STRUCTURE AND METAMORPHISM OF BLUESCHIST ECLOGITE-FACIES ROCKS FROM THE NORTHEASTERN OMAN MOUNTAINS

The northern part of the Saih Hatat window, Oman shows high-pressure m etamorphic rocks derived from shelf sediments and pre-Premian continen tal 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 obduct ion, although now many contacts are backthrusts, normal faults or exte nsional shear zones. Metamorphic breaks exist across many unit boundar ies. The deepest unit (As Sifah) has eclogite-facies assemblages in me tabasites and metapelites which record evolution of P-T conditions alo ng a clockwise path culminating at 23 +/- 2.5 kbar, 540 +/- 75-degrees -C, contrasting markedly with overlying units (5-10 kbar, 200-500-degr ees-C), although separated by <10 km on the ground. The dominant penet rative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-fac ies extensional fabric. Phengites from eclogite-facies schist show dis cordant Ar-40/Ar-39 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 emplacemen t against other units at c. 20-25 km depth, and (ii) exhumation of the entire high-P zone by culmination collapse after obduction.