Fluid circulation related to deformation in the Zabargad gneisses (Red Sea rift)

Abstract

A fluid inclusion study in relation to textures has been performed on Pan-African granulite-facies gneisses from Zabargad Island (Red Sea rift, Egypt). The gneiss samples were collected at a maximum distance of 60 m above the contact with mantle peridotites with which they are co-structured by a high- T plastic deformation during an isothermal decompression starting from conditions estimated at 35 km depth and 850°C (base of the Pan-African crust). A first metasomatism of the crustal section during decompression is represented by CO 2-fluid inclusions trapped at minimum conditions of 15 km depth and 800°C. The CO 2-fluid inclusion planes are controlled in orientation by the high- T plastic flow structures, but have been subsequently deformed during medium- T plastic deformation in quartz. The second metasomatism is represented by solid-rich aqueous fluid inclusions which were trapped at minimum P-T conditions of 3.7 km depth and 450°C corresponding to a very high thermal gradient. They trace the introduction either of seawater or of fluids resulting from leaching of the Middle Miocene evaporites, within the gneisses during their residence at a shallow depth below the seafloor. Two models are proposed in the literature in order to reconstruct the geodynamic history of Zabargad Island: one [Brueckner, H.K., Elhaddad, M.A., Hamelin, B., Hemming, S., Kröner, A., Reisberg, L., Seyler, M., 1995, A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: a Nd, Sr, Pb and Os isotope study, J. Geophys. Res. 100, 22283–22297] suggests that most of the decompression P-T path is Pan-African in age and only the aqueous metasomatism is related to the rifting history of the Red Sea; the second [Nicolas, A., Boudier, F., Montigny, R., 1987, Structure of Zabargad Island and early rifting of the Red Sea, J. Geophys. Res. 92, 461–474; Boudier, F., Nicolas, A., Ji, S., Kienast, J.R., Mevel, C., 1988, The gneiss of Zabargad Island: deep crust of a rift, Tectonophysics 150, 209–227] supposes the existence of an asthenospheric diapir intrusive through the deep Pan-African crust leading to a continuous deformation event during early rifting of the Red Sea. Although it is impossible to date trapping of fluid inclusions and to know with certainty the origin of the fluids, our preference is for the second model but slightly modified. The high- T decompression path traced by CO 2-fluid inclusions, relayed by solid-rich aqueous inclusions, traces a continuous process of crustal thinning. Such a continuous path would assign to a single event the thinning and uplift of the deep crust and uppermost lithosphere to shallow basin bottom during the Red Sea early rifting. In such an interpretative model, the northern and central peridotite bodies are representative of the uppermost continental lithosphere and only the southern body could belong to an ascending asthenospheric diapir centred on the southeast of Zabargad Island.