The Ntem complex (Sud-Cameroon) is one of the major tectonic units exposed in the northwestern Congo craton. The northern part of the complex in the Ebolowa area is underlain by three distinct granitoid suites: (a) charnockite-tonalite suite, (b) K-rich granitoids, (c) syenites. These
granitoids show variable degrees of deformation and intruded into metasediments with Nd model ages (TDM) ranging from 3.0 to 3.5 Ga.
Major element compositions indicate that the Ebolowa charnockite-tonalites are similar to the tonalite-trondhjemite-granodiorite (TTG) suites of other Archean cratons. They show low-K2O calc-alkaline or trondhjemitic differentiation trends. Based on zircon U-Pb, Rb-Sr and Sm-Nd whole rock isochrons the charnockite-tonalite suite intruded at ca. 2.9 Ga. Initial eNd and initial 87Sr/86Sr ratios (ISr) range from +0.29 to +3.3 and 0.7005 to 0.7016, respectively. These results suggest that the charnockite-tonalite suite is derived from a depleted mantle similar to that proposed by Stein and Hofmann model (1994; Fig. 1).
The K-rich granitoids define a Rb-Sr and a Sm-Nd whole rock isochrons of 2.6 Ga. This suite defines a classical calc-alkaline trend being clearly distinct from TTG-types. These granitoids are characterised by negative eNd (-2.9 to -5) at 2.6 Ga and values identical to the TTG suite at 2.9 Ga. These rocks display a large range of ISr: ranging from 0.7011 to 0.7028 in the granites and from 0.7149 to 0.7246 in the leucogranites. Field relationships and isotope data are consistent with formation of the K-rich granitoids by anatexis of TTG members and metasediments.
The syenites yield a Sm-Nd whole rock isochron of 2.3 Ga. They define an alkaline trend and are characterised by high K2O + Na2O (8 to 12%), LIL and REEs and large negative Nb and Ti anomalies. Initial isotope compositions of the syenites are characterised by low negative eNd (-6.7 to -7.5) and high ISr (0.7013 to 0.7041) at 2.3 Ga and initial values identical to the granitoids and the TTG at 2.9 Ga. These isotope signatures indicate that the syenites are derived from the pre-existing continental crust. They are distinct from syenites of rift or hotspot related syenites which are thought to be derived from depleted asthenospheric mantle. The isotope data of the three granitoid groups
indicate that the crust in the Ebolowa area was formed and differentiated rapidly (< 100 My) at ca. 2.9 Ga. The younger granitoids were generated by partial remelting of this existing crust without noticeable mantle contribution.
De Paolo, Nature 291, 193-196 (1981).
Stein & Hofmann, Nature 372, 63-68 (1994).
Fig. 1: Initial *Nd of the Ebolowa granitoids. Shown for reference is average depleted mantle (DM), and bulk earth (CHUR). The K-rich granitoids and syenites have the appropriate isotopic compositions to have been formed by anatexis of TTG.