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 crust

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 precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (100 °C/Ma), which could result from heat advection by large-scale fluid circulation.Editorial responsibility: W. Schreyer     

Stability of phlogopite in granitic melts, an experimental investigation

Water-saturated and water-undersaturated experiments (a _{H2 O} = 1.0 and 0.5) were performed in the temperature range 780–1040°C at 2 and 5 kbar in order to determine the upper thermal stability of phlogopite in granitic melts. Starting compositions were: (A) subaluminous mixtures of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous haplogranitic glass; (B) peraluminous mixtures (normative corundum  = 4 %) of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous peraluminous haplogranitic glass. The molar quartz: albite: orthoclase ratio of the glasses of the 2␣kbar runs was 35:39:26 and that of the 5 kbar runs 30:42:28. In the subaluminous system, phlogopite is stable up to 820°C at a _{H2 O} = 1.0 and up to 780°C at a _{H2 O} = 0.5. At higher temperatures, it is replaced by enstatite. In the peraluminous system phlogopite has a remarkably higher thermal stability (up to 1000°C at 5 kbar and a _{H2 O} = 1.0) and there is a temperature interval of 80°C at a _{H2 O} = 1.0, and 90–100°C at a _{H2 O} = 0.5 between the first appearance of enstatite and the disappearance of phlogopite. In the peraluminous system, phlogopite is a solid solution (ss) of phlogopite, muscovite, talc and eastonite components. The crystalline product of the phlogopite_ss breakdown reaction is an aluminous enstatite. The MgO-content of the melt depends on the normative corundum content of the starting material and the run temperature. It is independent of pressure. In the subaluminous system, the MgO-content ranges between 0.05 and 0.3 wt % in the temperature interval 780–880°C at both investigated water activities. The MgO-content of the peraluminous melts at a _{H2 O} = 1.0 ranges between 0.4 and 1.7 wt % and at a _{H2 O} = 0.5 between 0.2 and 1.4 wt % in the temperature range 780–980°C. Received: 28 August 1995 / Accepted: 6 August 1996