喀麦隆马鲁阿大学Gountié Dedzo Merlin研究组取得一项新突破。他们报道了Baïbokoum正长岩体(乍得南部)的岩相学和地球化学特征，对岩浆的成因进行了解读。相关论文发表在2024年9月9日出版的《地球化学学报》上。

据介绍，Baïbokoum正长岩体(BSP)位于乍得南部Adamawa-yadé域东北部，是乍得中非褶皱带(CAFB)南部为数不多的强钾质岩浆体之一。已有研究对它进行过岩石学研究，但人们对其的岩石成因仍然知之甚少。本文提供的岩石学和全岩地球化学资料，突出了其岩浆成因和地球动力学演化。

结果显示，BSP由中-粗粒正长岩和少量微碎屑组成，以同深成岩脉和基性微粒包体(MME)的形式出现，粗粒正长岩和中粒正长岩分别出露于BSP的岩心和边缘。正长岩具有高钾碱性和碱性与反碱性亲和关系。岩石学和地球化学资料表明，中-粗粒正长岩来自于岩浆储层中的单一岩浆源，由分异结晶作用演化而成。稀土谱显示，轻稀土元素富集(La_N/Yb_N=6.19- 45.55)，而重稀土元素谱几乎平坦(Dy_N/Yb_N=1.0-2.23)， La/Sm和Sm/Yb比值表明上述岩石来源于，石榴子石-尖晶石-二辉橄榄岩地幔源的部分熔融。

负Nb和负Ta异常表明，该地幔源区被与被俯冲相关物质的加入所改变。Th/Yb比值和高Ba/La比值表明，源的富集可能与板源流体有关。BSP的母岩浆是由交代岩石圈地幔的部分熔融，在俯冲环境下变质为弧岩浆活动物质而产生的。它的侵位经历了两个连续的阶段：一个是在深部岩浆源区分离结晶和结晶沉降的静态阶段，另一个是在剪切变形环境中分层岩浆向上火山口上升的动态阶段，演化的正长岩岩浆先侵位，闪长岩后侵位。BSP的侵位可能受泛非造山运动时期，Tcholliré-Banyo断裂和M'Béré剪切带的演化控制。

附：英文原文

Title: Petrography and geochemical characterization of the Baïbokoum syenitic pluton (Southern Chad): Implication for the magma genesis

Author: Baïssemia Ronang, Gustave, Gountié Dedzo, Merlin, Tcheumenak Kouémo, Jules, Klamadji, Moussa Ngarena, Mbaguedjé, Diondoh, Kwékam, Maurice

Issue&Volume: 2024-09-09

Abstract: The Babokoum syenitic pluton (BSP), located in southern Chad, to the NE of the Adamawa-Yadé domain, is one of the few strongly potassic magmatic bodies in the southern part of the Central African Fold Belt (CAFB) in Chad. It has been previously studied petrologically, but its petrogenesis has remained poorly known. Petrographic and whole-rock geochemical data presented in this article highlight their magma genesis and geodynamic evolution. The BSP consists of medium- to coarse-grained syenites associated with minor microdiorites, which occur as syn-plutonic dikes and mafic microgranular enclaves (MME) coarse- and medium-grained syenites outcrop respectively to the core and the border of the BSP. The syenite displays high-K and alkaline to trans-alkaline affinity. Petrographic and geochemical data suggest that medium- to coarse-grained syenites are from single magma source that evolved and differentiated by fractional crystallization in a magma reservoir. REE profiles show enriched LREEs (LaN/YbN=6.19–45.55) while HREEs show an almost flat profile (DyN/YbN=1.0–2.23), and the La/Sm and Sm/Yb ratios have led to propose that the aforementioned rocks derived from the partial melting of a garnet-spinel-lherzolite mantle source. Negative Nb and Ta anomalies indicate that this mantle source was modified by the addition of subduction-related material. Th/Yb ratios associated with high Ba/La ratios indicate that enrichment of the source could be related to slab-derived fluids. The parental magma of the BSP was generated by partial melting of the metasomatized lithospheric mantle that was modified into arc-magmatism material in a subduction setting. Its emplacement took place in two successive stages: a static stage of fractional crystallization and crystal settling in a deep magma source and a dynamic stage in a shear deformation setting during which stratified magma rises towards the upper crust, with evolved syenite magma being emplaced first and diorite later. The emplacement of the BSP was probably controlled by the evolution of the Tcholliré-Banyo Fault and M'Béré Shear Zone during the Pan-African orogeny.

DOI: 10.1007/s11631-024-00733-y

Source: https://link.springer.com/article/10.1007/s11631-024-00733-y