The ∼132 Ma-old Messum igneous complex, a circular structure of ∼18 km diameter, belongs to the Damaraland intrusive complexes that intruded into the Pan-African Damara basement of Namibia. The plutonic rocks of the core of the complex comprise olivine-gabbro, biotite-gabbro, nepheline-syenite and quartz-syenite as well as minor alkali gabbro and coexist with the Etendeka volcanic sequences of the Goboboseb Mountains, comprising basalts (Tafelkop and Tafelberg types) that are overlain by deposits of several quartz latite eruptions. New major and trace element and Sr, Nd and Pb isotope data are reported for nepheline-syenite and quartz-syenite. Major and trace element data are consistent with the development of nepheline-syenites through fractional crystallization processes. The nepheline-syenite samples have initial 87Sr/86Sr of 0.7046–0.7049 and initial εNd ranging from +0.53 to +1.47, indicating a mantle source with time-integrated lithophile element depletion. Initial Pb isotopes are rather uniform (206Pb/204Pb:17.89–18.14, 207Pb/204Pb: 15.56–15.58, 208Pb/204Pb: 37.78–38.00). Uniform initial Sr and Pb isotope ratios for nepheline-syenites suggest that the syenite magmas evolved through fractional crystallization from an inferred mafic alkaline magma without crustal contamination. However, the range in Nd isotope composition is too large to be related to crystal fractionation alone and imply a heterogeneous source with respect to the Nd isotope composition. The quartz-syenites show a large range in initial εNd (+1.0 to −4.4) and initial 87Sr/86Sr (0.7047–0.7115). Initial Pb isotope ratios are also heterogeneous (206Pb/204Pb: 18.29–18.74, 207Pb/204Pb: 15.60–15.65, 208Pb/204Pb: 37.43–38.49). These data, together with major and trace element constraints and results from AFC modeling substantiate the production of the quartz-syenites by fractional crystallization of clinopyroxene, amphibole and K-feldspar accompanied by assimilation of up to about 30 % pre-Pan African lower crust. Isotope and chemical data do not support derivation from a single batch of magma undergoing contamination, but suggest that a large magma body at depth evolved largely by fractionation with batches of melt extracted from this chamber being variably contaminated at lower crustal levels.

中文翻译：

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关于与裂谷相关的二氧化硅欠饱和和二氧化硅饱和碱性岩的共存 – 重新审视白垩纪 Messum 碱性杂岩体（纳米比亚）


∼132 马 年龄的Messum火成杂岩，直径∼18公里的圆形结构，属于侵入纳米比亚泛非达马拉基底的达马拉兰侵入杂岩体。该复合体核心的深成岩包括橄榄石-辉长岩、黑云母-辉长岩、霞石-正长岩和石英-正长岩以及少量碱辉长岩，并与 Goboboseb 山脉的 Etendeka 火山序列共存，包括玄武岩（Tafelkop 和 Tafelberg 类型），上面覆盖着几个石英板岩喷发的沉积物。报告了霞石-正长岩和石英-正长岩的新主量和痕量元素以及 Sr、Nd 和 Pb 同位素数据。主量和微量元素数据与通过分馏结晶过程形成的霞石-正长岩一致。霞石-正长岩样品的初始 87Sr/86Sr 为 0.7046–0.7049，初始 εNd 范围为 +0.53 至 +1.47，表明地幔源具有时间积分亲岩元素耗尽。初始 Pb 同位素相当均匀（206Pb/204Pb：17.89–18.14,207Pb/204Pb：15.56–15.58,208Pb/204Pb：37.78–38.00）。霞石-正长岩的均匀初始 Sr 和 Pb 同位素比值表明，正长岩岩浆是通过从推断的镁铁质碱性岩浆分馏结晶演变而来的，没有地壳污染。然而，Nd 同位素组成的范围太大，不能单独与晶体分馏有关，并且意味着 Nd 同位素组成的来源不同。石英-正长岩的初始 εNd（+1.0 至 −4.4）和初始 87Sr/86Sr （0.7047–0.7115） 的范围很大。初始 Pb 同位素比率也是异质的（206Pb/204Pb：18.29-18.74,207Pb/204Pb：15.60-15.65,208Pb/204Pb：37.43-38.49）。 这些数据，连同主量和微量元素的约束条件以及 AFC 建模的结果，证实了石英-正长岩的生产是通过斜辉石、闪石和 K-长石的分馏结晶，并伴有高达 30% 的前泛非前下地壳的同化。同位素和化学数据不支持从单批受到污染的岩浆中推导出来，但表明深处的大型岩浆体主要通过分馏演化，从该腔室中提取的一批熔体在较低的地壳水平上受到不同程度的污染。