TCHABAL-NGANHA VOLCANO IN ADAMAWA (CAMEROON) - PETROLOGY OF A CONTINENTAL ALKALINE LAVA SERIES

The Mio-Pliocene Tchabal Nganha volcano in the Adamawa Plateau (Camero on) contains a broad range of rock types from ankaramites to phonolite s and trachytes. Tchabal Nganha is located on large N70-degrees-E stri ke-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, ph onolite and trachyte lava flows and plugs, and finally tiny basaltic a nd intermediate outpourings. Olivine is Fo-85 in basaltic lavas and ha s altered fayalitic compositions in trachytes. Clinopyroxene compositi ons range from Ti-rich salite in basaltic rocks to hedenbergite in pho nolites and aegirine in trachytes. Kaersutite is present as megacrysts (almost completely replaced by rhonite + phlogopite + salite + olivin e + nepheline) in ankaramites and as phenocrysts in phonolites. The Fe -Ti oxides indicate decreasing equilibration temperatures and oxygen f ugacities from basalts to trachytes. Spinels occur as inclusions in ol and/or cpx phenocrysts in ankaramitic basalts where Al-rich Cr-poor s pinels arc found exclusively in clinopyroxene phenocrysts. Feldspar oc curs as phenocrysts of plagioclase in intermediate lavas, and as sanid ine and anorthoclase in phonolites and trachytes. Apatite is ubiquitou s in intermediate and felsic lavas. Sphene is common in some phonolite s 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 phon olites. Transition elements decrease strongly with differentiation in the basaltic lavas. Zr/Ta is constant along the series except in sphen e-bearing phonolites, which are depleted in Ta. REE patterns are simil ar for basaltic and intermediate lavas. Some phonolites exhibit overal l-enriched REE patterns, whereas peralkaline phonolites are strongly d epleted in MREE. Major- and trace-element variations confirm the domin ance of crystal fractionation in the differentiation of the basaltic a nd phonolite series. Due to high Ta contents along the series, and to low Sr-isotope ratios (< 0.07035) in basalts and phonolites, crustal c ontamination is not suspected to have contributed significantly to the ir genesis. Similar La/Ta ratios in Tchabal Nganha and St. Helena coul d reflect mixing of a hot-spot component and N-MORB source. Hornblende fractionation in the magma source allowed the development of the comp lete basaltic-felsic series. Partial melting of phlogopite-bearing per idolite in the lithosphere is likely to have produced the parental mag mas to the Tchabal Nganha suite.