Petrographic Characterisation of the Mounio "Younger Granites" Province, Central Portion of the Pan-African Mobile Chain at Gouré, South-East Niger

Abstract

The petrography of the Mounio province, the subject of this study, is poorly understood, as very few geological studies have been undertaken. This province is an integral part of the Pan-African mobile zone located to the east of the West African craton, between the Tuareg and the Benino-Nigerian Shields. In the study area, the Pan-African formations were intruded by anorogenic magmatism, resulting in the emplacement of the Younger granites. Most of the petrographic data in this area of the Niger basement come from geological correlations. The aim of this study is to update the geological map of the Mounio province through a petrographic analysis of the various lithological formations in the study area. The methodology used consisted of a macroscopic description of outcrops in the field. In the laboratory, thin sections of rock were processed and interpreted. The results of this work enabled us to distinguish three types of geological formations. (i) Pan-African formations made up of metasediments (schists and volcanosclastites) and “ancient granites”.  The cardinal minerals in these granites are quartz (30%), potassium feldspar (40%), plagioclase (10%) and biotite (20%). (ii) Younger granites have intruded ancient formations and form three groups: (a) A volcanic group, comprising several occurrences: pyroclastites, alkaline and hyperalkaline rhyolites and trachytes.  Alkaline rhyolites contain green hornblende-type amphibole, which differentiates them from hyperalkaline rhyolites, characterized by the presence of riébeckite-type amphibole and ægyrine-type pyroxene. (b) A hypovolcanic group represented by alkaline and hyperalkaline microgranites. Alkaline microgranites are characterized by the presence of amphibole, represented by hornblende, while hyperalkaline microgranites specifically contain ribeckite-type amphibole, aegyrine-type pyroxene and sanidine phenocrysts. (c) A plutonic group made up of alkaline and hyperalkaline granites and quartzite syenites. The alkaline granite is characterized by clinopyroxene pyroxene and hornblende amphibole, while the hyperalkaline granite is characterized by aegyrine pyroxene. (iii) A sedimentary cover composed of sandstone and alluvium, overlies ancient formations throughout the study area.