Neoarchaean crustal evolution in the Congo Craton: evidence from K rich granitoids of the Ntem Complex, southern Cameroon

The Neoarchaean (2.6 Ga) K rich granitoids of the Ebolowa area (Ntem Complex, Congo Craton) in southern Cameroon form small massifs trending north-northwest-south-southeast to north-south and contain xenoliths of the surrounding tonalite-trondhjemite-granodiorite (TTG) series and of the greenstone belt country rocks. The granitoids range in modal composition from granodiorite to granite and leucogranite. The rocks display high SiO2 (69-76.5%), Na2O + K2O (5.5-9.1 %) and K2O/Na2O (0.5-1.9). They are metaluminous to slightly peraluminous (0.9<A/ CNK<1.1) and define a classic calc-alkaline suite. Their trace element distribution diagrams are characterised by pronounced enrichment in LILE and LREE (93 < La-N < 705), and negative Nb, P and Ti anomalies. The intrusion temperatures, based on the major elements and zircon saturation, indicate a range from 700 degrees C to 800 degrees C. The Sr and Nd isotopes, combined with the trace element signatures and intrusion temperatures, are consistent with a derivation of the granitoids by partial melting of the surrounding or underlying Archaean gneisses during granulite-facies metamorphism. The Sr and Nd isotopes of the high K granitoids do not show any evidence for a new mantle contribution in their petrogenesis. Pb-Pb zircon evaporation ages of 2666+/-2 and 2687+/-3 Ma indicate that this final magmatic episode post-dates the main crust formation by more than 200 Ma. The formation of these late granitoids in the Ebolowa area is attributed to a large-scale tectonothermal event around 2.6 Ga that led to the formation of similar granitoids throughout the extensive Ntem Complex. The intrusion of these granitoids is probably the terminal tectono thermal episode following the Archaean accretion and differentiation of the northwest Congo Craton. (C) 2000 Elsevier Science Limited. All rights reserved.