Abstract Rhyolites compose an important record in the volcanic history of Earth, with significant occurrences in volcanic arcs, large igneous provinces and post-collisional terranes, often associated with explosive events. In several geologic provinces, rhyolites dominate as the most expressive geologic units (e.g., Silicic Large Igneous Provinces - SLIPs). Despite their importance, several aspects related to the emplacement of rhyolite flows are still enigmatic. Recent studies in modern rhyolite lavas suggest similar emplacement mechanisms to basaltic lavas, implying a more dynamic growth model for silicic flows, including outbreak lobes and outpour structures. Despite these advances, studies related to the recognition of these features in ancient flows are still rare. In this work we perform a multi-proxy study of an ancient (Neoproterozoic) rhyolitic lava flow combining fieldwork, petrography, geochemistry, rheology and magnetic fabric analysis. The Cerro do Perau outcrop (CP, southern Brazil) consists of a natural laboratory for the study of rhyolite lavas, presenting excellent exposure of a partially preserved flow with distinct flow features and folds. CP flow consists of a high-silica and low-crystal content rhyolite, suggesting its emplacement as an obsidian flow. Rheology data indicates high liquidus temperatures (>957 °C), with maximum viscosities of 108.5 Pa s and glass transition temperatures (Tg) of 750 °C. The absence of brittle features suggests little to none displacement below Tg. Structural analysis indicates the predominance of sub-vertical foliation planes, including axial planes of folds, indicative of proximal (near-vent) regions in rhyolite flows. The absence of lineations favors a predominantly planar accommodation of the flow-induced deformation, which is confirmed by the shape of the magnetic fabric ellipsoids. Several of these ellipsoids display a high degree of anisotropy, mostly related to an oblate fabric, indicative of the development of high-strain zones within the flow. Our data suggest that CP flow presents some similarities with recently proposed field-based emplacement models for rhyolitic flows, highlighting the significant data that can be extracted from a combination of magnetic fabrics and rheological analyses.

中文翻译：

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巴西最南端新元古代流纹岩流的多代理案例研究：对古长英质熔岩的侵位机制和意义

摘要 流纹岩是地球火山历史上的重要记录，在火山弧、大型火成岩区和碰撞后地体中大量出现，通常与爆炸事件有关。在几个地质省，流纹岩作为最具表现力的地质单元占主导地位（例如，硅质大型火成岩省 - SLIPs）。尽管它们很重要，但与流纹岩流就位相关的几个方面仍然是个谜。最近对现代流纹岩熔岩的研究表明，与玄武岩熔岩类似的侵位机制，意味着硅质流的更动态生长模型，包括爆发裂片和流出结构。尽管取得了这些进展，但与识别古代流动中的这些特征相关的研究仍然很少见。在这项工作中，我们对古代（新元古代）流纹质熔岩流进行了多代理研究，结合了实地考察、岩石学、地球化学、流变学和磁性织物分析。Cerro do Perau 露头（CP，巴西南部）由一个天然实验室组成，用于研究流纹岩熔岩，展示了部分保留的流的极好暴露，具有明显的流特征和褶皱。CP 流由高二氧化硅和低晶体含量的流纹岩组成，表明它是黑曜石流。流变学数据表明液相线温度高 (>957 °C)，最大粘度为 108.5 Pa s，玻璃化转变温度 (Tg) 为 750 °C。脆性特征的缺失表明在 Tg 以下几乎没有位移。结构分析表明亚垂直叶理平面占优势，包括褶皱的轴向平面，指示流纹岩流中的近端（近喷口）区域。没有划线有利于流动引起的变形的主要平面适应，这由磁性织物椭圆体的形状证实。这些椭球中的一些表现出高度的各向异性，主要与扁圆结构有关，表明流动中高应变区的发展。我们的数据表明，CP 流与最近提出的基于场的流纹岩流就位模型有一些相似之处，突出了可以从磁性织物和流变分析的组合中提取的重要数据。这是由磁性织物椭球的形状所证实的。这些椭球中的一些表现出高度的各向异性，主要与扁圆结构有关，表明流动中高应变区的发展。我们的数据表明，CP 流与最近提出的基于场的流纹岩流就位模型有一些相似之处，突出了可以从磁性织物和流变分析的组合中提取的重要数据。这是由磁性织物椭球的形状所证实的。这些椭球中的一些表现出高度的各向异性，主要与扁圆结构有关，表明流动中高应变区的发展。我们的数据表明，CP 流与最近提出的基于场的流纹岩流就位模型有一些相似之处，突出了可以从磁性织物和流变分析的组合中提取的重要数据。