A new ophiolite occurrence in NW Sudan – constraints on Late Proterozoic tectonism

Mafic–ultramafic sequences in the Jebel Rahib fold‐and‐thrust belt of NW Sudan comprise the metamorphosed equivalents of obducted oceanic lithosphere. Primary features like pillow‐structures, quenchtextures, igneous layering and primary minerals like chromian spinels and pyroxenes are partly preserved. Geochemical investigations confirm the field evidence and reveal the occurrence of harzburgites, ophiolitic chromites, gabbros, gabbroic cumulates and basalts of primitive tholeiitic compositions resembling transitional MORBs. The age of low‐grade overprint of the basaltic rocks can be limited to 860–740 Myr (K–Ar). The existence of an ophiolite, as well as its close interfingering with highly deformed basin sediments, provides unambiguous evidence for a cycle of extensional and compressional tectonics in NW Sudan during Pan‐African times. Consequently, the concept of an older, stable Proterozoic, or even Archaean, ‘African Craton’onto which the juvenile Pan‐African Nubian Shield assemblages were accreted, needs revision. Copyright © 1990, Wiley Blackwell. All rights reserved