Is the Arabian Nubian Shield with Westward Subduction Polarity? Clues from Prograding Metamorphism in Mantle Peridotites, Eastern Desert of Egypt.

Abstract:

Neoproterozoic arc mantle beneath the Arabian Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt exhumed due to intra-oceanic upthrusting are represented mainly by exposed ophiolitic peridotites serpentinized to different degree. Metamorphism is related to the Pan-African collision and the subduction of oceanic lithosphere. However, polarity of the Pan-African intra-oceanic subduction is still questionable. We here trace the variation of the degree of serpentinization and regional metamorphism of six serpentinite masses, widely distributed in the ED (from the east to the west: W (Wadi). Alam, W. Igla, W. Mubarak, G. El-Maiyit, W. Um El Saneyat and W. Atalla). This is based on their mineralogy, textures and mineral chemistry.

Much difference in the degree of serpentinization is obvious among these rocks. They are mainly partly serpentinized containing primary olivine and orthopyroxene at W. Alam and W. Igla, while they are completely serpentinized in the other localities. With the increased degree of metamorphism, textures were transformed from the pseudomorphic to the non-pseudomorphic. The most common retrograde assemblage is composed of lizardite _ chrysotile_ brucite_ magnetite. The serpentine prograde textures can be viewed as a continuum from retrograde lizardite pseudomorphic textures, to very fine-grained transitional texture of lizardite and chrysotile, to chrysotile antigorite interlocking texture and finally to antigorite interpenetrating texture. These textures appear to represent successive stages in a recrystallization event. In late subduction-related metamorphism and early collisional emplacement stage, mylonitic-antigorite serpentinites formed and antigorite became the major phase in G. El-Maiyit, Um El-Saneyat andW. Atalla.

Compositional zoning in spinel grains in all serpentinites reflect variation in the degree of alteration. The biggest variation of spinel compositions are among serpentinites from Um El-Saneyat and W. Atalla. The alteration zones were formed in a temperature < 400 °C to 550 °C corresponding to the low green-schist to the lower amphibolite facies.

So, we propose that this is concordant with a westward polarity of the subducting oceanic lithosphere, associating the intra-oceanic arc ophiolite during the closure of the Mozambique ocean.