Retrograde evolution of eclogite facies rocks from NE Oman: evidence from fluid inclusions and petrological data

Eclogite facies rocks from eastern Saih Hatat (Oman), metamorphosed under p eak conditions of P > 12 kbar, and T of 550-580 degrees C, have several typ es of fluid inclusions in quartz, clinopyroxene, and garnet. Most of these inclusions are two-phase at room T, and contain an aqueous fluid belonging to the system NaCl-MgCl2-H2O. Final melting temperatures for most inclusion s range from -4 to 0 degrees C, but values as low as -13 degrees C have bee n recorded, indicating salinities of < 17 wt.% equivalent NaCl. Homogenizat ion temperatures for these inclusions range from 112 to > 490 degrees C. In clusions with three phases at room I also contain a predominantly low salin ity aqueous fluid with XCO2 < 0.05. None of the isochores calculated for is olated inclusions in quartz or clinopyroxene intersect the peak metamorphic conditions of these rocks, suggesting that these inclusions stretched or r e-equilibrated during near isothermal decompression from > 12 kbar to simil ar to 4 kbar or less ar T similar to 500-550 degrees C. This conclusion is supported by the scatter in homogenization temperatures for these inclusion s, and their large variety of shapes and sizes which range from irregular w ith hair-like fractures surrounded by satellite inclusions to negative crys tals. The replacement of ferroan dolomite/ankerite and quartz by calcite talc, and of rutile + calcite by titanite or rutile + glaucophane by ilmeni te + albite in these eclogites are also consistent with a stage of isotherm al decompression. Following near isothermal decompression to depths of simi lar to 10-15 km, brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz +/- ca lcite. This stage was followed by near isobaric cooling recorded by the iso chores for fluid inclusions in vein quartz and by the occurrence of dendrit ic networks of decrepitated and 'imploded' fluid inclusions in these veins, (C) 1999 Elsevier Science B.V. All rights reserved.