Le complexe volcano-plutonique calco-alcali de la rivière daléma (Est Sénégal): discussion de sa signification géodynamique dans le cadre de l'orogénie eburnéenne (protérozoïque inférieur)

Abstract

This suite is located in the Kedougou inlier where a Lower Proterozoic sequence is surrounded by unconformable Upper Proterozoic. This suite is a member of the Daléma supergroup which also includes some epicontinental sediments with numerous carbonate layers. In contrast, the Mako supergroup (located further to the west) is characterized by submarine volcanic or plutonic rocks (mainly tholeiitic) intruded by trondhjemites at around 2200 Ma. After deposition of carbonate rocks alternating with sandstones or greywackes, the Dalé with some volcanism. The volcanic belt is characterized by dacites or andesites, by a lot of hypovolcanic rocks (accumulative microdiorites) and by its elongation along a N-S trend. This belt was intruded 1990 Ma ago by a plutonic intrusion with petrographic composition ranging from diorite to monzogranite. The two magmatic events are spatially linked but the granitoid develops a strong thermic metamorphism in volcanic and sedimentary rocks which contrasts with the very low grade regional metamorphism. Despite a hydrothermal alteration, petrographic and geochemical studies show that the Dalé,a volcano-plutonic suite is calc-alcaline. Some large amounts of iron ore have been found in skarnoid rocks along the belt near the granitoid. The geotectonic significance is discussed; the Daléma suite could result from the subduction of oceanic crust as shown by Mako supergroup. However, it is more probable that the suite has spatial relations with large transcurrent faults according to a geodynamic model proposed for the Stephano-Permian calc-alcaline volcanism of western Europe. The sedimentary and tectonic context, the strongly linear position and the coincidence of a linear positive gravity anomaly with the volcano-plutonic belt are more consistent with the second interpretation.