Mineralogical evolution of the anorogenic granites of Tirmini (Damagaram Southeast, Niger)

The present study focuses on the anorogenic granites of Tirmini. The latter are part of the younger granites of the pan-African province of Damagaram-Mounio, which constitutes a connection between the younger granites of the Aïr in the north and those of the Jos Plateau in the south. The objective of this study is to determine the mineralogical characteristics of the anorogenic granites of Tirmini. The methodological approach used includes X-ray diffraction (XRD) analysis on the one hand, and scanning electron microscope (SEM) and cathodoluminescence munimicroscope observations on the other. Two magmatic lineages form the Tirmini granite complex: one alkaline and the other peralkaline. Qualitative analysis at XRD shows that alkaline rocks include orthose, albite, quartz with little or no arfvedsonite. In addition to these minerals, peralkaline rocks contain aegyrin and arfvedsonite. SEM observation of all the Tirmini rocks shows that ilmenite (FeTiO3) is xenomorphic and luminescent in the yellow-orange range. Fluorapatite Ca5(PO4)F, which is stocky or elongated in shape, is present only in granites. Zircon (ZrSiO4) occurs in an elongated prism or quadratic section. Thus, at cathodoluminescence, monazite appears in luminescent zoned crystals in dark gray. Titanite is luminescent and appears in yellow-oranges. Semi-quantitative analysis of alkaline rocks shows the percentage of orthotics 50-65%, albite 6-8%, arfvedsonite quasi-absent 6%. Quartz is present between 30-34% in all rocks compared to 11% in the quartz alkaline syenite of Tirmini. In the peralkaline line, arfvedsonite and aegyrin account for 13% and 7%, respectively. Orthoclase 40-46%, albite 4% and biotite 2%. Quartz is present at 30%. The high orthotic contents show that the Tirmini rocks are persolvated and that the exsolution dome in the albite-orthotic system was well separated from the solidus.