Short-lived Eburnian orogeny in southern Mali. Geology, tectonics, U-Pb and Rb-Sr geochronology

Abstract

This work is based on a comprehensive field survey, including mapping of the Massigui degree sheet in southern Mali. The investigated area belongs to the northern part of the Man shield which is mainly composed of Lower Proterozoic volcano-sedimentary sequences (Birimian) and of large areas of granitoids, the whole being affected by the Eburnian orogeny. Structural and geochronological (U-Pb and Rb-Sr) results from this study point to a short period of time (∼ 30 Ma) for the Eburnian orogenesis between 2100 and 2070 Ma, the events recorded later (1984 ± 30 Ma) being due to tectonic reactivations. On the other hand, no trace of events older than 2100 Ma have been found. Coupled with geochemical and Rb-Sr isotopic systematics, this leads to invoke a fundamental link between Birimian volcanics and Eburnian granitoids, both being high-K calc-alkaline in composition and coming from a single, evolving mantle source in a palaeo-environment which has some of the characteristics of modern subduction (including back-arc) or docking zones. The main regional structure of the area strikes roughly NNE-SSW with an important strike-slip component, escorted by a greenschist metamorphism. The deformation is the most intense in the N40° to N10° 5 km large Banifin shear-zone where metamorphism reached the lower amphibolite facies. Here one observes subvertical strong foliation, non-cylindrical folding (locally sheath folds) and subhorizontal pencil lineations, the deformation being regionally recorded by light subisoclinal folds with subvertical axes compatible with a sinistral movement. This regional shear tectonics (D 2,2074+9/-8 Ma) is preceded by an isoclinal deformation (D 1, bracketed between 2098 ± 5 Ma and D 2) only known as relics, from which it likely derived. A later phase (D 3) occurred along the Banifin shear belt, at a more superficial level, even if still ductile, generated by a dextral movement and having induced retrograde metamorphism. This last tectonic phase, which is not accompanied by plutonism nor volcanism, has been dated at 1982±30 Ma being responsible for the complete resetting of the Rb-Sr isotopic system in the plutons present in the Banifin corridor.