STABLE ISOTOPIC RESULTS FROM METAMORPHOSED ARABIAN PLATFORM ROCKS FROM THE SAIH HATAT DOME: EVIDENCE FOR FABRIC DEVELOPMENT ASSOCIATED WITH EXHUMATION

The Saih Hatat dome is a window through the allochthonous Samail ophiolite complex to the underlying metamorphosed Arabian platform of Oman. The dome refolds a Cretaceous nappe structure whose upper limb is pinned to unmetamorphosed Arabian platform rocks to the south. The lower limb of the regional scale nappe is truncated against a low angle shear zone that juxtaposes carpholite-bearing rocks of the upper plate nappe against a complex of lower plate retrogressed blueschist and eclogite facies rocks. The lower plate consists of a sequence of quartz mica schist, mafic schist and calcschist. Stable isotopic analyses were performed on numerous whole rocks and their coexisting minerals to infer conditions of metamorphism and length scales of exchange during deformation. There is a general lowering of carbonate whole-rock d ¹⁸O values going from upper plate rocks of Wadi Meeh (22.7 < d ¹⁸O < 29.0; mean 25.6 per mil) to lower plate carbonates of As Sifah (12.1 < d ¹⁸O < 28.8; mean +20 per mil). These measurements are consistent with higher strain and exchange with infolded silicate rocks at the higher temperatures experienced by the lower plate rocks. Upper plate carbonate carbon isotope ratios are generally >0 consistent with their Permian depositional age whereas highest grade lower plate marble and calcschist generally have d ¹³C <0 inconsistent with a Permian age or possibly modified by metamorphism. The lowest ¹⁸O carbonate is from a marble boudin adjacent to an eclogite pod; the isotope data from coexisting calcite, phengite and quartz suggest temperatures of equilibration >450 °C. Quartz-pyroxene and quartz-glaucophane pairs from eclogite boudins give D values of 3.8 and 4.0 per mil, respectively, spread along a slope 1 array some 4 per mil long. These measured D values are consistent with peak metamorphic temperatures in excess of 550 °C. In contrast, quartz-white mica pairs cluster along a D = + 3.5 line over a 8 per mil range. Quartz-phengite pairs are consistent with fabric growth at lower temperatures (» 450 °C) during deformation associated with exhumation. The oxygen isotope results support the Ar geochronology data that indicates older ages for fabrics formed at higher temperatures.