THE ECLOGITES OF THE MARUN - KEU COMPLEX, POLAR URALS (RUSSIA): FLUID CONTROL ON REACTION KINETICS AND METASOMATISM DURING HIGH P METAMORPHISM

Abstract

The Marun–Keu complex (Polar Urals, Russia) is a poorly known member of a group of high P complexes outcropping along the length of the Uralide orogen. The central and southern parts of the complex are metamorphosed at high P, medium T conditions (Tmax∼600–650 °C and P∼14–17 kbar) and differ from its northern part and the rest of the Uralian high P complexes which are metamorphosed at blueschist-to-low T eclogite-facies conditions.The Marun–Keu complex consists of Neoproterozoic to Cambrian volcanic-sedimentary sequences with a large variety of intrusive mafic to felsic rocks. Based on the nature of protoliths and the mode of occurrence, it is distinguished: (1) eclogite-facies rocks after intrusive protoliths, which vary from metagabbros to metagranites; (2) eclogitic quartzofeldspathic gneisses; (3) metasomatic eclogites; and (4) amphibolite–eclogite alternations produced by fluid infiltration during uplift.Igneous textures and mineralogy are preserved in non-sheared, intrusive, dry rocks, whereas eclogitization may be complete along centimetre to 10-m-thick shear zones and in domains infiltrated by H2O-dominated fluids. The most important reaction features are: (1) transformation of igneous diopside (Na∼0.04–0.09 apfu, based on 6 oxygens) into Na-rich diopside and omphacite (Na∼0.24–0.45 apfu) across microfractures and grain boundaries; (2) replacement of plagioclase by tiny aggregates of zoisite/clinozoisite+white mica+garnet (Alm32–43, Pyr7–19, Gro38–60)+kyanite; and (3) double coronas of garnet and orthopyroxene at olivine–clinopyroxene and olivine–plagioclase pseudomorph interfaces and garnet coronas at clinopyroxene–plagioclase pseudomorph interfaces.Eclogitic quartzofeldspathic gneisses display a fine-scale layering of intermediate and felsic rock compositions with omphacite-bearing and omphacite-free assemblages, respectively. Oligoclase is abundant in this type of rocks, coexisting with omphacite in the intermediate rock compositions.The studied area presents a large variety of veins and metasomatic mineral sequences developing in the host-rock adjacent to the veins. High P minerals (garnet (Alm40–60, Pyr25–40, Gro11–23), omphacite, phengite, paragonite) occur in both vein infillings and wall-rock mineral sequences, indicating that metasomatism was caused by the infiltration of out of equilibrium fluids (mostly silica-rich, alkali-rich compositions) during the eclogite-facies metamorphism. In zones of high fracture density, vein networks divide the host-rock into decimetre-scale blocks with omphacite-rich rinds replacing amphibolite cores. In contrast, metasomatic replacement of mafic eclogites by dendritic amphibole with barroisite composition (M4Na∼0.64 apfu, based on 23 oxygens) also occurred at eclogite-facies conditions. We outline the importance of considering such metasomatic processes for the correct evaluation of P–T–t metamorphic histories.The Marun–Keu complex is another example that highlights the important control exerted by fluids on reaction kinetics during high P metamorphism and further points to an important metasomatic effect of these fluids.