K-AR AGES, MINERALOGY, MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF THE TERTIARY-QUATERNARY LAVAS FROM THE NDU VOLCANIC RIDGE N.W. CAMEROON

Abstract

The Ndu Volcanic Ridge occupies the central position amongst the continental sector volcanoes of the Cameroon Volcanic Line. K-Ar age determination coupled with field observations show that there were three episodes of volcanic manifestation: 31-28 Ma; 23-21 Ma and <1 Ma. Basaltic magmas were erupted during the first phase while the second phase was characterised by the eruption of basaltic flows followed by trachyte, rhyolite and sporadic welded rhyolitic tuffs. The last episode, comprises weathered scoria and ash. The ridge is composed of a spectrum of alkali basalt/trachyte-rhyolite suite with a marked Daly gap within the intermediate compositional range. Petrographic, mineralogical and major/trace element (including Rare Earth Elements) geochemical studies, suggest that fractional crystallisation was accompanied by magma mixing processes during evolution of the magmas. Olivine, clinopyroxene, plagioclase, apatite and titanomagnetite were the dominant fractionating phases. The mafic rocks are characterised by high Fe2O3, TiO2, MgO, CaO and variable enrichment in compatible trace elements (Cr, Ni, Co) all of which abruptly decrease with increase in fractionation. Na2O, K2O and Al2O3 and the LILE increase with magma differentiation. Normalised incompatible trace elements show marked K and Sr negative anomalies probably reflecting the involvement of a K-rich amphibole at the source. Mafic rocks are enriched in light-rare-earth elements (LREE) and depleted in heavy-rare-earth elements (HREE) for the whole spectrum. A slight positive Eu anomaly in the mafic rock and the strong negative Eu anomalies in the felsic rocks indicate plagioclase accumulation and fractionation consistent with petrographic observations

African Journal of Science and Technology Vol.5(1) 2004: 47-56