Revised petrology and new chronological data on the peralkaline felsic lavas of Ngaoundéré volcanism (Adamawa plateau, Cameroon, Central Africa): evidence of open-system magmatic processes
Journal Title: Analele stiintifice ale Universitatii "Alexandru Ioan Cuza" din Iasi - seria Geologie - Year 2012, Vol 58, Issue 2
Abstract
The petrology of the peralkaline felsic lavas from the Ngaoundéré region was re-examined, together with new chronological data obtained on them, in order to determine their petrogenesis and investigate their peralkaline features. The results showed that all lavas have a microlitic porphyritic texture. They are composed of more than 70% phenocrysts and microlites of alkali feldspar, with diopside, hedembergite, augite, aegirine-augite for trachytes, and nosean, nepheline, kaersutite, titanite, apatite and oxides¸ set in a matrix of the same minerals, in phonolite. All lavas, Miocene in age, are the result of the fractional crystallization processes undergone by the surrounding basaltic lavas. The titanite crystallization phase is responsible for the concave downward appearance of the REE pattern of peralkaline phonolite, as well as the slight depletion of Nb and Ta. AFC modeling using Pan-African basement rock as contaminant was also intended. The genesis of peralkaline trachytes from less-evolved lavas requires a Ma/Mc ratio of 0.1, and a fractionation process (F ≈ 0.9) involving only feldspar so as to comply with the (87Sr/86Sr) 9Ma values of ≈ 0.710. Additionally, hydrothermal fluid effects are suspected. The peralkalinity of the lavas is the result of a pronounced fractionation of the Al2O3 component by aluminous-enriched phases, in combination with alkali- and volatile-enriched metasomatized fluids, which have acted through the network of Pan-African cracks.
Authors and Affiliations
Aminatou M. Fagny, Oumarou F. Nkouandou, Bernard Déruelle, Ismaila Ngounouno
Keywords
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