ABSTRACT

The Kelle Bidjoka Iron Formations (IFs) occur at the northmost margin of the Neoarchean to Paleoproterozoic Nyong group greenstone belts in SW Cameroon. This contribution presents for the first time the petrographical, geochemical, and geochronological data of the Kelle Bidjoka IFs and associated metaigneous (metatonalite) and metasedimentary (pyroxene gneiss) rocks. The studied IFs form a thin unit of 3.21 m thick and comprise magnetite pyroxene gneiss, garnet-bearing magnetite pyroxene gneiss, pyroxene BIFs, and grunerite BIFs. Petrographic studies have revealed mineral assemblages indicating retrograde amphibolite facies metamorphism. Magnetite pyroxene gneiss and pyroxene BIFs show signatures of both seawater and hydrothermal fluids, with magnetite pyroxene gneiss deposited closer to the vent source. In contrast, garnet-bearing magnetite pyroxene gneiss and grunerite BIFs are devoid of hydrothermal characteristics and show significant crustal contamination. The lack of Ce anomaly in most samples indicates anoxic to suboxic conditions during their deposition. The geochemical characteristics of the associated metaigneous and metasedimentary rocks indicate that Kelle Bidjoka IFs were deposited in a submarine volcanic arc environment, similar to the Algoma-type BIFs. Detrital zircon LA-ICP-MS U-Pb dating suggests that the Kelle Bidjoka BIFs were deposited during early Paleoproterozoic (ca. 2423 Ma) and were metamorphosed up to granulite facies during the Eburnean/Transamozian (ca. 2050 Ma) orogeny.

摘要

Kelle Bidjoka 铁组 (IF) 位于喀麦隆西南新太古代至古元古代 Nyong 群绿岩带的最北边缘。这一贡献首次提供了 Kelle Bidjoka IF 和相关变火成岩（变质岩）和变沉积岩（辉石片麻岩）的岩石学、地球化学和地质年代学数据。所研究的 IF 形成了一个 3.21 m 厚的薄单元，包括磁铁矿辉石片麻岩、含石榴石的磁铁矿辉石片麻岩、辉石 BIF 和花岗岩 BIF。岩石学研究揭示了表明逆行角闪岩相变质作用的矿物组合。磁铁矿辉石片麻岩和辉石 BIF 显示出海水和热液流体的特征，磁铁矿辉石片麻岩沉积在更靠近喷口源的地方。相比之下，含石榴石的磁铁矿辉石片麻岩和花岗岩 BIF 缺乏热液特征，并显示出明显的地壳污染。大多数样品中缺乏 Ce 异常表明它们在沉积过程中处于缺氧到低氧状态。相关变火成岩和变沉积岩的地球化学特征表明，Kelle Bidjoka IFs 沉积在海底火山弧环境中，类似于 Algoma 型 BIF。碎屑锆石 LA-ICP-MS U-Pb 测年表明 Kelle Bidjoka BIF 沉积于古元古代早期（约 2423 Ma），并在 Eburnean/Transamozian（约 2050 Ma）造山运动期间变质为麻粒岩相。大多数样品中缺乏 Ce 异常表明它们在沉积过程中处于缺氧到低氧状态。相关变火成岩和变沉积岩的地球化学特征表明，Kelle Bidjoka IFs 沉积在海底火山弧环境中，类似于 Algoma 型 BIF。碎屑锆石 LA-ICP-MS U-Pb 测年表明 Kelle Bidjoka BIF 沉积于古元古代早期（约 2423 Ma），并在 Eburnean/Transamozian（约 2050 Ma）造山运动期间变质为麻粒岩相。大多数样品中缺乏 Ce 异常表明它们在沉积过程中处于缺氧到低氧状态。相关变火成岩和变沉积岩的地球化学特征表明，Kelle Bidjoka IFs 沉积在海底火山弧环境中，类似于 Algoma 型 BIF。碎屑锆石 LA-ICP-MS U-Pb 测年表明 Kelle Bidjoka BIF 沉积于古元古代早期（约 2423 Ma），并在 Eburnean/Transamozian（约 2050 Ma）造山运动期间变质为麻粒岩相。