The alkaline-peralkaline granitic post-collisional Tin Zebane dyke swarm (Pan-African Tuareg shield, Algeria): prevalent mantle signature and late agpaitic differentiation

The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogen y along a mega-shear zone separating two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid terrane, but inside the adjacent Tassendjanet terrane, strongly remo bilized at the end of the Precambrian. The Tin Zebane swarm was emplaced du ring post-collisional sinistral movements along the shear zone at 592.2 +/- 5.8 Ma (19WR Ph-Sr isochron). It is a dyke-on-dyke system consisting of dy kes and stocks of gabbros and dykes of metaluminous and peralkaline granite s. All rock types have Sr and Nd isotopic initial ratios (Sr-i = 0.7028 and epsilon(Nd) = +6.2) typical of a depleted mantle source, similar to the pr evalent mantle (PREMA) at that period. No crustal contamination occurred in the genesis of the Tin Zebane swarm. Even the samples showing evidence of fluid interaction (essentially alkali mobility) have the same isotopic sign ature. The peralkaline granites have peculiar geochemical characteristics t hat mimic subduction-related granites: this geochemical signature is interp reted in terms of extensive differentiation effects due to late cumulates c omprising aegirine, zircon, titanite, allanite and possibly fergusonite, se parated from the liquid in the swarm itself due to magmatic flow turbulence . The Tin Zebane dyke swarm is thus of paramount importance for constrainin g the differentiation of mantle products to generate highly evolved alkalin e granites without continental crust participation, in a post-collisional s etting. (C) 1998 Elsevier Science B.V. All rights reserved.