle. This inferred mantle source is refractory in major element terms but has incompatible element levels and Sr-Nd-Pb isotope ratios similar to an enriched (EM1-type) mantle component originated by ancient metasomatic event(s). Spinel peridotite xenoliths in the potassic rocks are believed to represent fragments of the uppermost lithospheric mantle in the region. The xenoliths have low concentrations of basaltic components in their bulk composition and constituent phases, exhibiting a refractory nature. On the other hand, they display heterogeneously enriched incompatible element and Sr-Nd isotopic signatures, corresponding to the inferred garnet peridotite source for the potassic rocks. The general overlap in geochemical signatures between the xenoliths and the inferred lower lithospheric mantle suggests a common evolutionary history for the whole lithospheric mantle column. This implies broad similarities in the composition of metasomatic melts as well as the style of mantle metasomatism at different levels of the lithospheric mantle. The geochemical data for the Chinese potassic rocks and entrained mantle xenoliths further suggest that the lithospheric mantle beneath the Songliao Block is a Precambrian (Archaean (?) to early Proterozoic) mantle terrane extending well beyond the present boundary of the Songliao Basin into the eastern flank of the Great Xing'an Mountains. These data also demonstrate that the subcontinental lithospheric mantle is at least partially responsible for the ubiquitous Dupal signature in the Cenozoic mafic volcanic rocks in the eastern China. The upwelling asthenospheric mantle, which is another important mantle source for the Chinese basalts, may have heterogeneously acquired the Dupal signature by thermal erosion of the thick lithospheric mantle beneath the Gondwana-derived Sino-Korean Craton during late Mesozoic times.