Petrology, geochronology and Sr-Nd isotopic geochemistry of the Konso pluton, south-western Ethiopia: implications for transition from convergence to extension in the Mozambique Belt

Granites were shown to be excellent geochronological, structural and geodynamic markers. Among several generations of granites described in the Neoproterozoic of Ethiopia, we studied the post-tectonic Konso pluton to characterise the post-Pan-African evolution of the Mozambique Belt (MB) of southern Ethiopia. The Konso pluton is a composite intrusion of slightly peraluminous and ferro-potassic, bt (biotite)-leucogranites, bt-hbl (hornblende)-granites and subordinate coeval metaluminous monzodiorites, intruded into high-grade gneiss-migmatite associations of the MB. The whole suite displays chemical features of A-type granites. It is LIL- and HFS-elements enriched with Y/Nb and Yb/Tagreater than or equal to1.2. The granites and leucogranites show non-fractionated to fractionated REE patterns [(La/Yb)(N)=0.3-9.4] with strong negative Eu anomalies. The monzodiorites show fractionated REE patterns [(La/Yb)(N)=5.5-7.4] with negligible negative Eu anomaly. The low initial (Sr-87/Sr-86)(450) ratios (0.70113-0.70441) and positive epsilonNd((450)) values (+1.8 to +3.3) suggest an isotopically primitive source. The Konso granites are likely to be derived from a basaltic parent, with minor contamination by crustal material with high Y/Nb and low Sr initial isotopic ratios. Age of pluton emplacement is constrained by a Rb-Sr isochron and zircon U-Pb data at 449+/-2 Ma. The Konso pluton is, therefore, the witness of an Ordovician A-type magmatic event, which marks a change from convergence, related to the Pan-African collision, to extension in the Mozambique Belt of southern Ethiopia.