Résumé
The Arabian-Nubian Shield (ANS) represents a mosaic of more than 15 Neoproterozoic juvenile island-arc terranes (850-600 Ma) formed in the Mozambique ocean realm. In spite of amalgamation events that subsequently led to accretion of the ANS, most exposed terranes are characterized by low- to moderate metamorphic overprint, greenschist-facies rocks being prevalent in the shield. It is thus difficult to examine how and when the lower crust of the ANS formed since it is mostly inaccessible. In this study, we analysed 3 gneisses from Zabargad Island (Egypt) corresponding to lower crustal units exposed at surface level during the OligoMiocene thinning of the ANS lithosphere related to the Red Sea rifting. Zircons from a felsic granulite have both low and high Th/U ratios (0.3 and 0.07), but yield undistinguishable ages of 652 ± 10 Ma, which we interpret as dating the age of the protolith and its conversion to granulite. Some zircons however yield low Th/U ratios and a significantly younger age of 597 ± 4 Ma, reflecting a second metamorphic event, possibly related to collision between island arc terranes of the ANS and the Saharan craton on the West. Monazites from agranulitic gneiss yield a much younger age of 497 ± 2 Ma which may reflect far-field stress associated to the latest stages of amalgamation of eastern Gondwana. Lastly, zircons extracted from a granulitic gneiss sampled at the contact with the Northern peridotite massif yield an age of 25.1 ± 0.4 Marelated to contact metamorphism during juxtaposition of the hot peridotite and lower crustal units. Gneisses from Zabargad island constitute a unique window to look at the polymetamorphic evolution of high-grade lower crustal units of the ANS.