Tchabal Nganha volcano in Adamawa (Cameroon): petrology of a continental alkaline lava series

Abstract

Abstract The Mio-Pliocene Tchabal Nganha volcano in the Adamawa Plateau (Cameroon) contains a broad range of rock types from ankaramites to phonolites and trachytes. Tchabal Nganha is located on large N70°E strike-slip faults that have been active since the Cretaceous and delimit the Adamawa Plateau horst. Geological mapping revealed a succession of basaltic-hawaiitic lava flow units, mugearite-benmoreite breccias, phonolite and trachyte lava flows and plugs, and finally tiny basaltic and intermediate outpourings. Olivine is Fo-85 in basaltic lavas and has altered fayalitic compositions in trachytes. Clinopyroxene compositions range from Ti-rich salite in basaltic rocks to hedenbergite in phonolites and aegirine in trachytes. Kaersutite is present as megacrysts (almost completely replaced by rhonite+phlogopite+salite+olivine+nepheline) in ankaramites and as phenocrysts in phonolites. The Fe-Ti oxides indicate decreasing equilibration temperatures and oxygen fugacities from basalts to trachytes. Spinels occur as inclusions in ol and/or cpx phenocrysts in ankaramitic basalts where Al-rich Cr-poor spinels are found exclusively in clinopyroxene phenocrysts. Feldspar occurs as phenocrysts of plagioclase in intermediate lavas, and as sanidine and anorthoclase in phonolites and trachytes. Apatite is ubiquitous in intermediate and felsic lavas. Sphene is common in some phonolites and chevkinite in some trachytes. Differentiation indices vary from 17 to 93 with a gap in the range 60–75. Ba may attain 4650 ppm in phonolites. Transition elements decrease strongly with differentiation in the basaltic lavas. Zr/Ta is constant along the series except in sphene-bearing phonolites, which are depleted in Ta. REE patterns are similar for basaltic and intermediate lavas. Some phonolites exhibit overall-enriched REE patterns, whereas peralkaline phonolites are strongly depleted in MREE. Major- and trace-element variations confirm the dominance of crystal fractionation in the differentiation of the basaltic and phonolite series. Due to high Ta contents along the series, and to low Sr-isotope ratios (