Chronology of nappe assembly in the Pan-African Dahomeyide orogen, West Africa: Evidence from 40Ar/39Ar mineral ages

The Pan-African Dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the West African Craton was incorporated into Gondwana. Orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in Ghana and Togo. External nappes include the Atacora, composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c. 2.0 Ga foreland basement. The suture zone is represented by high-pressure granulite facies garnet homblende gneiss. We have analyzed muscovite and homblende concentrates from external and suture-zone nappes in incremental heating 40Ar/39Ar experiments. Muscovites which define the penetrative foliation in the Atacora nappes and a shear foliation in the basement parautochthon record 40Ar/39Ar plateau ages of 579.4+/-0.8 Ma and 578.1+/-0.5 Ma respectively. In contrast, c. 600 km to the north, muscovite in quartz schist near the base of the Atacora nappes displays a spectrum which gives a minimum age of 608.1+/-1.2 Ma, whereas, muscovite in the overthrust Kara nappes yields a plateau age of 633.8+/-0.5 Ma. The c. 30 m.y. difference between the Atacora nappes in the northern Dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication, however, forward-imbricating thrusting is also suggested by the older age of Kara nappes relative to Atacora nappes in the northern segment. The youngest ages are inferred to date orogen parallel nappe transport at c. 575 Ma. Hornblende from the structurally lower sections of the suture-zone nappes record a Ar-36/40Ar versus 39Ar/40Ar isotope correlation age of 587+/-4.3 Ma. Hornblende from a tonalitic vein in the host garnet-mafic gneiss yields an isotope correlation age of 581.9+/-2.4 Ma. These ages are interpreted to date the exhumation of the suture-zone nappes and thus provide a minimum age of crustal thickening in the Dahomeyide orogen. The age data are consistent with coeval forward- and hindward- propagating thrusting and nappe imbrication, and constrain the amalgamation in northwest Gondwana to have occurred in the latest Proterozoic rather than Cambrian. (C) 1997 Elsevier Science B.V.