Abstract The ca. 3.07 Ga volcanic rocks of the Dominion Group, South Africa, represent the oldest example of intracontinental, rift-related volcanism on the Archaean Kaapvaal craton. The volcanic assemblage comprises a >2 km-thick succession of mafic-intermediate lavas interlayered with felsic lavas and pyroclastic rocks. Textural and geological features indicate emplacement in a subaerial environment probably in an incipient intracontinental rift. We report SHRIMP U Pb zircon ages, elemental and Nd-isotope bulk-rock analyses of drill core samples and interpret their petrogenesis in the context of a Mesoarchaean continental setting. The U Pb zircon ages of four felsic samples from different stratigraphic levels yielded the same dates, resulting in a pooled 207Pb/206Pb age of 3074 ± 5 Ma. Primitive mantle-normalised incompatible trace element concentrations show enriched patterns with fractionated rare earth elements over high field-strength elements and negative anomalies of Nb and Ta relative to La. Initial eNd values for mafic and felsic rocks from −1.0 to −0.2 indicate melting of sources comprising time-integrated incompatible element-enriched mantle. The combined trace element and Sm Nd isotopic data suggest that the enrichment of incompatible elements and the low eNd values in the most primitive basalt samples (Mg# of 65–67) can be explained with contamination of asthenosphere-derived melts with crustal material or melting of an incompatible element-enriched upper mantle. The chemical compositions of the Dominion Group and Pongola Supergroup represent a significant petrogenetic departure from earlier Archaean (>3.6–3.1 Ga) magmatism as recorded in the Kaapvaal craton, which was dominated by komatiite-basalt volcanism and tonalite-trondhjemite-granodiorite intrusions. This change reflects the transition from a “greenstone belt type” tectonic setting to a failed intracontinental rift setting shortly after stabilisation of the Kaapvaal craton.

中文翻译：

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中太古代统治群和 Kaapvaal 克拉通大陆内火山活动的开始——地质、地球化学和时间限制

摘要约。南非 Dominion Group 的 3.07 Ga 火山岩代表了太古代 Kaapvaal 克拉通上与裂谷相关的大陆内火山作用的最古老例子。火山组合包括连续 2 公里厚的基性中质熔岩，中间夹有长英质熔岩和火山碎屑岩。构造和地质特征表明可能在早期大陆裂谷中的地下环境中就位。我们报告了钻芯样品的 SHRIMP U Pb 锆石年龄、元素和 Nd 同位素散装岩石分析，并在中古古代大陆背景下解释了它们的岩石成因。来自不同地层水平的四个长英质样品的 U Pb 锆石年龄产生相同的日期，导致汇总的 207Pb/206Pb 年龄为 3074 ± 5 Ma。原始地幔归一化的不相容微量元素浓度显示富集模式，其中分馏稀土元素高于高场强元素，Nb 和 Ta 相对于 La 呈负异常。基性和长英质岩石的初始 eNd 值从 -1.0 到 -0.2 表明熔融来源包括时间积分不相容的富含元素的地幔。结合微量元素和 Sm Nd 同位素数据表明，在最原始的玄武岩样品（Mg# 为 65-67）中不相容元素的富集和低 eNd 值可以用软流圈衍生的熔体与地壳物质的污染或熔化来解释不相容的富含元素的上地幔。Dominion Group 和 Pongola Supergroup 的化学成分代表了与早期太古代 (>3.6-3. 1 Ga) 记录在 Kaapvaal 克拉通的岩浆活动，主要是科马提岩-玄武岩火山作用和方长岩-长闪长岩-花岗闪长岩侵入体。这种变化反映了 Kaapvaal 克拉通稳定后不久从“绿岩带型”构造环境向失败的陆内裂谷环境的转变。