Abstract

The Tilemsi magmatic arc, preserved along the suture zone of the pan-African trans-Saharan belt of northern Mali, crops out as a series of northeast- to north-northeast-trending strips along the Tilemsi Mesozoic trough and is about 100 km in width. The volcanic arc series includes pillowed metabasalts of tholeiitic character and associated with rhyodacites. Overlying sedimentary rocks are turbiditic volcanic greywackes. They are progressively recrystallized into grey gneiss in the vicinity of gabbro-noritic and dioritic intrusions. U/Pb zircon dating of a crosscutting metaquartz diorite gives a nearly concordant age of graphic, while that of a plagiogranite mobilizate associated with the grey gneiss is graphic. Initial Nd and Sr isotopic compositions of two metaquartz diorites (εNd730 = +6.6, +6.3; (87Sr/86Sr)i = 0.7024) are in a agreement with a depleted mantle source similar to modern intraoceanic arcs. Isotopic compositions of two Tilemsi metagreywackes (εNd730 = +5.8, +4.3; 87Sr/86Sr ≈ 0.7027) exclude any significant derivation from an older sialic source and support the ensimatic origin of the magmatic arc. A U/Pb zircon age of 635 ± 5 Ma has also been obtained on a pretectonic granodiorite batholith at the eastern margin of the arc. Isotopic composition of this intrusion (εNdi = −6.0, −6.4; (87Sr/86Sr)i = 0.7046) illustrates the lack of a genetic link between the 730 Ma old, mantle-derived magmas and these granitoids, which originated from a crustal reservoir. This change in magmatic source is interpreted as the result of accretion of the ensimatic arc along the eastern continent, preceding continent–continent collision during the pan-African event.

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