Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

Abstract

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of P – T conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by <l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant 40 Ar/ 39 Ar apparent ages. Our tectonic model relates all the high- P units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at c . 20–25 km depth, and (ii) exhumation of the entire high- P zone by culmination collapse after obduction.