Geochemical characteristics and U-Pb zircon LA-ICPMS ages of granitoids from the Pan-African Dahomeyide orogen, West Africa

摘要

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses in Ghana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatitic gneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for ～2500 km from the Sahara to the Gulf of Guinea. The granitoids are characterized by 1-type signatures, display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a subduction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss) that forms a sliver just east of the suture zone, yielded U-Pb zircon ages ranging from 615 to 589 Ma. This age range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U-Pb zircon crystallization ages ranging from 2.19 to 2.14 Ga. These are the first U-Pb zircon ages to document Paleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages of rocks in the suture zone are also consistent with the existence of widespread involvement of older crust in the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain of the northwest Borborema province in Brazil.