Félix M. Tchoua

The Ntui-Betamba high-grade gneisses: a northward extension of the Pan-African Yaoundé gneisses in Cameroon

Abstract The Ntui-Betamba area (southern Cameroon) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (kyanite-biotite-garnet gneisses, biotite-muscovite-garnet gneisses, calc-silicate rocks and quartzites) interpreted as a continental margin sedimentary series; and (ii) meta-igneous rocks comprising alkaline ultramafic to mafic pyroxene gneisses and amphibolites and amphibole-bearing alkaline orthogneisses. These units recrystallised under HP-HT conditions ( T =750–800 °C, P ≥0.9–1.3 GPa) and were deformed in relation to a major tangential tectonic event with a north-northeast-south-southwest kinematic direction. This lithological association and its tectono-metamorphic evolution show striking similarities with the Yaounde gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther north, towards the Adamawa Shear Zone (Lorn series). The contrasted metamorphic evolution between areas located to the south of the Adamawa (high pressure: Yaounde, Ntui-Betamba), and those located to the north (low pressure: Banyo), along with widespread remains of a Palaeoproterozoic crust, suggest important crustal thickening during tangential tectonics in southern Cameroon. As a consequence, the Adamawa Shear Zone is not simply a late Pan-African transcurrent or transpressive shear zone but appears to have been formerly a major (possibly intracontinental) thrust zone.

La bande mylonitique Foumban-Bankim, Ouest Cameroun. Une zone de cisaillement de haute température

Abstract The mylonitized rocks between Foumban and Bankim constitute a granodiorite suite. Microtextures and mineralogic changes are consistent with a deep-crustal (15–20 km) mylonitization, under T and P conditions estimated at 730–800 °C and 5±1 kbar.

Olivine phenocrysts in some Cameroonian Basalts—implications for primary magma composition

SummaryThe chemical composition of olivine phenocrysts was determined for 13 basaltic samples taken from the central part of the Cameroon Volcanic line to estimate the compositions of the primary olivine phenocrysts and the primary magmas. The Mg/(Mg + Fe) ratios of the olivines attain 0.91, though many of the magnesian olivines are identified on a textural basis as xenocrysts from peridotite inclusions. Most magnesian olivine phenocrysts have Mg/(Mg + Fe) ratios of 0.87–0.88 and NiO contents of 0.32–0.35 wt %. The NiO versus Mg/(Mg + Fe) ratios of the olivines can be used to estimate the compositions of the primary phenocrysts in a range of Mg/(Mg + Fe) = 0.88–0.90. Assuming the Fe(3)/(total Fe) ratios of the magmas are 0.17–0.25, which is estimated from the Fe-Mg partitioning between plagioclase and groundmass, the compositions of the primary magmas were obtained from the Fe2+-Mg partition equilibrium with the primary olivine phenocrysts. The primary magmas of the Cameroonian basalts have 13.5 ± 4.0 wt MgO, which is similar to the compositions of basalts with highest MgO contents reported so far from the Cameroon volcanic line.ZusammenfassungDie chemische Zusammensetzung von Olivin Phenokristallen wurde in 13 Basalt-Proben, die vom Zentralteil .des Kamerun-Vulkangürtels stammen, bestimmt, um Hinweise auf die Zusammensetzung der primären Olivineinsprenglinge and des primären Magmas zu erlangen. Die Mg/(Mg + Fe) Verhältnisse der Olivine reichen bis 0.91, obwohl viefle dieser Mg-Olivine auf Grund textureller Kriterien als aus Peridotit-Einschlüssen stammende Xenokristalle zu identifizieren sind. Die Mg/(Mg + Fe) Verhältnsse der Hauptmasse der Olivin-Phenokristalle liegen im Bereich von 0.87 bis 0.88, die Ni0 Gehalte zwischen 0.32 und 0.35 Gew %. Die Ni0 gegen Mg/(Mg + Fe) Verhältnisse eignen sich zur Abschätzung der Zusammensetzung der primären Phenokristalle, solange die Mg/(Mg + Fe) Verhältnisse im Bereich von 0.88 bis 0.90 liegen. Die Zusammensetzung des Primärmagmas wurde mittels der Fe2+-Mg-Verteilung zwischen dem Magma und den sich mit dem Magma im Gleichgewicht befindlichen primären Olivinkristallen unter der Annahme eines Fe3+/Fe total Verhältnisses von 0.17–0.25, welches sich aus der Mg-Fe Verteilung zwischen Plagioklas und Grundmasse ergibt, bestimmt. Demnach ergibt sich für das primäre Magma der Kamerun Basalte ein MgO Gehalt von 13.5 + 4.0 Gew%. Dieser Wert ist mit der Zusammensetzung von Basalten mit höchsten MgO-Gehalten, wie sie bisher vom Kamerun-Vulkangürtel berichtet worden sind, vergleichbar.

The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line

Abstract About 60 volcanic-plutonic alkaline complexes of Tertiary age are located along the continental part of the Cameroon Line. This paper reports the chemical nature and evolution of six of them (Gwenfalabo, Ntumbaw, Nkogam, Nda Ali, Bana and Koupe), all located in the southern part of this line. The geochemical data (major, trace and REE) of these six complexes reveal the following general behaviour: • -differentiation from a primary magma of intermediate composition situated in the well-known Daly gap. The absence of any discrimination between the different complexes in geochemical diagrams suggests that they have a unique source. This model is defined by three co-magmatic rock-types: cumulative basic rocks composed of anorthositic gabbros, residual acidic rocks with abundant syenites and granites and primitive intermediate rocks mainly represented by monzodiorites and monozonites of the Ntumbaw complex; • -emplacement of the acidic rocks by successive pulses, as illustrated in the Rb/Sr versus Sr diagram by three subparallel trends, corresponding (with increasing Sr contents) to fayalite syenites, other syenites, other syenites and granites respectively. From Rb/Sr isotopic data, the Nkogam granite is 67 Ma-old, while the fayalite syenite of the Mount Koupe is 46 – 50 Ma-old. Rb/Sr isotopic data of acidic formations of the other complexes and stable isotopes are needed.