The deformed alkaline Balda granite (Northern Cameroon): A witness of back-arc basin in the northern part of Central African Orogenic Belt

IF 3.2 2区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Precambrian Research Pub Date : 2024-06-28 DOI:10.1016/j.precamres.2024.107490

Aboubakar Bello , Daouda Dawaï , Paul Yves Jean Antonio , Oscar Laurent , Carmen Irene Martinez Dopico , Rigobert Tchameni , Olivier Vanderhaeghe

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Abstract

Located on the border between the NW Cameroon and Mayo-Kebbi domains in northern Cameroon, the Balda pluton is a deformed alkaline granite. Due to its unique location and geological features, it holds crucial information for understanding the evolution of the Central African Orogenic Belt (CAOB), which remains poorly understood. This multidisciplinary study combines field data, microscopic observations, Anisotropy of Magnetic Susceptibility (AMS), and U-Pb zircon dating to constrain the geodynamic evolution of the CAOB.

The Balda pluton is a NNE-SSW elongated pluton consists mainly of aegirine-riebeckite granite and leucogranite. It displays diverse magnetic susceptibility values (Km) spanning from 0.19 x 10^-3 to 21.98 x 10^-3. Notably, 93 % of the Km values exceed 5 × 10^-4SI, indicating a prevalence of mixed paramagnetic and ferromagnetic mineralogy (such as magnetite-rich). This observation is supported by microscopic examinations and K-T curves. An analysis of AMS unveils high P% values (2.8 to 58.7 %, with a mean of 15 %) and prevalent oblate magnetic fabrics (68 %). The magnetic foliation generally aligns with the field foliation and has steep dips ranging from NNE to NE or from SSW to SW and magnetic lineations exhibit shallow plunges (<35°) from SSW to SW. Microscopic analysis reveals high-temperature, solid-state deformation microstructures, indicating post-emplacement deformation processes and the presence of kinematic markers consistent with sinistral shear. Zircon grain analysis reveals two distinct types: “bright” and “dark.” These types differ in both their appearance under cathodoluminescence and their chemical composition. Bright zircons yield a U-Pb age of 732.7 ± 7.5 Ma, interpreted as the pluton emplacement age. Dark zircons are younger (ca. 680 Ma), suggesting a later tectono-metamorphic or alteration event.

The integration of these results in a larger geodynamic context suggests that the Balda pluton formed within a syn-orogenic extensional back-arc basin. This emplacement was followed by significant post-emplacement deformation characterized by sinistral simple shear-dominated transpression likely related to the continental collision (ca. 680 Ma) of the NW Cameroon and Mayo-Kébbi domains.

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变形的碱性巴尔达花岗岩（喀麦隆北部）：中非造山带北部弧后盆地的见证者

巴尔达柱岩位于喀麦隆北部西北部和马约-克比地区的交界处，是一种变形的碱性花岗岩。由于其独特的地理位置和地质特征，它为了解中非造山带（CAOB）的演化过程提供了重要信息。这项多学科研究结合了野外数据、微观观察、磁感应强度各向异性（AMS）和 U-Pb 锆石年代测定法，对中非造山带的地球动力学演化进行了约束。巴尔达岩体是一个 NNE-SSW 长条形岩体，主要由egirine-ribeckite 花岗岩和白榴石组成。它的磁感应强度（Km）从 0.19 x 10-3 到 21.98 x 10-3 不等。值得注意的是，93%的 Km 值超过 5 × 10-4SI，表明顺磁性和铁磁性混合矿物（如富磁铁矿）的普遍存在。这一观察结果得到了显微镜检查和 K-T 曲线的支持。AMS 分析揭示了高 P% 值（2.8% 至 58.7%，平均值为 15%）和普遍的扁圆形磁性结构（68%）。磁性剖面一般与磁场剖面一致，具有从东北到东北或从西南到西南的陡峭倾角，磁力线从西南到西南呈浅倾角（<35°）。显微分析显示了高温固态变形微结构，表明了置换后的变形过程，并存在与正弦剪切一致的运动标记。锆石晶粒分析显示出两种截然不同的类型：明 "和 "暗"。这两类锆石在阴极荧光下的外观和化学成分都有所不同。亮锆石的 U-Pb 年龄为 732.7 ± 7.5 Ma，可解释为深成岩的形成年龄。将这些结果与更大的地球动力学背景相结合，表明巴尔达柱状构造形成于同步成因的伸展后弧盆地中。这些结果表明，Balda岩体形成于同步成因的后弧延伸盆地中，形成后发生了显著的变形，其特征是以正弦简单剪切为主的转位，这可能与喀麦隆西北部和马约-凯比域的大陆碰撞（约680Ma）有关。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Precambrian Research 地学-地球科学综合

CiteScore

7.20

自引率

28.90%

发文量

325

审稿时长

12 months

期刊介绍： Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.