Abstract One of the most debated issues about the Carajas Mineral Province in Brazil is the tectonic setting that prevailed at the time of the major Neoarchean (ca. 2.7 Ga) volcanic event, with views ranging from subduction- to rift-related. During this event, thick lava flows of mainly basaltic and basaltic-andesitic composition extruded at the Earth's surface, cooled and underwent significant alteration by hydrothermal fluids rising through deep-seated crustal faults. Archean settings have been inferred for hydrothermally altered volcanic rocks using as a proxy their whole-rock compositions in discrimination diagrams for modern geotectonics. Despite its debatable application to Archean volcanic sites, the same working approach is used in reconsidering the geodynamic view about the Carajas volcanic event. We use an artificial neural networks-based method to perform a full mass balance calculation on a large compiled database of metavolcanic rocks from Carajas. Predicted precursors reproduce a linear and limited FeO*/MgO increase with SiO2 typical of modern calc-alkaline arc environment. Moreover, the identified least-altered rocks show that they may petrogenetically involve subducted water-bearing sediments. But contrary to what is expected for such a volcanic arc environment, amphibole and magnetite fractionations have not been observed in the precursors. Discrimination diagrams using incompatible trace elements were unable to rule out the subduction hypothesis so other geological and geochemical evidence from the granitoid record of Carajas was taken into consideration. In combination with the Archean granitoid evolution of CMP, we conclude that crustal contamination associated with continental rifting appears to be the most likely process for inducing the calc-alkaline affinity of the Carajas Neoarchean metavolcanic rocks. This calc-alkaline affinity in least-altered volcanic rocks appears to be best reproduced by petrogenetic modelling involving wall-rock assimilation and melting of the Carajas basement (TTG and other K-rich granitoids) by parental magmas derived from depleted mantle source, prior to their fractional crystallization. This volcanic event was coeval with the intrusion of A-type syntectonic granitoids at the transition from a stagnant lid to mobile plate tectonic regime associated with localized translithospheric transpressional rift.

中文翻译：

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恢复 Carajás 矿产省（巴西）热液蚀变太古代变质火山岩的原始成分：从盖子到移动构造过渡的地球动力学意义

摘要 关于巴西卡拉哈斯矿产省最具争议的问题之一是在新太古代（约 2.7 Ga）主要火山事件发生时盛行的构造环境，其观点范围从俯冲到裂谷相关。在这次事件中，主要由玄武岩和玄武岩-安山岩组成的厚熔岩流在地球表面被挤压出来，被通过深部地壳断层上升的热液流体冷却并发生显着变化。已将热液蚀变火山岩的太古代环境用作现代大地构造判别图中的全岩成分的代理。尽管它对太古代火山遗址的应用存在争议，但在重新考虑有关 Carajas 火山事件的地球动力学观点时使用了相同的工作方法。我们使用基于人工神经网络的方法对来自 Carajas 的变质火山岩的大型编译数据库执行完整的质量平衡计算。预测的前体重现了线性和有限的 FeO*/MgO 增加，SiO2 是现代钙碱性电弧环境的典型特征。此外，已确定的变化最小的岩石表明，它们可能在岩石成因上涉及俯冲的含水沉积物。但是与对这种火山弧环境的预期相反，在前体中没有观察到角闪石和磁铁矿的分馏。使用不相容微量元素的判别图无法排除俯冲假说，因此考虑了来自 Carajas 花岗岩记录的其他地质和地球化学证据。结合CMP的太古代花岗岩演化，我们得出结论，与大陆裂谷相关的地壳污染似乎是引起 Carajas 新太古代变质火山岩钙碱性亲和力的最可能过程。这种在变化最小的火山岩中的钙碱性亲和力似乎最好通过岩石成因模型重现，该模型涉及围岩同化和 Carajas 基底（TTG 和其他富含钾的花岗岩）由来自耗尽地幔源的母岩浆在它们的分步结晶。这一火山事件与 A 型同构造花岗岩在从停滞盖过渡到活动板块构造体制的入侵同时发生，与局部的跨岩石圈挤压裂谷相关。