Petrogenesis of Lava from Wainama West, Mount Oku (CVL): Source Characterization and Magma Evolution

摘要

The Wainama West area belongs to the Cameroon volcanic Line (CVL). Three petrographic types of lavas are identified: basanites, trachytes and rhyolites. Basanites with porphyritic texture are made up of minerals: plagioclase, clinopyroxene, olivine phenocrysts and opaque minerals within a groundmass; trachytes with microlitic-porphyritic texture are composed of phenocrysts of sanidine, hornblende, plagioclases, pyroxene and opaque minerals enclosed in a fine groundmass showing a preferred orientation. Rhyolite shows a microlitic-porphyritic texture made up of quartz, sanidine and opaque minerals in the groundmass. According to their geochemical behaviour, major elements show an enrichment in SiO2 (42.7–70.7 wt.%), Al2O3 (13.7–16.6 wt.%), Na2O (2.3–6.3 wt.%), K2O (1.3–4.9 wt.%) and an impoverishment in MgO (8.3–0.02 wt.%) and CaO (10.0–0.5 wt.%) from basanite to rhyolite. Some binary diagrams indicate a good correlation with some minor elements (Cr, Ba, Zr, Sr, Rb, and Nb) against SiO2. REE patterns of the rocks are characterized by a negative anomaly in Eu (0.3Eu/Eu*0.4) in basanite and weak negative anomaly in Eu (0.1Eu/Eu*0.2) from trachytes to rhyolites, with parallel profiles. The Wainama West lavas are found to have originated from a single primary melt similar to that of OIB and the continental basalt but with slightly higher Nb/Ta content. From the Dy/Yb vs La/Yb diagrams, the major processes resulting to the generation of this primary melt is the partial melting of garnet peridotite, of the high degree (11-12%). The Wainama West alkali lavas were formed in intraplate setting of continental part of the CVL.