ECLOGITES AND BLUESCHISTS FROM NORTHEASTERN OMAN - PETROLOGY AND P-T EVOLUTION

Eclogites and blueschists occur in the basement and continental shelf deposits in the Saih Hatat tectonic window of northeastern Oman, where they structurally underlie the Semail ophiolite. These eclogites and blueschists constitute part of a coherent high-P/T metamorphic terrain comprised of interlayered metabasites, metapelites, and quartzofeldspathic and calcareous mica schists. The metabasites record a continuous regional trend of increasing P and T from west to east, where crossite-epidote schists in central and western Saih Hatat grade into blueschists and eclogites at As-Sifah, allowing for the subdivision of this terrain into three metamorphic zones. P-T conditions range from 4·5 to 5·5 kb atT <340°C for zone A to >10·2 kb at 500-580°C for zone C.Zoning patterns of garnets, pyroxenes, and phengite, and the compositions of pyroxene and amphibole inclusions in garnet, indicate that the eclogites and enclosing high-grade blueschists of zone C followed a 'clockwise' P-T rpath of increasing P and T followed by increasing T and decreasing P and a final stage of retrogression. Garnets and pyroxenes crystallized at different stages of P-T evolution of the terrain in the various eclogite pods and boudins. This conclusion and the contrasting mineralogies of the eclogites and the enclosing blueschists are attributed to differences in bulk-rock chemistry, fluid availability, or fluid compositions within the terrain.The blueschists and eclogites of this terrain formed as a result of A-type subduction and crustal thickening of the Oman continental margin along an east-northeast-dipping thrust or 'subduction zone' which was initiated by the change in plate motion between Africa and Eurasia at 131 Ma. High P/T metamorphism continued until the final stages of ophiolite emplacement in the late Cretaceous, as indicated by the metamorphism of the thrust sheets overlying the coherent terrain. © 1990 Oxford University Press.