Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China

The mechanisms of continental intraplate magmatism were investigated with respect to Cenozoic basaltic shield lavas from the Changbaishan area, northeast China, using a comprehensive data set of major and trace elements, Sr, Nd, and Pb isotopic compositions, and K–Ar ages. The shield lavas show a significant variation in TiO2 content (1–4 wt.%), which exhibits a positive covariance with K2O and a negative covariance with SiO2 for most samples. The compositional variation of the lavas cannot be explained solely by magma chamber processes, such as fractional crystallization and crustal assimilation, and may in part be attributed to processes in the magmatic source region. The variation in TiO2 is considered to mainly reflect variations in the degree of partial melting of the source mantle, and the negative covariance of SiO2 and TiO2 suggests that lower-degree melts were produced at greater depths. On the basis of the observed negative covariance of TiO2 with K–Ar ages in the main lavas, it would appear that magmas segregated at progressively greater depths with time, since at least 4.2 Ma. This feature cannot be explained in terms of temporal changes in the mantle potential temperature and/or bulk source composition. Instead, it appears likely that the rate of asthenosphere upwelling beneath the Changbaishan area decreased with time, whereby the melting depth gradually increased, resulting from a reduction in the heat flux from the upwelling mantle to the overlying lithosphere. The inferred deceleration of the asthenospheric mantle is considered to have been related to a change from an extensional to compressive tectonic stress regime during the late Pliocene. Our study suggests that long-term volcanic activity in the Changbaishan area will continue to decrease unless there is an increase in the upwelling rate of the asthenospheric mantle.