Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continentalshelf deposits in the Saih Hatat tectonic window of northeasternOman, where they structurally underlie the Semail ophiolite.These eclogites and blueschists constitute part of a coherenthigh-P/T metamorphic terrain comprised of interlayered metabasites,metapelites, and quartzofeldspathic and calcareous mica schists.The metabasites record a continuous regional trend of increasingP and T from west to east, where crossite-epidote schists incentral and western Saih Hatat grade into blueschists and eclogitesat As-Sifah, allowing for the subdivision of this terrain intothree metamorphic zones. P-T conditions range from 4?5 to 5?5kb atT <340?C for zone A to >10?2 kb at 500–580?Cfor zone C. Zoning patterns of garnets, pyroxenes, and phengite, and thecompositions of pyroxene and amphibole inclusions in garnet,indicate that the eclogites and enclosing high-grade blueschistsof zone C followed a ‘clockwise’ P-T rpath of increasingP and T followed by increasing T and decreasing P and a finalstage of retrogression. Garnets and pyroxenes crystallized atdifferent stages of P-T evolution of the terrain in the variouseclogite pods and boudins. This conclusion and the contrastingmineralogies of the eclogites and the enclosing blueschistsare attributed to differences in bulk-rock chemistry, fluidavailability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a resultof A-type subduction and crustal thickening of the Oman continentalmargin along an east-northeast-dipping thrust or ‘subductionzone’ which was initiated by the change in plate motionbetween Africa and Eurasia at 131 Ma. High P/T metamorphismcontinued until the final stages of ophiolite emplacement inthe late Cretaceous, as indicated by the metamorphism of thethrust sheets overlying the coherent terrain.