EVOLUTION OF THE NEOPROTEROZOIC DELGO SUTURE ZONE AND CRUSTAL GROWTH IN NORTHERN SUDAN - GEOCHEMICAL AND RADIOGENIC ISOTOPE CONSTRAINTS

In the Delgo basement area of northern Sudan, low to medium grade meta morphosed volcanic, sedimentary and plutonic rocks are surrounded by h igh grade gneisses. A NNE-SSW trending suture zone can be defined by t he lithological, chemical and structural characteristics of several di stinct units. The early Proterozoic gneiss terrain is overlain by meta sedimentary units, the metamorphism of which has been dated by the Sm- Nd whole rock - mineral technique (702 +/- 27 Ma in the west, 592 +/- 16 Ma in the east). In the central part, the Abu Sari volcanic rocks s how geochemical signatures of formation at an arc, with a protracted t holeiitic, calc-alkaline and shoshonitic evolution. The overlying El H amri ophiolite contains chemical features of a back-arc tectonic envir onment. The ophiolite was dated by the Sm - Nd whole rock method on me tagabbros at 752 +/- 48 Ma. The further extension of this oceanic basi n into the Jebel Rahib in the south-west was dated at 707 +/- 54 Ma (S m - Nd whole rock and minerals). A widespread suite of syn-tectonic gr anitoid intrusives displays subduction-related characteristics. They w here emplaced between 650 to 760 Ma (Pb-zircon evaporation method). Th eir Nd and Sr isotopic compositions indicate a changing pattern of isl and arc to active continental margin character along an east - west tr ansect and suggest a west to north-west dipping subduction zone. All u nits were juxtaposed at the minimum age of almost-equal-to 600 Ma and rearranged during an extensional event, which was dated by the Rb-Sr t hin slab technique (546 +/- 19 Ma) on a migmatite. The Delgo suture pr ovides evidence of a complex terrane pattern in north-east Africa and crustal growth during the Pan-African event by the addition of oceanic material to pre-existing continental crust.