Metasomatic hydrous fluids in amphibole peridotites from Zabargad Island (Red Sea)
Résumé
Combining textural, petrological, chemical and isotopic (Sr, H and O) data for amphiboles and whole rocks from the Zabargad peridotite diapir allows three different events to be distinguished. During each event, which can be related to a specific tectonic process of the rifting of the Red Sea, hydrous fluids produced amphiboles.
The first and the second generations of amphiboles have characteristics consistent with the involvement of mantle-derived hydrous fluids. The first generation consists of scarce Ti-pargasites which crystallized from small amounts of fluid at temperatures of around 900–1000°C. Their growth was linked to magma percolation in the peridotites before their deformation during diapiric uplift. The second generation consists of Cr-pargasites which crystallized locally (and abundantly) during reaction between the peridotites and a sodium/potassium-bearing hydrous fluid at temperatures of around 700–800°C. These amphiboles grew synchronously with the diapiric uplift. The hydrous fluids probably originated in the sub-continental mantle and were released during the diapiric uplift of the peridotites.
The third generation consists of amphiboles (pargasitic hornblende, hornblende sensu lato and tremolite) which are localized in shear zones and veins. They crystallized at temperatures estimated between 700°C and 450°C, again from a sodium/potassium-bearing hydrous fluid. However, this fluid is extraneous to the peridotites, as shown by the Sr, H and O isotope compositions which suggest seawater penetration either during or after the final emplacement of the peridotite diapir.
Although the peridotite diapir was emplaced in granulitic gneisses of the pan-African deep continental crust, no evidence was found for a contribution of hydrous continental fluids in the production of the amphiboles present in the peridotite bodies of Zabargad Island.