Major and trace element geochemistry of the Archaean Ntem plutonic complex (south Cameroon): petrogenesis and crustal evolution

Abstract

The Ntem complex crops out in south Cameroon and belongs to the Congo craton. It corresponds to a 2.8 Ga basement which in the northern part is made up of plutonic rocks: (1) Charnockites and (2) Soo tonalites. Both have similar compositions and define a typical tonalite-trondhjemite and granodiorite (TTG) suite. The study of isotopes (ISr), major elements and REE allows us to propose a petrogenetic model that may summarized as follows: 1. (1) partial melting of the mantle gave rise to tholeiites; 2. (2) tholeiites transformed into garnet-bearing amphibolites underwent partial fusion thus generating the parental magma of the TTG suite; 3. (3) fractional crystallization mainly of hornblende and plagioclase produced the differentiated TTG suite. This scheme is typical of Archaean TTG suites and is supposed to have occurred in a subduction-like geodynamic environment, where, because of the high Archaean geothermal gradients, the subducted oceanic crust underwent fusion before dehydration operated. It is also possible to show that the TTG suite was not emplaced during the granulite-facies metamorphism, but that this occurred later, affecting only some parts of the basement. Consequently the Ntem complex appears to be an ‘Archaean high-grade terrane’, where the granulitic assemblages may be Early Proterozoic in age, characteristic that seems to be more widespread among Archaean high-grade terranes.