Neoproterozoic Ophiolites of the Arabian-Nubian Shield

Abstract

Ophiolites of mid-Neoproterozoic age are abundant in the Arabian-Nubian Shield (ANS) of NE Africa and Arabia. The ANS ophiolitic mantle was mostly harzburgitic, containing magnesian olivines and spinels that have compositions consistent with extensive melting. Cumulate ultramafics transition upwards into layered gabbro. Several crystallization sequences are inferred from ANS transition zones and cumulate gabbro sections. In all samples studied, olivine and spinel crystallized first, followed by cpx-plag, cpxopx-plag, and opx-cpx-plag. The ANS in NE Africa and W. Arabia is the largest tract of juvenile continental crust of Neoproterozoic age on Earth. The best preserved ophiolites in the northern ANS contain all or most of the components of complete “Penrose” ophiolites, including pillowed basalts, gabbros and tectonized harzburgites. Fragments from dismembered ophiolites are common in the ANS, and it becomes more difficult to interpret these as once being allochthonous pieces of oceanic crust as these fragments become more deformed and metamorphosed.