Basalt platforms in Inner Mongolia and Hebei Province, northeastern China: New K-Ar ages, geochemistries, and revision of palaeomagnetic results

Abstract

Summary K–Ar ages were determined for the basalt samples from northeastern China of which palaeomagnetic results were reported in (Zheng et al. 1991, Geophys, J. Int.,104, 29–40). Samples used for dating were collected at 42 sites from 12 basalt platforms near Chifeng in Inner Mongolia, and 9 sites from 3 platforms near Zhangjiakou in Hebei Province, all of which were stratigraphically dated to the Neogene Period in the previous study. K–Ar dating showed that most of the platforms with a supposed age of Neogene were formed in either the Miocene or the Oligocene except one platform, which turned out to be about 90 Ma in age. Hence, minor revision was made to the palaeomagnetic data by discarding the relevant Cretaceous palaeodirections, and updated palaeomagnetic poles were reported. Differences in the new and previous poles are not significant for both Tertiary and Late Cretaceous, but some of the site-mean directions were combined due to their similarity, providing a new data set to be used for the study of palaeosecular variation. Chemical compositions of major and trace elements were measured in some of the Tertiary basalt samples. Secular variation was observed in both the incompatible element concentrations and the normative compositions. The secular variation occurred in a similar trend before and after the volcanic hiatus of 10–25 Ma. It is suggested that the observed secular variation in chemical compositions could be caused by differences both in degrees of partial melting and depths of magma segregation.