Peridotites from the Island of Zabargad (St. John), Red Sea: Petrology and geochemistry

Abstract

Exceptionally fresh peridotite bodies outcrop on Zabargad Island, an uplifted fragment of sub‐Red Sea lithosphere. The peridotites are associated with basaltic dikes and are in tectonic contact with a metamorphic unit and with post‐Mesozoic sedimentary units. The peridotites can be divided into three main groups: (1) protogranular spinel Iherzolites (sp‐lherzolites), with average modal composition ol 65%, opx 16%, cpx 16%, spinel 3% (2) amphibole peridotites (amph‐peridotites), containing >2% magnesio‐hornblende (3) plagioclase peridotites (pl‐peridotites), containing >2% Ca‐plagioclase. Minor outcrops of dunite and wherlite were also observed. The pl‐peridotites and amph‐peridotites, which are found in localized zones or bands within the sp‐lherzolite, show textures ranging from por‐phyroclastic to cataclastic, indicating varying degrees of tectonic deformation. Olivine and opx have a rather constant composition in the three groups, Fo ranging between 87.3% and 90.5% and En between 88% and 89%, respectively. Clinopyroxene is chromian diopside but contains less Na in the pl‐peridotites than in the sp‐lherzolites. Both opx and cpx are moderately Al and Cr‐rich, as is typical of mantle‐equilibrated pyroxenes. Spinel has a very low Cr/Al ratio in the sp‐lherzolites, lower than in the pi‐ and amph‐peridotites. Plagioclase in the pl‐peridotites ranges between An 80% and 93%, while traces of it rimming spinel in some of the sp‐lherzolites are more sodic. The amph‐peridotites contain up to 28% magnesio‐hornblende and, in some cases, traces of phlogopite and apatite; opx, cpx, and spinel are scarce. The major element composition of the Zabargad sp‐lherzolites, their slight light rare earth element (LREE)‐depleted pattern, transition elements Sc, Ti, Cr, Mn, Fe, Co, and Ni data, together with modal and mineral chemistry data, are all consistent with the sp‐lherzolites having last equilibrated in the sp‐lherzolite stability field (>9 kbar, >30 km deep) and representing essentially undepleted parental mantle material, though some samples might have undergone minor partial melting. The pl‐peridotites may represent localized incor‐poration of a melt component by the ascending Iherzolite body. The amph‐peridotites are enriched in K, LREE, and occasionally in P relative to sp‐lherzolites; they were probably formed by localized contamination with H2O‐rich metasomatic fhids injected through the Iherzolite body during its ascent. The Zabargad peridotites were probably emplaced from the upper mantle into the crust during the development of the Red Sea rift, i.e., in post‐Mesozoic time. They show affinity with some mantle‐derived oceanic ultramafics, particularly with St. Paul Rocks in the Atlantic. They could be considered a sample of oceanic mantle before extraction of the basaltic oceanic crust.