Pierre Dogsaye DOURWE^1,, Aminatou Mefire FAGNY², Lise Carole Atouba OKOMO³, Atour Mey MAHAMAT², Jacques-Marie BARDINTZEFF⁴, Oumarou Faarouk NKOUANDOU²

The mineralogical study carried out on the doleritic formations of Mbaoussi shows that the rocks are characterized by a typical mineralogy of tholeiitic and calc-alkaline lavas. The rocks consist of clinopyroxene (diopside and augite), plagioclase (andesine and oligoclase), alkaline feldspar (anorthosite and sanidine), iron-titanium oxides (titano-magnetite and ilmenite), black mica (biotite and phlogopite), amphibole (tschermakite and magnesio-sadanagaite) and quartz (in the dolerites of group 2). The higher values of Al^IV (0.13 < Al^IV < 0.25) in the dolerites of group I (orientcompared to those of group II (0.06 < Al^IV < 0.13), suggest that the clinopyroxenes of Group II dolerites would have crystallized under conditions of high silica activity. The values of higher ratios of Al^IV (0.19<Al^IV/Al^VI <1.13) in the clinopyroxenes of the group II dolerites suggest that these crystals with tholeiitic affinity would have crystallized under lower pressures and in relatively hydrated magma conditions (<10% H₂O). The magma from clinopyroxene crystals origin of the group I dolerites would be poor in water, even anhydrous. Amphibole crystals from group I dolerites crystallized at maximum depths between 90 and 70 km and amphibole crystals from group II dolerites would have crystallized at shallower depths, between 25 and 18 km. The different crystallization temperature variations of clinopyroxenes (400-500°C, 500-800°C and 1000-1100°C) suggest that the crystallization of these minerals resulted in an evolution of more calcium diopside crystals towards those clinopyroxene crystals with weakly calcic augite composition which would be the last crystals to have crystallized in these rocks.

mineralogy, dolerites, continental tholeiites, Pan-African chain, Adamawa Plateau, Mbaoussi, Cameroon