MICROTHERMOMETRY OF FLUID INCLUSIONS IN FE-MG CARPHOLITE AND QUARTZ FROM SAIH HATAT, OMAN: CONSTRAINTS ON P-T EVOLUTION

Fe-Mg carpholite bearing metasediments occur in several thrust sheets of basement and continental shelf units in the central part of the Saih Hatat window, NE Oman. These thrust sheets, which structurally underlie the Semail ophiolite, are thrust onto a lower plate of coherent basement and shelf units, metamorphosed under blueschist to eclogite facies conditions. Rocks of the upper plate thrust sheets exhibit a down-section increase in metamorphic grade from < 300°C, 3 – 6 kbar for the structurally highest units to 380 – 430°C, 6.5 – 9 kbar for the deepest ones. P-T estimates for the lower plate crossite-epidote schists are 400 – 460°C, 6.5 – 8.5 kbar. All rock types had clockwise P-T paths.

Several types of fluid inclusions were identified in quartz and Fe-Mg carpholite (X_Mg ~ 0.35) in Upper plate metasediments. Most inclusions are two-phase at room T, and contain an aqueous NaCl – H₂O or NaCl – MgCl₂ – H₂O solution. Texturally early fluid inclusions (isolated or in clusters) display a wide range of final melting temperatures (T_mf) and homogenization (T_h). The lowest T_mf and T_h values of –12.4 and 106°C were recorded for moderately sized inclusions occurring in clusters. Some of the highest T_mf (> 0) and T_h values (> 320°C) are recorded for isolated inclusions in Fe-Mg carpholite and quartz. In lower plate crossite-epidote schists, most fluid inclusions in quartz are decrepitated along grain boundaries. Most of the surviving inclusions are 3 – phase, CO₂ – bearing filled with a low salinity fluid with XCO₂ < 0.05. For all samples from both plates, isochores calculated for the few texturally early inclusions with low T_h values are consistent with their respective P-T estimates.

We conclude that most inclusions with high T_h experienced post-entrapment stretching ± loss of H₂O during exhumation based on (i) the scatter in T_h and T_mf values, (ii) that most inclusions with T_h > 250°C have lower degrees of fill and relatively high shape factors, and (iii) that plots of T_h vs. T_mf for most samples show weak negative trends. Larger inclusions and inclusions in Fe-Mg carpholite were more strongly affected by these changes. Small isolated inclusions in relatively unstrained quartz have escaped these modifications and are useful for constraining P & T. Post-entrapment changes therefore depend on strain, P –T conditions and the host mineral.