Petrology of the bimodal Cenozoic volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa)

The Kapsiki Plateau is the northernmost volcanic zone of the Cameroon Line. The volcanism (27-35 Ma) is of alkaline type and has a typical bimodal lav a series diversity with basalts and hawaiite as mafic lavas and phonolites, trachytes, and rhyolites as felsic lavas. Mg-rich olivine phenocrysts occu r only in basalts. The hawaiite contains andesine, olivine and Ca-rich pyro xene phenocrysts, and sanidine and quartz xenocrysts. The phonolites contai n alkali-feldspar and Na-rich clinopyroxene phenocrysts. Two types of trach ytes occur: peralkaline trachytes, with Ti-, Na- and F-rich aegirine augite , richterite, arfvedsonite phenocrysts and non-peralkaline trachytes, with an aenigmatite-type undetermined mineral, Ti-rich biotite and zircon phenoc rysts. Similarly, two types of rhyolites occur: peralkaline with quartz and arfvedsonite phenocrysts and non-peralkaline with quartz and biotite pheno crysts. Differentiation indices (Thornton and Tuttle, 1960) of the lavas range from 22 to 97 with a large gap in the range 34-82. Some basalts are primitive ( 530 ppm Ni, 1100 ppm Cr). In basaltic lavas, phonolites and non-peralkaline trachytes and rhyolites, Zr and Nb covary with approximately constant rati os (3.1 < Zr/Nb < 5.3). However, peralkaline trachytes and rhyolites have h igh concentrations in Zr (up to 2180 ppm) and Nb (up to 780 ppm), with corr elative higher Zr/Nb ratios (6.3-8.3). Some rhyolites have abnormal REE pat terns (with kinks), depleted in light-REE, probably resulting from stabilit y of Na-REE-F complexes under hydrothermal conditions. Despite a large gap between basaltic and felsic lavas, major- and trace-element distributions i ndicate co-magmatism for both the basaltic and felsic lavas. The differenti ation of the lava series is dominated by crystal fractionation, the role of fluids in rhyolite genesis being of minor effect, as evidenced by constant values of Y/Ho and Zr/Hf throughout the series. The Kapsiki Plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors o f the Cameroon Line. The continental crust appears to have no significant r ole in their genesis. The hawaiite mineralogical and geochemical characteri stics are consistent with an origin by mixing of basaltic and felsic (phono litic) magmas. The Kapsiki Plateau basaltic magmas may have originated from an infra-asthenospheric reservoir similarly to other basaltic magmas gener ated throughout the Cameroon Line. The Sr-isotope variations observed in tr achytes and rhyolites point to some contamination of the magmas by crustal materials, while the Nd isotopic composition is only slightly affected. (C) 2000 Elsevier Science B.V. All rights reserved.