华北克拉通东部含金刚石金伯利岩的侵位年龄和Sr-Nd-Hf同位素地球化学特征

Emplacement age and Sr-Nd-Hf isotopic characteristics of the diamondiferous kimberlites from the eastern North China Craton

本文首次报道了我国华北克拉通内部金伯利岩中金云母巨晶的Ar-Ar同位素年龄和含金刚石金伯利岩的Hf同位素组成特征,并结合金伯利岩的Sr-Nd同位素组成特征对华北金伯利岩的岩石成因及其构造背景进行了探讨。金伯利岩中金云母巨晶的年代学分析显示华北克拉通内部的蒙阴和复县金伯利岩具有一致的侵位年龄,约465±2Ma。这与以前发表的金伯利岩中钙钛矿的U-Pb年龄和金云母的Rb-Sr等时线年龄一致。Sr-Nd同位素结果表明蒙阴和复县金伯利岩皆具有很小的Nd同位素变化范围(ε_(Nd)分别为-0.4～0.2和-3.4～-2.3)。然而,这些金伯利岩却具有极大的Sr同位素组成范围(I_(Sr)分布为-0.2～28.1和0～75)。其中复县比蒙阴金伯利岩具有较低的Nd同位素和更宽的Sr同位素组成。Hf同位素分析结果表明蒙阴和复县两岩区内部各金伯利岩岩管具有非常一致的铪同位素组成,类似于Nd同位素组成特征,其中蒙阴金伯利岩的^(176)Hf/^(177)Hf初始值为0.282474～0.282416,相应的ε_(Hf)值为-2.37～-0.30;复县金伯利岩的^(176)Hf/^(177)Hf初始值为0.282305～0.282369;相应的ε_(Hf)值为-6.29～-4.04。这说明蒙阴金伯利岩与复县金伯利岩相比具有较高的Hf同位素组成。结合Sr-Nd同位素组成,暗示蒙阴和复县金伯利岩的Hf同位素组成的不同可能反映金伯利岩岩浆形成时软流圈、岩石圈地幔以及俯冲的洋壳物质参与比例的不同。Sr-Nd-Hf同位素特征说明我国华北古生代金伯利岩的形成同样与华北用边的古大洋岩石圈向华北内部俯冲作用有关。

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 （БNd 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 nf ratios of the Mengyin kimberlites are in the range of 0.282474 ~ 0.282416 with corresponding БHf values of -2.37 ~ -0.30 and the initial ^176Hf/^177Hf ratios of the Fuxian kimberlites are of 0.282305 ~ 0.282369 with corresponding БHf 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.