Eclogite-facies vein systems in the Marun-Keu complex (Polar Urals, Russia) : textural, chemical and thermal constraints for patterns of fluid flow in the lower crus

Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogite- facies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434 °C (relicts) to 526–668 °C (vein precipi- tates). Vein and metasomatic assemblages formed dur- ing a single fluid-rock interaction process, implying high heating rates (‡100 °C/Ma), which could result from heat advection by large-scale fluid circulation.