Are pressures for blueschists and eclogites overestimated? The case from NE Oman

Abstract

The high pressure, low temperature metamorphic rocks of the structurally deepest regions of Saih Hatat, NE Oman exhibit an eastward increase in metamorphic grade from epidote blueschist facies conditions in central Saih Hatat to eclogitic conditions along the eastern coast. Mineral assemblages in these rocks allow the application of one or two reliable experimentally calibrated geobarometers, which can only constrain either minimum or maximum pressures of metamorphism. Thermodynamic calculations of experimentally uncalibrated pressure-sensitive equilibria, considered useful geobarometers by some authors, and their application to these high pressure–low temperature rocks, yields unreasonably high pressures which are inconsistent with phase relations. Calculation of stable and metastable multivariant equilibria also seldom yields a unique P– T point of equilibration for each sample, and results in widely variable P– T estimates for rocks from the same outcrop. Calculated activity corrected petrogenetic grids for mineral assemblages from different interlayered and cofacial lithological units, combined with jadeite+quartz barometry and garnet–clinopyroxene thermometry, lead to the conclusion that the epidote–blueschist facies rocks formed at T=400–460°C, P=6.5–8.5 kbar, whereas the eclogite facies rocks formed at T=550–580°C, P=12–16 kbar. These results suggest that the use of experimentally uncalibrated pressure-sensitive equilibria or of intersections of stable and metastable multivariant equilibria for thermobarometry in blueschists and eclogites is fraught with uncertainties, and could lead to large errors. Such results should not be accepted unless found to be consistent with phase relations in all interbedded cofacial lithological units.