The Palaeoproterozoic Birimian Supergroup of the West African Craton (WAC) consists of volcanic belts composed predominantly of basaltic and andesitic rocks and intervening sedimentary basins composed predominantly of wackes and argillites. Mafic metavolcanic rocks and granitoid-hosted enclaves from the Palaeoproterozoic Lawra Belt of Ghana were analysed for geochemical and Sr–Nd isotopic data to constrain the geological evolution of the southeastern part of the WAC. The metavolcanic rocks display mainly tholeiitic signatures, whereas the enclaves show calc-alkaline signatures. The high SiO2 contents (48.6–68.9 wt%) of the enclaves are suggestive of their evolved character. The high Th/Yb values of the samples relative to that of the mantle array may indicate derivation of their respective magmas from subduction-modified source(s). The rocks show positive εNd values of +0.79 to +2.86 (metavolcanic rocks) and +0.79 to +1.82 (enclaves). These signatures and their Nd model ages (TDM2) of 2.31–2.47 Ga (metavolcanic rocks) and 2.39–2.47 Ga (enclaves) suggest they were probably derived from juvenile mantle-derived protoliths, with possible input of subducted pre-Birimian (Archean?) rocks in their source(s). Their positive Ba–Th and negative Nb–Ta, Zr–Hf and Ti anomalies may indicate their formation through subduction-related magmatism consistent with an arc setting. We propose that the metavolcanic rocks and enclaves from the Lawra Belt formed in a similar island-arc setting. We infer that the granitoids developed through variable degrees of mixing/mingling between basic magma and granitic melt during subduction, when blobs of basic to intermediate parental magma became trapped in the granitic magma to form the enclaves.

中文翻译：

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加纳东南西非克拉通古元古代Birimian地体变质火山岩和镁铁质飞地的成因和构造意义

西非克拉通（WAC）的古元古代Birimian超群由主要由玄武岩和安山岩组成的火山带和主要由碎屑和泥岩组成的沉积盆地组成。分析了加纳古元古代劳拉带的镁铁质变质火山岩和花岗岩包裹体的地球化学和 Sr-Nd 同位素数据，以约束 WAC 东南部的地质演化。变质火山岩主要表现出拉斑沸石特征，而飞地则表现出钙碱性特征。高 SiO2飞地的含量（48.6-68.9 wt％）暗示了它们的进化特征。样品相对于地幔阵列的高 Th/Yb 值可能表明它们各自的岩浆来自俯冲改性源。岩石显示正 ε钕+0.79 至 +2.86（变质火山岩）和 +0.79 至 +1.82（飞地）的值。这些签名及其 Nd 模型年龄 (吨DM2) 的 2.31-2.47 Ga (变质火山岩) 和 2.39-2.47 Ga (飞地) 表明它们可能来自于幼年地幔衍生的原岩，在它们的来源中可能输入了俯冲的前Birimian (太古代？) 岩石。它们的正 Ba-Th 和负 Nb-Ta、Zr-Hf 和 Ti 异常可能表明它们是通过与弧设置一致的俯冲相关岩浆作用形成的。我们提出劳拉带的变质火山岩和飞地形成于类似的岛弧环境中。我们推断花岗岩是通过俯冲过程中碱性岩浆和花岗岩熔体之间不同程度的混合/混合而发育的，当时碱性到中间母岩浆团块被困在花岗岩浆中形成飞地。