Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record.

中文翻译：

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锆石中的微量元素记录了岩浆过程的变化和太古宙原大陆地壳的多阶段形成

锆石微量元素地球化学已成为追踪地壳随时间演化的越来越流行的工具。这在大部分地壳已经消失的早期地球环境中尤其重要，但在一些偶然的情况下，源自消失地壳的碎屑锆石被保存在较年轻的沉积物中。为了研究从冥宙到古太古代大陆地壳的形成和地球化学演化，南部非洲的 3.6 至 3.2 Ga 巴伯顿绿岩带是一个极好的目标，因为它保存完好，并且存在跨越近 10 亿年的碎屑锆石。在这里，我们使用穆迪组和无花果树组的 3.65 至 3.22 Ga 碎屑和凝灰质锆石的微量元素以及铪和氧同位素，并将其地球化学与之前研究的来自绿砂岩床的 4.2 至 3.3 Ga 碎屑锆石进行比较。 Onverwacht 集团。前者的主要碎屑锆石年龄簇位于 3.55 Ga、3.46 Ga 和 3.26-3.23 Ga，与 Barberton 地区的 TTG 侵位和长英质火山活动重叠，表明其起源于当地。相比之下，穆迪组和无花果树组的 3.65 Ga 和 3.29 Ga 年龄簇以及来自绿砂岩层的 4.2 至 3.3 Ga 碎屑锆石没有已知的侵入源，很可能来自当今的巴伯顿带之外。这表明碎屑锆石记录中超过一半的长英质火成岩事件不具有可直接研究的全岩表征。无花果树群和穆迪群以及绿砂岩层的锆石中记录的相似成分和推断的地壳演化历史表明，它们源自历时经历类似地壳过程的相连地体。它们共同显示了长英质大陆形成的三个阶段，反映了不同的地壳过程：（1）在冥古宙中由未耗尽的地幔源形成的长寿原壳。这些锆石与较年轻的锆石有很大不同，因此，巴伯顿 TTG 不能很好地模拟冥宙地壳的形成。 (2) 在 3.8 Ga 时，通过周期性的幼体添加和含水熔融，地壳开始显着生长，可能是在火山高原环境中，但根据该数据不能排除弧状环境。 (3) 在 3.4 至 3.3 Ga 之间，长英质地壳是通过镁铁质、地幔来源的浅层熔融产生的地壳生长过程而产生的，这一过程以前未被认识到。紧接着，通过将表面改变的水合物质输送到地壳深处，地壳开始增厚。由于当时存在延伸和缩短的地质证据，这可能反映了水平运动的开始。无论最后的地球动力学背景是否反映了现代板块构造，巴伯顿地区的大陆形成和板块构造的开始是通过跨越近十亿年的复杂的多阶段过程发生的，其中大部分过程只能通过碎屑锆石记录才能获得。