Pan-African plutonism of the Damagaram inlier, Niger Republic

Abstract

This paper presents the first petrological, geochemical and geochronological data on the basement granitoids of Damagaram (southernNiger Republic) which cut a supracrustal sequence subjected to Barrovian metamorphism in upper greenschist to amphibolite facies. The petrographic study points to the great variety of granitoids although they all belong to the high-K calc-alkaline series and are late-kinematic. Six plutonic units and their enclaves were studied in some detail, five of which yielded Rb-Sr isochrons with good alignment. All the ages obtained are Pan-African with a mean of 575±12 Ma . A striking feature is the wide range in Sr initial ratios (0.705 to 0.720) indicating complex and strong interactions with crust, which is reflected in the geochemistry of the enclaves and the chemical evolution of the micas. This is in contrast with the Iforas (Mali) next to the suture with the WestAfrican craton where Sr initial ratios of the calc-alkaline rocks are less than 0.705. The calc-alkaline granitoids of the Damagaram cannot be related to the far distant suture bordering the West African craton nor the older SE Aïr collision (680 − 670 Ma) nevertheless probably represented in the Damagaram by the thrust sheets of eastern Damagaram and further east by the outcrops of the Gazamni Group similar to the Proche-Ténéré-Molassic Group which extends to the east of Hoggar and Aïr. The solution we now envisage to explain the presence of 610−570 Ma old subduction-collision granitoids in Damagaram and central aïr is the existence of a Pan-African suture close to long. 7°E, the key area to solve this problem being in Nigeria or in central Hoggar.