Emplacement age and Sr-Nd-Hf isotopic characteristics of the diamondiferous kimberlites from the eastern North China Craton
Emplacement age and Sr-Nd-Hf isotopic characteristics of the diamondiferous kimberlites from the eastern North China Craton
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作者：
Zhang Hong
本文首次报道了华北克拉通东部金伯利岩金云母巨晶Ar-Ar年代学和含金刚石金伯利岩Hf同位素组成，并结合Sr-Nd同位素资料，讨论了金伯利岩成岩作用及其构造背景。金云母巨晶Ar-Ar年代学表明，蒙阴金伯利岩和抚县金伯利岩的侵位年龄一致，约为465±2Ma，这与前人金伯利岩钙钛矿U-Pb年龄和金云母Rb-Sr等时线年龄结果一致。Sr-Nd同位素数据表明，蒙阴金伯利岩和抚县金伯利岩的Nd同位素组成(e_(Nd)均限制在分别为-0.4～0.2和-3.4～-2.3)。然而，这些金伯利岩具有极大的Sr同位素组成(I_(Sr)分别在-0.2～28.1和0～75范围内)，其中相对于蒙阴，抚仙金伯利岩具有更低的Nd同位素和更宽的Sr同位素。Hf同位素结果表明，个别蒙阴和抚仙地区金伯利岩田的Hf同位素组成相当均匀，与Nd同位素情况相似，蒙阴金伯利岩初始~(176)Hf/~(177)Hf比值范围为0.282474～0.282416，对应的e_(Hf)值为-2.37～-0.30，抚仙金伯利岩初始~(176)Hf/~(177)Hf比值为0.282305～0.282369，相应的e_(Hf)值为-6.29～-4.04。这表明蒙阴金贝岩具有相对较高的Hf同位素组成。结合Sr和Nd同位素结果表明，蒙阴金伯利岩和抚县金伯利岩的Hf同位素组成差异可以反映金博拉岩浆成因中软流圈、岩石圈地幔和俯冲洋壳的参与比例的差异。这些Sr-Nd-Hf同位素特征表明华北古生代金伯利岩的生成克拉通与古大洋岩石圈从环华北克拉通向克拉通内部俯冲有关。
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(e_(Nd)in the range of -0.4～0.2 and -3.4～-2.3, respectivey).However,these kimberlites have extremely large Sr isotopic compositions(I_(Sr)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 ~(176)Hf/~(177)Hf ratios of the Mengyin kimberlites are in the range of 0.282474～0.282416 with correspondinge_(Hf)values of -2.37～-0.30 and the initial ~(176)Hf/~(177)Hf ratios of the Fuxian kimberlites are of 0.282305～0.282369 with correspondinge_(Hf)values of -6.29～-4.04.This manifests that Mengyin kimbedites have relatively higher Hf isotopic compositions than those of Fuxian kimborlites.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 lithospherie mantle and the subdueted oceanic crust in the origin of the kimborlitic 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.