Significance of high-grade metamorphic events in the Tuareg Shield and its implication for evolution of the Columbia Supercontinent

Using innovative methodological developments applied to the geosciences, such as "split-stream" LASS-HR-MC-ICP-MS technique, we present new geochemical results focusing on high-grade granulite facies rocks from the Iforas Granulitic Unit (IGU, Hoggar Mountains, NE Mali). This unit represents a high grade metamorphic terrane that has been only poorly studied yet. The aim of this study is to understand how granulites provide information on crustal growth processes and, in the context of the IGU, to test whether the processes identified can be integrated into global geodynamic reconstructions during the Palaeoproterozoic. Granulites can contain various U-Pb datable minerals such as zircon, monazite, rutile or apatite. These minerals record, to varying degrees (influence of temperature and/or fluids, nature of the protolith), the events that the granulites have undergone. From a global point of view, the main results of this study show that the major events recorded within the IGU have affected this unit uniformly. Five major events were identified on the basis of the nine rocks studied in detail, and correspond respectively to the following periods: c. 2.06-1.99 Ga, c. 1.95-1.96 Ga, c. 1.86-1.90 Ga, c. 1.76 Ga and c. 1.70 Ga. Present results suggest that the basement is mostly formed by leptynite and orthogneiss of Late Rhyacian age at c. 2.05 Ga. This basement was affected by two major phases corresponding to two successive but distinct granulite facies events at c. 2.0 Ga and c. 1.90 Ga. The first high-grade event is followed by an exhumation event concomitant to erosional processes and sedimentary deposition. The second high-grade metamorphic event occurred at c. 1.90 Ga and is contemporaneous to gabbonoritic magma intrusion. A retrograde amphibolite/greenschist facies metamorphism followed at 1.76-1.78 Ga. This study also highlighted the influence of fluid phases in granulitic rocks and their potential importance in supercontinent break-up. The thermal evolution of the IGU ended with cooling until c. 1.7 Ga. Finally, the Pan-African event did not affect the U-Pb system of zircon, monazite and especially rutile and apatite in the granulitic rocks studied, suggesting that the IGU represents a preserved segment of Paleoproterozoic granulitic crust.