Fluid inclusions in carpholite-bearing metasediments and blueschists from NE Oman: Constraints on P-T evolution

Abstract

Thrust sheets of the upper plate of the Saih Hatat window, NE Oman contain metasediments formed under lawsonite albite facies conditions. Quartz and Fe-Mg carpholite in these metasediments contain several types of fluid inclusions; most are two-phase at room T with an aqueous NaCl - H2O or NaCl - MgCl2 - H2O solution. Texturally early fluid inclusions display a wide range of final melting (Tmf) and homogenization (Th) temperatures. The lowest Tmf and Th values were recorded for moderately sized inclusions occurring in clusters. Some of the highest Tmf and Th values (> 320°C) are recorded for isolated inclusions in Fe-Mg carpholite and quartz. In the lower plate epidote-blueschist facies metasediments, many fluid inclusions in quartz are decrepitated, with most of the surviving inclusions being 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 Th values are consistent with their respective P-T estimates. We conclude that inclusions with high Th experienced post-entrapment stretching ± loss of H2O during exhumation. 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 peak conditions.