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

Two petrographically different syenite suites can be distinguished in the m ostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites an d a suite of clinopyroxene syenites. The syenites occur as discrete intrusi ons surrounded by Archean gneisses. Based on structural relationships the s yenites are of late to post-orogenic origin. The Pb-Pb zircon evaporation a ges indicate that the syenites intruded at ca. 2.3 Ga and thus postdate sig nificantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the tr ace elements provide unambiguous evidence that the two syenite suites are o f crustal origin. The syenite suites do not show simple chemical trends, ha ve highly variable trace element contents and display only a small variatio n in their major element contents, particularly in their SiO2 contents. The isotope characteristics, including Nd-143/Nd-144, Sr-87/Sr-86 and common p h, point to a heterogeneous source or multiple sources for the syenites. Th e 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 rati os are more homogenous with epsilon Nd-(2320) from -5.8 of 0.690 to high va lues of 0.713. In contrast the initial to -7.5. The Pb isotopes from leache d 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 character istics as the surrounding gneiss terrane, thus the deep continental crust o f the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace element s 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 crus tal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibil ity that the syenites were generated in the deep crust during a time of add ition of mantle material to the continental crust, The cause for the genera tion of these large syenite bodies in the Ntem Complex is most likely an ep isode of deep-seated thermal activity that was so far not known from this p art of the Congo Craton. (C) 2001 Elsevier Science B.V. All rights reserved .