ORIGIN AND AGE OF PALEOPROTEROZOIC CONGLOMERATES AND SANDSTONES OF THE TARKWAIAN GROUP IN BURKINA-FASO, WEST-AFRICA

The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup wh ich is considered to be part of a juvenile crust formed during the Ebu rnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, cher t and schist), embedded in an acid volcaniclastic matrix, During a sin istral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoc linal folding. Fold axes are nearly horizontal, parallel to a stretchi ng lineation and oriented 035-215 degrees. A typologic study of zircon s in the volcaniclastics, coupled with radiometric datings, enables th e volcanism to be characterized and its age to be determined. The typo logic study has shown that: (1) zircons from the rhyolitic pebbles, th e matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to al kaline evolutionary trend implying three main episodes of volcanism; ( 3) the morphology of this zircon population is very similar to those u sually observed in post-orogenic metasediments or sediments. The tecto nic setting is one of intracontinental extension associated with rifti ng and crustal thinning in the: course of a major period of crustal gr owth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments. The radiometric study has sho wn that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma, Three main volcanic episodes may be defined 2170, 21 50 and 2124 Ma; the last one was probably contemporaneous with the sed imentation. Deformation of Tarkwaian rocks took place between 2124 +/- 9 Ma (the age of the youngest inherited zircon) and 1991 +/- 12 Ma (t he age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fit s well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also re cognized in South America. The Tarkwaian conglomeratic rocks of Burkin a Faso do not have gold concentrations similar to those in Ghana and G uyana. With respect to the Tarkwaian rocks of Ghana, where gold is con centrated in the quartz pebbles, they also differ because: (1) there i s a volcaniclastic contribution; (2) the zircons from the Tarkwaian ro cks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing. With respect to Guyana, the rocks are very similar and probably have the same origi n; the short transport distance may not allow a secondary concentratio n of gold. These differences, at least, may help explain the lack of e conomic gold concentration in the Tarkwaian metasediments.