Abstract
The mafic-ultramafic igneous complex occupying some 40 km (super 2) in west-central Saudi Arabia was tectonically dismembered and emplaced as thrust slices into an island-arc volcano-sedimentary sequence >680 m.y. ago. It contains tectonized harzburgite with dunitic lenses, layered mafic and ultramafic plutonic rocks, as well as gabbros, tonalites and plagiogranites, and also sheeted dykes and pillow lavas. Major-oxide and trace-element analyses suggest that the complex was originally oceanic lithosphere formed above a subduction zone. During a subsequent collison, this ophiolite was thrust onto an Upper Proterozoic island-arc sequence of andesitic-rhyodacitic rocks of tholeiitic transitional to calc-alkaline affinity. This mass lies in a NE-SW zone of ophiolite complexes extending from Bir Umq (150 km to the NE) to NE Sudan that marks a suture between island-arc systems subsequently accreted to form the continental crust of the Arabian-Nubian Shield.