PACROFI VI - Electronic Program

Fluid Inclusion Evidence for Exhumation of Eclogites and Blueschists from Northeast Oman

El-Shazly*, Aley El-Din and Sisson**, Virginia B.

*Department of Earth Sciences, Sultan Qaboos University, P.O. Box 36, Al-Khod, PC123, Sultanate of Oman

**Department of Geology and Geophysics, MS-126, Rice University, Houston, Texas 77005-1892

Both fluid inclusion and retrograde mineral reactions imply an isothermal decompression of eclogites and blueschists following peak metamorphism in the eastern part of the Saih Hatat window of northeastern Oman. In this region, blueschists, eclogites and epidote amphibolites are interlayered on a millimeter to decimeter scale in metabasalts. These were metamorphosed during the Cretaceous reaching peak metamorphic conditions of temperatures of 500 - 580oC at pressures greater than 1000 -1200 MPa. According to El-Geochemical and stable isotope data indicate that the eclogites and blueschists represent different protolith compositions whereas the epidote amphibolites represent secondary assemblages related to channelized fluid flow during exhumation. Fluid inclusions were observed in the mafic eclogites as well as in late quartz veins which cross cut the complex. They have a range in textural types from solitary inclusions in clinopyroxene to secondary inclusions in quartz. Solitary inclusions in clinopyroxene are characterized by tubular of negative crystal shapes, with a few inclusions surrounded by satellite inclusions. This texture is probably indicative of overpressure during exhumation. Most of the fluid inclusions are aqueous with final melting temperatures between -2 to 0oC indicating low salinity brines. Initial melting temperatures suggest they probably belong to either the MgCl2-NaCl-H2O or NaCl-H2O system. Some later fluid inclusions in quartz veins contain minute quantities of CO2. These probably have less than 5 vol. % CO2 as only clathrate melting could be observed and there is not enough CO2 to form a second immiscible phase at room conditions. Homogenization temperatures cluster into three groups: >380oC for fluid inclusions in garnet or quartz in eclogites, 220-260oC in clinopyroxene and quartz segregations, and 170-180oC in vein quartz and rarely in eclogite. None of the isochores for these fluids intersect the metamorphic conditions. However, they do intersect retrograde reactions at pressures of 200-400 MPa at temperatures slightly below peak metamorphic conditions. This combined with the textural evidence for overpressuring and stretching of fluid inclusions supports an isothermal decompression path for the exhumation of the eastern Saih Hatat window.