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 ( T max∼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 H 2O-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 (Alm 32–43, Pyr 7–19, Gro 38–60)+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 (Alm 40–60, Pyr 25–40, Gro 11–23), 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.