K-cymrite pseudomorphs in high-ultrahigh- pressure rocks

Abstract

Formation of new minerals in rocks of specific composition during prograde metamorphism depends mostly on pressure and/or temperature changes. However, some of these minerals are very sensitive to reverse reactions when the rocks are subject to decompression and/or cooling. This is well known from high- or ultrahigh-pressure rocks which poorly preserve or even lack a number of minerals, like lawsonite or phengite, whose former presence is expected based on the results of experimental data or thermodynamic modelling applied to a given rock composition. Another such mineral is K-cymrite that is stable at UHP conditions but has not been observed in UHP rocks returned to and exposed at the surface. It likely decomposes to K-feldspar or K-mica during temperature increase or pressure decrease. This study examines the textural and compositional relationships of pseudomorphs in blueschist and eclogite facies felsic and mafic lithologies from the Meliata unit (Western Carpathians) and the Bohemian Massif, concluding that they are best interpreted as alteration products of former K-cymrite. The calculated P-T conditions for the host rocks plot at or very near the experimentally constrained stability field of K-cymrite. The occurrence of these pseudomorphs across a range of lithologies suggests that K-cymrite may have formed abundantly and in a diverse range of bulk rock compositions during subduction under a low-temperature geothermal gradient of ~ 7 °C/km. However, due to its hydrous nature and narrow P-T stability field, it is most commonly transformed into other phases during exhumation and is rarely preserved as shape relics (pseudomorphs) because of ongoing deformation.