Geochronology of granitoid and metasedimentary rocks from Togo and Benin, West Africa: Comparisons with NE Brazil

Abstract

The westernmost part of the Pan-African (c. 600Ma) Dahomeyide orogen in Togo and Benin consists of a complex stack of thrust sheets, mainly composed of Palaeoproterozoic (‘Eburnean’, c. 2000Ma) granitoid rocks and younger volcanic and metasedimentary successions. This thrust stack comprises the suture between the West-African craton and the Benino-Nigerian province to the east. This eastern province consists largely of migmatites, supracrustals and granitoid rocks. Rb–Sr and Sm–Nd isotope data for four granites from the western domain confirm the Palaeoproterozoic age of these rocks found by earlier studies. One of these granites yielded a zircon U–Pb age of c. 2060Ma. Twenty samples from the eastern region yield Rb–Sr and Sm–Nd data indicating that both migmatites and granitoid rocks are of Neoproterozoic (Pan-African) age. Zircon U–Pb data on six of these samples yielded ages ranging from c. 650 to c. 550Ma. Suggestions that parts of the migmatites might be of Eburnean age could not be confirmed. The Neoproterozoic granitoid rocks may represent a Pan-African arc overlying an eastward dipping subduction zone, and Sm–Nd data indicate that they contain significant proportions of older crustal material. Age probability diagrams for detrital zircons from three metasedimentary rocks (one from Nigeria) and one S-type quartz diorite from the eastern region show prominent peaks at c. 1900Ma and 2400–2200Ma. A provenance of the zircons from sources in the Borborema province in NE Brazil or lateral equivalents thereof is suggested. A few Neoproterozoic zircons are present in three of the samples. On the basis of Th/U ratios a distinction can be made between Neoproterozoic zircons of metamorphic and detrital origin. The zircon age probability diagram of a quartzite from northern Benin exhibits one peak, at c.1900Ma, different from the other metasedimentary samples. This quartzite might be correlated with sedimentary strata of the Volta basin that overlie the Eburnean basement in Ghana, Togo and Burkina Faso. Correlation of the rocks in the study area with those in the Borborema province is discussed.