A zircon trace element and Hf isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline carbonatite complex, Namibia

IF 1.2 4区 地球科学 Q2 GEOLOGY
M. Tshiningayamwe, R. Bolhar, P. Nex
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引用次数: 0

Abstract

The Epembe Complex is one of the Mesoproterozoic (~1200 Ma) carbonatite alkaline complexes situated along the southern margin of the Congo Craton in northwestern Namibia. Nepheline syenites and minor syenites constitute the main lithologies, cross-cut by a calcite-carbonatite dyke. In order to constrain zircon forming-processes and magma sources, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Syenite zircons are generally unaltered and are characterised by positively sloping REE patterns in a chondrite-normalised diagram, with positive Ce anomalies. Syenite zircon further displays significant negative Eu anomalies attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognised. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon underwent low temperature fluid alteration and displays a cloudy appearance. Type 2 zircon is characterised by enrichment in LREE, Nb and Ti when compared to magmatic type 1 zircon. Carbonatite zircon displays a variety of textures and variable chemical compositions suggestive of the presence of both xenocrystal, altered and magmatic zircon. The Hf concentration and Hf isotope composition of type 1 and type 2 zircon are similar suggesting that zircon alteration did not affect the Hf isotope systematics. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+1.5 ± 0.2 to +1.9 ± 0.1) is consistent with the melts having been derived from a moderately Depleted Mantle.
正长岩和碳酸岩的锆石微量元素和Hf同位素地球化学研究,以纳米比亚埃彭贝碱性碳酸岩杂岩为例
埃彭贝杂岩是中元古代(~1200 Ma)碳酸盐岩碱性杂岩之一,位于纳米比亚西北部刚果克拉通南缘。霞石正长岩和少量正长岩构成主要岩性,由方解石-碳酸岩脉横切。为了限制锆石的形成过程和岩浆来源,本文介绍了从一个正长岩、五个霞石正长岩样品和一个碳酸岩样品中提取的锆石颗粒的阴极发光(CL)成像与微量元素(包括REE)以及Hf同位素组成相结合。正长锆石通常没有变化,其特征是球粒陨石归一化图中的REE正倾斜模式,具有正Ce异常。正长岩锆石进一步显示出明显的负Eu异常,这归因于早期斜长石的形成和分馏。这些特征与岩浆环境中锆石的形成一致。在霞石正长岩样品中,识别出两种锆石类型。1型锆石是岩浆岩,CL中具有均匀的灰色、未分区和振荡分区域,而2型锆石经历了低温流体蚀变,并显示出多云的外观。与岩浆型1锆石相比,2型锆石的特征是LREE、Nb和Ti富集。碳酸盐岩锆石显示出多种结构和可变的化学成分,表明存在异晶锆石、蚀变锆石和岩浆锆石。1型和2型锆石的Hf浓度和Hf同位素组成相似,表明锆石蚀变不影响Hf同位素系统学。正长岩(+0.5±0.4至+1.5±0.4)、霞石正长岩和碳酸岩(+1.5±0.2至+1.9±0.1)锆石的ƐHf(t)值的相似性与来自中度贫化地幔的熔体一致。
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
期刊介绍: The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.
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