Alkaline intrusives at the east foot of the Taihang-Da Hinggan Mountains: Chronology, Sr, Nd and Pb isotopic characteristics and their implications

Based on the Rb-Sr isochron dating results, this paper suggests that the al kaline intrusive belt at the east foot of the Taihang-Da Hinggan Mountains were formed between 135 and 122 Ma. And the alkaline intrusives in the nort h and south sections of this belt have entirely different Sr, Nd and Pb iso topic characteristics, i.e., all the rocks in the south section have positi ve epsilon (Sr)(t) and negative epsilon (Nd)(t) values and all those in the north have the opposite values. On the epsilon (Sr)(t) versus epsilon (Nd) (t) correlation diagram, the samples from the south are concentrated along the enriched mantle evolution trend lines and nearby, while those from the north fall along the depleted mantle trend lines and nearby. On the Pb isot ope composition diagram, most of the samples from the south section fall on the mantle Pb evolution line and nearby, while those from the north lie be tween the Pb evolution lines of the mantle and the orogenic belt. The above -stated isotopic characteristics not only indicate that the source rocks of the alkaline intrusives in the south section have a close connection to ma terials from the enriched mantle reservoir, while those in the north are re lated to materials from the depleted one, but also reveal that the upper ma ntle below the North China platform is enriched and that below the Inner Mo ngolia geosyncline is depleted.