Meaning of Rocks Basic Enclaves in the Eburnean Granitoids of Mako Area (Kédougou-Kéniéba Inlier, Senegal): Petrogenetic Implications

The Mako area, located in eastern Sénégal, constitutes a segment of Paleoproterozoic (Birimian) formations of the Kédougou-Kéniéba Inliers (KKI) in the western part of the West African craton. The basic volcanism of the Birimian formations of the KKI has long been considered to be related to a single magmatic event associated with a Mid Oceanic Ridge Basalts (MORB) setting. The aim of this paper is to demonstrate on the basis of the architectural characteristics of the granitoids dark enclaves, the occurrence of at least two distinct phases of basic Magamtism in the Birimian of the KKI. The methodology consists of a cartography, a petrographic and architectural characterization of the dark enclaves within the Mako granitoids, in order to constrain their spatial and temporal relationships with the granitic magma. The results obtained are compared with those of the bibliography. We have pinpointed two types of enclaves depending on their shapes and the characteristics of their edge with the enclosing rocks: angular enclaves with straight edges (ante-granitoids) and the soft enclaves with uneven edges (syn-granitoids). The enclaves with straight and well-defined rectilinear edges (ante-granitoids) would be linked to a basic magma which is already consolidated before being fragmented, torn and carried away as enclaves in the granitoids. These enclaves would come from enclosing outcrops of metabasalts and metagabbros which are locally cross-cut by the granitoids. The soft enclaves with uneven edges (syn-granitoids) were co-magmatic and not solid during their incorporation into the granitoids. They come from a basic magma which is contemporaneous to the granitic one. Thus, the occurrence of two generations of dark enclaves is related to, at last, two different phases of basic magmatism in the Birimian formations of the KKI.

Tele-Analytical Characterization of the Eburnean Tectonic Structures within the Birimian Formations of Mako Area, Eastern Senegal

The tectonic structures of the Mako area in the Kédougou-Kéniéba inlier (KKI) were mapped with tele-analytical investigation which is validated by field data. This study is based on different images processing in particular: 1) the colored combinations (1 to 5) and panchromatic (8) thematic mapping bands of the Landsat-8 (ETM+) satellite;2) Digital Elevation Model (DEM) image of Space Shuttle Radar Topography Mission (SRTM);3) airborne geophysics (aeromagnetic and radiometric) images. In these images, four major directions of lineaments NS, NNE-SSW, NW-SE and EW would be related to the tectonic structures, have been identified and mapped. Field data confirm these four structural directions and show that most of these lineaments are related to faults, shear and/or thrust zones, or even basic rock dykes. N-S to NE-SW oriented lineaments are more frequent followed by those NW-SE oriented. These two directions of lineaments are generally in correlation with trajectories of major sinistral shear zones in the Mako area. They would be associated with the S2a and S2b schistosities relative to the D2 Eburnean major deformation. They often intersect the E-W oriented lineaments which are less frequent and sometimes folded and crenulated. This E-W oriented lineament would be prior and are associated with the S1 schistosity of the D1 Eburnean thrust phase of deformation. At the scale of the studied area, these major linear structures (shear zones) are conjugated and create a partitioning of deformation through an anastomosed network of mylonitic shear zones which surround weakly deformed rock bodies. All these structures would be related to the Eburnean or post-Eburnean orogenic events in this Mako area.