MODE OF OCCURRENCE AND TECTONIC SETTING OF NEOPROTEROZOIC OPHIOLITES OF THE CENTRAL EASTERN DESERT OF EGYPT: IMPLICATIONS FOR COLLISIONAL TECTONICS

Numerous ophiolite occurrences are reported in central eastern desert of Egypt. Based on geological setting, ophiolites occur as intact ophiolite succession, exotic masses, blocks and fragments in mélange matrix and dismembered slices and sheets along inherent planes of weakness essentially within host rocks other than mélanges. Some ophiolitic rocks are closely associated with arc volcanic and not included in mélanges. They are made up of serpentinized peridotite and metamorphosed ultramafics, metagabbros, pillowed and massive metabasalts, sheeted dykes and pelagic rocks.

Most of the serpentinites and metamorphosed ultramafic ophiolite blocks within the melange, display mainly supra-subduction zone (SSZ) tectonic setting but other minor blocks show MORB tectonic setting. There are two groups of ophiolitic metagabbros, a high-Ti, high- Zr group and appears to posses N-MORB features and a low-Ti, Low Zr group displays arc-like features which may suggest a supra-subduction zone setting (SSZ). The basaltic rocks at the top of intact ophiolites and the blocks of metabasalts within the mélange matrix apparently displaying N-MORB affinity and are closely comparable to MORB or back-arc basin basalt (BABB) ophiolites. The basaltic rocks associated with arc volcanics and boninites have arc affinity and display SSZ tectonic setting.

The rocks of the ophiolites have undergone two main phases of deformation and metamorphism: The first deformation phase (D₁) was manifested in imbricate thrusting (T₁) and folding (F₁), cleavage formation (S₁), stretched lineation (L₁), shearing and stratal disruption was related to the tectonic emplacement of the ophiolites. During D₂the rocks are later disrupted and tectonically emplaced along low angle thrust faults (T₂) and strike-slip shear zones. There are features indicative of two phases of metamorphism; ocean-floor event and regional metamorphism.

Ophiolites of the central eastern desert fall into two groups, MORB or BABB and SSZ ophiolites, which are spatially and temporally unrelated and thus it seems likely that the two types are not petrogenetically related. Ophiolites underwent multiple phases of deformation and metamorphism, and successive tectonic emplacement as a consequence of a collisional events during back arc–arc-continent collision.