Crustal origin of Early Proterozoic syenites in the Congo Craton (Ntem Complex), South Cameroon

Two petrographically different syenite suites can be distinguished in the mostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites and a suite of clinopyroxene syenites. The syenites occur as discrete intrusions surrounded by Archean gneisses. Based on structural relationships the syenites are of late to post-orogenic origin. The Pb-Pb zircon evaporation ages indicate that the syenites intruded at ca. 2.3 Ga and thus postdate significantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin. The syenite suites do not show simple chemical trends, have highly variable trace element contents and display only a small variation in their major element contents, particularly in their SiO2 contents. The isotope characteristics, including Nd-143/Nd-144, Sr-87/Sr-86 and common ph, point to a heterogeneous source or multiple sources for the syenites. The Sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low Sr-87/Sr-86 ratio Nd-143/Nd-144 ratios are more homogenous with epsilon Nd-(2320) from -5.8 of 0.690 to high values of 0.713. In contrast the initial to -7.5. The Pb isotopes from leached K-feldspars indicate that the sources of the syenites had a history of U depletion and Th/U enrichment. The syenites show the same isotope characteristics as the surrounding gneiss terrane, thus the deep continental crust of the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source. In addition, inherited zircons from the syenites yield Pb-Pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses. In contrast to many other syenites worldwide, the syenites in the Ntem Complex do not contain juvenile mantle material, but are the sole products of crustal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust, The cause for the generation of these large syenite bodies in the Ntem Complex is most likely an episode of deep-seated thermal activity that was so far not known from this part of the Congo Craton. (C) 2001 Elsevier Science B.V. All rights reserved.