Devonian alkaline magmatism in South Qinling, China: evidence from the Taohekou Formation, Northern Daba Mountain

The Langao area in South Qinling consists of a poorly dated sequence of volcano-sedimentary units (Taohekou Formation). This study focuses on volcanic edifice of Taohekou Formation. Detailed field mapping, section facies analysis, geochemistry, and Ar–Ar geochronology is presented here to develop a more accurate understanding of the tectonic evolution of the Qinling Orogen. Detailed field mapping and section facies analysis reveals the presence of a seamount volcanic edifice in western Xiaozhen area. Ar–Ar dating of alkaline basalts in the sequence indicates an Early Devonian magmatic age. Whole rock geochemistry of these alkaline basalts indicates that both potassic and sodic basalts were erupted respectively in the eastern and western areas. Both types of samples are typically characterized by high concentrations of TiO₂, Nb, Ta, and Zr. Ocean-island basalt (OIB)-like rare earth element distribution patterns, trace element spider diagrams, and similar Sr and Nd isotopic compositions (i.e. relatively low Sr and moderate Nd isotopic ratios) likely indicate that the samples were comagmatic but have experienced varying degrees of fractional crystallization. In addition, the samples appear to have all originated from the asthenospheric mantle and undergone mantle metasomatism, ultimately resulting in an OIB-type provenance. Most importantly, in combination with its regional tectonic evolution, the Taohekou Formation is constrained to a seamount environment. This formation does not represent a plume but, rather, an extensional fracture zone related to subduction. Thus, this geologic interpretation generates pivotal evidence for accurately investigating the properties and temporal constraints of the Mianlue oceanic basin.

南秦岭岚皋地区发育一套定年程度较低的火山-沉积岩系（陶河口组）。本研究聚焦于陶河口组的火山机构。本文通过详细的野外填图、剖面相分析、地球化学测试以及Ar-Ar年代学研究，以期更准确地认识秦岭造山带的构造演化历程。详细野外填图与剖面相分析结果显示，小镇西部地区发育海山火山机构。对该岩系中碱性玄武岩的Ar-Ar定年结果表明其形成时代为早泥盆世岩浆活动期。这些碱性玄武岩的全岩地球化学数据显示，钾质玄武岩与钠质玄武岩分别产出在研究区的东部与西部。两类样品均具有高TiO₂、Nb、Ta及Zr含量的典型特征。类洋岛玄武岩（OIB）型稀土元素配分模式、微量元素蛛网图特征，以及相似的Sr、Nd同位素组成（即较低的Sr同位素比值与适中的Nd同位素比值），均指示这些样品具有同源岩浆成因，但经历了不同程度的分离结晶作用。此外，所有样品均起源于软流圈地幔，并经历了地幔交代作用，最终形成OIB型物源。最为重要的是，结合区域构造演化背景，陶河口组的形成环境被限定为海山环境。该组并非形成于地幔柱环境，而是形成于与俯冲作用相关的伸展断裂带。因此，本次地质研究成果为准确探讨勉略洋盆的性质与时代约束提供了关键证据。