EMPLACEMENT AGE AND SR-ND-HF ISOTOPIC CHARACTERISTICS OF THE DIAMONDIFEROUS KIMBERLITES FROM THE EASTERN NORTH CHINA CRATON

Abstract

This paper first reports the Ar-Ar geochronology of kimberlite-borne phlogopite megacrysts and the Hf isotopic compositions of the diamondiferous kimberlites from the eastern North China Craton. The paper, combined with the Sr-Nd isotopic data, discussed the kimberlite petrogeneses and their tectonic settings. Ar-Ar geochronology of the phlogopite megacrysts demonstrates that both the Mengyin and Fuxian kimberelites have coherent emplacement ages, about 465 ± 2Ma. This is consistent with the previous results of the perivoskite U-Pb age and phlogopite Rb-Sr isochron ages of the kimberlites. The Sr-Nd isotopic data show that both the Mengyin and Fuxian kimberlites have restricted Nd isotopic compositions (εnd in the range of -0.4 - 0.2 and - 3.4 - - 2.3, respectivey). However, these kimberlites have extremely large Sr isotopic compositions (Isr in the range of - 0.2 - - 28.1 and 0-75, respectively), in which relative to the Mengyin, the Fuxian kimberlites have lower Nd isotopes and much wider Sr isotopes. Hf isotopic results show that individual kimberlite fields from both the Mengyin and Fuxian regions have quite uniform Hf isotopic compositions, similar to the situation for the Nd isotopes, in which the initial 176Hf/177 Hf ratios of the Mengyin kimberlites are in the range of 0.282474 - 0.282416 with corresponding εHf values of - 2.37 ~ - 0.30 and the initial 176Hf/177Hf ratios of the Fuxian kimberlites are of 0.282305 - 0.282369 with corresponding εHf values of - 6.29 ~ - 4.04. This manifests that Mengyin kimberlites have relatively higher Hf isotopic compositions than those of Fuxian kimberlites. In combination with the Sr and Nd isotopic results, it indicates that the difference in Hf isotopic compositions between the Mengyin kimberlites and the Fuxian kimberlites could reflect the difference in proportions of the involvement of the asthenosphere, the lithospheric mantle and the subducted oceanic crust in the origin of the kimberlitic magmas. These Sr-Nd-Hf isotopic features demonstrate that the generation of the Paleozoic kimberlites of the North China Craton was related with the inward subduction of Paleo-oceanic lithosphere from the circum North China Craton towards the interior of the craton.