Abstract Archean greenstone belts commonly consist of a stratigraphy that records subaqueous deposition capped by subaerial sedimentation, yet it remains unclear if the controls on this change were driven by horizontal or vertical processes. Clasts in conglomerates can provide important constraints on this question because they inform on the changing nature of sources. The 2.7 Ga Agnew Greenstone Belt, Yilgarn Craton, Australia, provides the ideal opportunity for such a study because it contains abundant and diverse conglomerate facies and its basal stratigraphy is well understood. This study focusses on the texture and composition of lithic clasts derived from three stratigraphic conglomerate units and their host successions. The lowermost conglomerate-bearing unit, hosted in the Vivien Formation, unconformably overlies the mafic-ultramafic lavas and intrusions of the Kalgoorlie Group (ca. 2720–2690 Ma) and is dominated by aphyric to porphyritic mafic and felsic volcanic clasts. The conformably overlying Maria Mine Formation conglomerates contain a higher proportion of felsic clasts with medium- and plutonic groundmass textures. The Vivien and Maria Mine Formations (ca. 2690–2670 Ma) are dominated by cherts, turbidites and conglomerates, and represent subaqueous sediments formed in the ring-plain surrounding an emergent volcanic island. The unconformably overlying Scotty Creek Formation (ca. 2665–2655 Ma) contains conglomerates with many plutonic felsic clasts, and is dominated by bedded and cross-bedded sandstones formed in a subaerial environment. Whole-rock geochemistry of mafic and ultramafic clasts matches the local Kalgoorlie Group rocks and indicates progressive erosion of the underlying supracrustal sequence. Undeformed conglomerates lack clasts with metamorphic foliations, indicating uplift by horizontal compression could not have been significant before ca. 2655 Ma, which is considered as the time of the first major compressional event in many published structural frameworks for the region. The systematic erosion of a felsic stratovolcano, its associated plumbing system, and any resultant upwarping of underlying sequences through vertically-driven diapirism, better explains the progressive changes observed in clast texture and composition, as opposed to horizontally-driven uplift, such as compression associated with subduction.

中文翻译：

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太古代砾岩中的碎屑及其对隆升的影响：来自 2.7 Ga Agnew 绿岩带（西澳大利亚）的证据

摘要 太古代绿岩带通常由记录水下沉积的地层组成，该地层记录了地下沉积作用，但目前尚不清楚对这种变化的控制是由水平过程还是垂直过程驱动的。砾岩中的碎屑可以为这个问题提供重要的限制，因为它们可以说明来源的变化性质。澳大利亚 Yilgarn Craton 的 2.7 Ga Agnew 绿岩带为此类研究提供了理想的机会，因为它包含丰富多样的砾岩相，并且其基底地层已被很好地了解。本研究侧重于来自三个地层砾岩单元及其寄主序列的岩屑的结构和组成。位于 Vivien 组的最低含砾岩单元，不整合地覆盖在 Kalgoorlie Group（约 2720-2690 Ma）的镁铁质-超镁铁质熔岩和侵入体上，并且主要由非晶质至斑状的镁铁质和长英质火山碎屑组成。一致上覆的 Maria Mine Formation 砾岩包含较高比例的长英质碎屑，具有中等和深成岩质质地。Vivien 和 Maria 矿组（约 2690-2670 Ma）以燧石、浊积岩和砾岩为主，代表了在火山岛周围环形平原中形成的水​​下沉积物。上覆不整合的 Scotty Creek 组（约 2665-2655 Ma）包含具有许多深成长英质碎屑的砾岩，主要是在地下环境中形成的层状和交错层状砂岩。镁铁质和超镁铁质碎屑的全岩地球化学与当地的卡尔古利群岩石相匹配，表明下伏上地壳层序逐渐侵蚀。未变形的砾岩缺乏具有变质叶理的碎屑，这表明在约 2655 Ma，这被认为是该地区许多已发表的结构框架中第一次主要压缩事件的时间。长英质层状火山的系统侵蚀、其相关的管道系统，以及通过垂直驱动的底辟作用导致的下伏序列的任何由此产生的向上翘曲，更好地解释了在碎屑纹理和成分中观察到的渐进变化，而不是水平驱动的隆起，例如相关的压缩与俯冲。