In situ perovskite Sr–Nd isotopic constraints on the petrogenesis of the Ordovician Mengyin kimberlites in the North China Craton
In situ perovskite Sr–Nd isotopic constraints on the petrogenesis of the Ordovician Mengyin kimberlites in the North China Craton
复制标题
DOI：
10.1016/j.chemgeo.2009.02.011
复制
发表时间：
2009-06
期刊：
影响因子：
3.9
通讯作者：
Yue-heng Yang;Fu-Yuan Wu;S. Wilde;Xiaoming Liu;Y. Zhang;Liewen Xie;Jin-Hui Yang
中科院分区：
文献类型：
--
作者：
Yue-heng Yang;Fu-Yuan Wu;S. Wilde;Xiaoming Liu;Y. Zhang;Liewen Xie;Jin-Hui Yang
准确的锶、钕、氢同位素数据是破译岩浆岩演化及其源区的重要标志。然而，对于金伯利岩来说，这样的数据很难获得，因为这些岩石通常同时受到地壳和地幔物质的污染，而且在侵位后显示出广泛的蚀变和风化作用。本研究选取了中国东北部蒙阴地区大量的金伯利岩样品，进行了钙钛矿的U-Pb定年和Sr-ND-Hf同位素分析。新鲜钙钛矿的原位U-Pb分析年龄为470±4 Ma，被认为是蒙阴金伯利岩侵位年龄。然而，蚀变钙钛矿显示铅的损失，并产生古生代-中生代年龄，表明钙钛矿并不像以前认为的那样具有很强的抗同位素修饰能力。用激光烧蚀采自蒙阴金伯利岩的钙钛矿颗粒进行的原位锶-钕同位素分析，测得平均初始~(87)Sr/~(86)Sr比值为0.70371±12，ε_(ND(T))为0.13±0.22，与溶液分析结果在不确定度内一致。然而，它们与从全岩样品获得的可比数据显著不同，这表明，由于地壳污染和(或)随后的蚀变，根据金伯利岩全岩测量计算的初始锶-钕同位素比值可能记录到混合的同位素特征。这一结论也得到了钙钛矿的Hf同位素数据的支持。因此，只有从钙钛矿等单一矿物中才能获得金伯利岩有意义的同位素比值。本研究中研究的蒙阴样品具有与南部非洲定义的I族金伯利岩相似的同位素组成，并被解释为反映了原始地幔来源的起源。结合蒙阴金伯利岩在∼470 Ma奥陶纪与区域岩石圈隆升共生的事实，提出了地幔热柱在北中国克拉通本区触发金伯利岩岩浆作用的观点。
Precise Sr, Nd and Hf isotopic data are important indicators in deciphering the evolution of magmatic rocks and their source. However, such data are difficult to obtain for kimberlite, since these rocks are commonly contaminated by both crustal and mantle materials and also show extensive alteration and weathering following emplacement. In this study, numerous kimberlitic samples from Mengyin in northeast China were selected for U–Pb dating and Sr–Nd–Hf isotopic analysis of perovskite. In situ U–Pb analyses of fresh perovskite yield an age of 470±4 Ma, which is considered the emplacement age of the Mengyin kimberlite. However, the altered perovskite shows Pb loss and yields Paleozoic-Mesozoic ages, indicating that perovskite is not as strongly resistant to isotopic modification as previously thought. In situ Sr–Nd isotopic analyses by laser ablation of perovskite grains collected from the main Mengyin kimberlite record uniform Sr and Nd isotopic compositions with an average initial87Sr/86Sr ratio of 0.70371±12 and εNd(t) value of 0.13±0.22, which are identical, within uncertainties, to the values obtained by solution analyses. However, they are significantly different from comparable data obtained from whole rock samples, indicating that initial Sr–Nd isotopic ratios calculated from whole rock measurements on kimberlites are likely to record mixed isotopic signatures, due to crustal contamination and/or subsequent alteration. This conclusion is also supported by the Hf isotopic data for perovskite. Meaningful isotopic ratios for kimberlite can therefore only be obtained from single minerals such as pervoskite. The Mengyin samples investigated in this study have isotopic compositions similar to the Group I kimberlite, as defined in Southern Africa, and are interpreted to reflect derivation from a primitive mantle source. Combined with the fact that the Mengyin kimberlites were emplaced coevally with regional lithospheric uplift in the Ordovician at ∼470 Ma, it is proposed that a mantle plume triggered kimberlite magmatism in this part of the North China Craton.