Late Eburnean granitization and tectonics along the western and northwestern margin of the Archean Kenema-Man domain (Guinea, West African Craton)

Recent mapping of the eastern part of Guinea has revealed a vast plutonic belt that rims the margin of the Archean Kenema-Man craton and that is made up of a variety of granitic rocks (granodiorite, biotite granite, monzogranite, two-mica granite), granodiorite, with common clinopyroxene, being the most abundant. Single-zircon dating by stepwise Pb-evaporation carried out on characteristic rock types (granodiorite, monzogranite, two-mica granite) reveals their age to be late Eburnean (c. 2090-2070 Ma), and indicates that their emplacement occurred within a relatively short time interval (< 20 Ma). The granodiorite, biotite granite and monzogranite of the plutonic belt all present a calc-alkaline and highly potassic nature and show numerous analogies with the Neogene calc-alkaline rocks of the central Andes; there is evidence to suggest strong contamination from the Archean crust. These analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone. The occurrence of peraluminous two-mica granite at the northern extension of the plutonic belt in the Siguiri Basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin. Contemporaneously with its emplacement, the late Eburnean plutonic belt accommodated regional convergence by major WNW-ESE sinistral movements along the northwestern margin of the Archean block. Horizontal shortening towards the WSW is envisaged at the scale of eastern Guinea in relation with these kinematics. The late Eburnean plutonic belt of eastern Guinea is a key element of the Archean/Proterozoic transition zone in the southern part of West Africa. Evidence along the western and northwestern border of the Archean Kenema-Man domain leads us to interpret this transition zone as an active margin, along which strike-slip tectonism accommodated regional convergence towards the end of the Eburnean cycle. (C) 2002 Published by Elsevier Science B.V.