Abstract

The Jebel Thurwah manc-ultramafic igneous complex occupies some 40 km2 in W-central Saudi Arabia and is a largely inverted, structurally dismembered mass of tectonized harzburgite with dunitic lenses, layered mafic and ultramafic plutonic rocks, varitextured gabbros, tonalites and plagiogranites, sheeted dykes and pillow lavas. Major oxide and trace element geochemistry suggest that the complex was originally oceanic lithosphere formed above a subduction zone. During a subsequent collision event, this ophiolite was thrust on to an Upper Proterozoic island arc sequence composed largely of andesitic-rhyodacitic volcanic rocks of tholeiitic transitional to calc-alkaline affinity. Regional studies indicate that the Thurwah mass lies in a NE-SW zone of ophiolite complexes extending from Bir Umq, 150 km to the NE, to the Khor Nakasib and Wadi Amur bodies in the NE Sudan, that marks a suture between island arc systems subsequently accreted to form the Upper Proterozoic continental crust of the Arabian-Nubian Shield.

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