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

Abstract

Eclogite facies rocks from eastern Saih Hatat (Oman), metamorphosed under peak conditions of P>12 kbar, and T of 550–580°C, have several types 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–MgCl 2–H 2O. Final melting temperatures for most inclusions range from −4 to 0°C, but values as low as −13°C have been recorded, indicating salinities of <17 wt.% equivalent NaCl. Homogenization temperatures for these inclusions range from 112 to >490°C. Inclusions with three phases at room T also contain a predominantly low salinity aqueous fluid with XCO 2<0.05. None of the isochores calculated for isolated inclusions in quartz or clinopyroxene intersect the peak metamorphic conditions of these rocks, suggesting that these inclusions stretched or re-equilibrated during near isothermal decompression from >12 kbar to ∼4 kbar or less at T∼500–550°C. This conclusion is supported by the scatter in homogenization temperatures for these inclusions, and their large variety of shapes and sizes which range from irregular with hair-like fractures surrounded by satellite inclusions to negative crystals. The replacement of ferroan dolomite/ankerite and quartz by calcite+talc, and of rutile+calcite by titanite or rutile+glaucophane by ilmenite+albite in these eclogites are also consistent with a stage of isothermal decompression. Following near isothermal decompression to depths of ∼10–15 km, brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz±calcite. This stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and `imploded' fluid inclusions in these veins.