Petrographic and structural analysis of the Precambrian rocks in the Zalbi Sector, northwest Léré, Chad

Abstract

The northwestern part of southwest Chad is one of the best regions to find exposed Precambrian terrains and their Phanerozoic cover. In this area, it is easy to establish the lithostratigraphic and structural relationships between sedimentary and granitic rocks, greenstone belts and dolerite dykes. Based on petrographic and structural studies, we distinguished the greenstone belts that belong to the Neoproterozoic Zalbi Series. The Neoproterozoic greenstone belts that are intruded by three generations of Pan-African granitoids (gabbro-diorite, biotite monzogranite and leucogranite) host the mineral resources in the region. The Precambrian rocks are covered by Cretaceous shale, sandstone and microconglomerate in the Kebbi Basin, and intruded by dolerite dykes. These late dolerite dykes, have a continental tholeiitic composition, which is supported by the presence of orthopyroxene in the mineralogical assemblage. Structurally, two major deformation events are present in the Zalbi Series. The earliest D₁ event is the most intensive and is associated with vertical to subvertical north-trending S₁ foliation, a vertical L₁ lineation and P₁ folds; the metamorphism associated with this phase is equilibrated in greenschist facies conditions. The second D₂ event is discrete and is characterised by large-amplitude folds and fractures.

乍得西南的西北部是寻找出露的前寒武纪地体及其显生宇覆盖体的最佳地区之一。在该区，易建立沉积岩和花岗岩石、绿石带和辉绿岩岩墙之间的岩石地层关系和构造关系。根据岩石和构造研究，我们分辨出属于新原生代Zalbi系的绿带。新原生代绿石带被三代泛非洲花岗岩类（辉长-闪长岩、黑云母二长花岗岩和淡色花岗岩）所侵入，因此是该地区矿物资源的载体。在Kebbi盆地，前寒武纪岩石之上覆有白垩纪页岩、砂岩和微砾岩，并有辉绿岩岩墙侵入。这些辉绿岩岩墙具有陆地拉斑玄武岩成分，矿物组合中有斜方辉石类的存在是其证明。在构造方面，Zalbi系中有两个主要变形事件。最早的D₁事件最强烈，与垂直和近垂直的北向S₁叶理、垂直的L₁线理和P₁皱褶相关；与这一事件相关的变质作用在绿片岩相环境中呈平衡状态。第二个D₂事件以大幅度皱褶和断裂为特征。

KEY WORDS:

• petrography
• structural
• Neoproterozoic Zalbi Series
• Pan-African plutonic rocks
• Cretaceous sedimentary rocks
• dolerite dykes

关键词:

• 岩石学
• 构造
• 新原生代 Zalbi系
• 泛非洲深成岩
• 白垩纪沉积岩
• 辉绿岩岩墙

ACKNOWLEDGEMENTS

This work was financially supported by the University of N’djamena in Chad and the University of Ngaoundere in Cameroon. We give special thanks to Dr Lanan Wassy Soromou for assistance with this manuscript.