Petrographic and Geochemical Characterization of the Fianga Granitoids (Mayo Kebbi) in the Pan-African Range of Central Africa in Chad: Geodynamic Implication
- Introduction
The Pan-African Range of Central Africa (CPAC), identified in the early 1960s, is a vast geological complex located north of the Congo Craton. It extends from the Central African Republic (CAR) to eastern Nigeria via Chad and Cameroon, and continues across the Atlantic in the Borborema Province in northeastern Brazil. This chain corresponds to an orogen that formed between 700 and 500 Ma. In Chad, the identified Precambrian formations are distributed between the Tibesti massifs in the north, Ouaddaï in the east, Yadé or Baïbokoum in the south, Mayo Kebbi in the southwest and Guéra in the center. These formations are part of the Pan-African mobile zone.
Granitoids, being a major component of the continental crust, appear to be extremely common in collisional belts, particularly in shear zones. They are intrinsically associated with the dynamics of mountain ranges and record deformation structures during their emplacement. It is through shear movements in the lithosphere that magmas in general are injected to lead to the complete formation of plutons.
The Fianga region, located on the peneplained surface of Paleo-Chadian formations, where the granitoids of the Fianga massif outcrop, is a geologically poorly known area that has been little studied in detail. This area has not been the subject of a detailed geological study. Many gaps remain in the petrography and geochemistry of the massif. Consequently, major questions remain about its petrogenesis and the geodynamic context of its formation.
- Geological Context
In Africa, there are several Pan-African belts, including those of Central Africa, or the Central African “mobile zone”. It corresponds to an orogenic megabelt elongated in an E-W direction, over 5000 km long and 300 km wide, extending across Nigeria, Cameroon, Chad, and the Central African Republic. The Precambrian formations identified in Chad cover between 15% and 20% of the territory’s surface.
- Materials and Methods
Data acquisition in the study area focused primarily on a petrographic study. The fieldwork phase allowed for the macroscopic identification of four (4) rock facies (pink granites, pink microgranites, syenites, and granodiorites) and the collection of key lithological parameters, namely color, texture, mineralogical composition, degree of alteration, and deformation. Samples were selected based on their location and lithological relevance.
- Petrographic Study
4.1. Host Rock
The metamorphic formations consisting of massive amphibolites and quartz chlorite schists constitute the host rock for the formations in the study area. These rocks are collectively known as greenstones.
4.2. Plutonic Formations
The plutonic formations that constitute the rocks of the study area are granitoids. Pink granites, syenites, and granodiorites outcrop as intrusive plutons in the metamorphic rocks.
- Classification
5.1. Elemental Distributions
The geochemical analysis of the major elements in the rocks studied presents the following four groups of rocks.
- Discussion
6.1. Petrogenetic Process
The enrichment in Rb, Ba, and K, and the high K/Nb and La/Nb ratios are consistent with crustal-derived enrichment fluids.
6.2. Crystallization
Harker diagrams and rare earth element distribution patterns suggest that fractional crystallization played an important role in the differentiation of the pluton rocks.
6.3. Geochemical Affinity
The Fianga pink granitoids are predominantly alkaline, sodic rocks with a K2O/Na2O ratio ≤ 1.
- Conclusion
The Fianga area consists of plutonic formations intruding into metamorphic (amphibolite) formations.
