Morphology and stratigraphy of Serra Geral silicic lava flows in the northern segment of the Torres Trough, Paraná Igneous Province

ABSTRACT: The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Paraná-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The São Joaquim silicic eruptive sequence is comprised of Palmas-Type aphyric dacites and is located in the northern segment of Torres Trough, a dissected mountainous region in southern Brazil. These dacite outcrops form plateaus and remnant hills that are scattered above a basaltic andesitic landscape. The original morphology of the silicic volcanic flows was constrained based on the internal architecture, geometry, and cyclicity of the lithofacies, in conjunction with petrographic and geochemical information. The data suggests the presence of ancient large tabular and lobate silicic lava flows (~100 m thick and ~10-40 km extents). No pyroclastic features were found. Eight interdigitating units were mapped. These large aspect ratios are similar to ones for basaltic flows and those of Snake River-type rhyolites, and they suggest high effusion rates and high temperatures as well as the presence of well-insulated cooled crusts. Lavas would have overflowed from long fissure vents that had created an extensive volcanic silicic surface body composed of conjugated and interdigitated flows above the north segment of the Torres Trough region.

摘要：本研究的缘起在于帕拉奈-埃滕德卡火成岩省（Paraná-Etendeka Igneous Province）内广泛分布的硅质火山岩单元的成因之谜：这些单元究竟是流变熔结凝灰岩（rheoignimbrites）、熔岩流，还是二者的混合体？圣若阿金硅质喷发序列由帕尔马斯型无斑英安岩（Palmas-Type aphyric dacites）构成，分布于巴西南部侵蚀山地区域——托雷斯海槽（Torres Trough）的北段。这些英安岩露头形成了散布于玄武安山岩地貌之上的高原与残丘。研究基于岩相的内部结构、几何形态与旋回性，并结合岩石学与地球化学数据，限定了硅质火山岩流的原始形貌。数据表明，存在过古代大型板状及叶状硅质熔岩流（厚度约100米，延伸范围约10至40千米），未发现火山碎屑特征。本次研究共圈定8个相互穿插的单元。这类熔岩流的高长宽比与玄武岩流及斯内克河型流纹岩（Snake River-type rhyolites）的特征相似，这表明其具有极高的喷发速率与温度，同时存在保温良好的冷却壳层。熔岩应是从长裂隙喷口溢出，在托雷斯海槽北段上方形成了由共轭且相互穿插的熔岩流构成的大面积硅质火山地表体。