Home – Home – Jornals – Russian Geology and Geophysics 2012 number 1

The paper focuses on the metamorphic geology of the oldest crustal eclogites discovered in the Late Archean tonalite-trondhjemite-granodiorite (TTG) complex of the Belomorian Mobile Belt on the Kola Peninsula. Eclogite bodies are, most likely, widespread. We studied one of the key objects, the Kuru-Vaara quarry, where several tens of retrogressed eclogite blocks randomly embedded in the TTG gneisses were stripped at the benches. Based on the field observations, two visually different types of eclogites have been recognized: "southern", strongly retrogressed coarse-grained, and "northern", well-preserved fine-grained. The southern eclogite blocks bear evidence for their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence for melting. Despite the significant mineralogic difference, both types of eclogites can be assigned to amphibole eclogite facies. The applied jadeite solubility geobarometers yielded the minimum pressures of ~12 kbar for the northern eclogites and ~14-15 kbar for the southern ones. The used geothermometers yielded ~700 and ~750?C, respectively. But the presence of quartz lamellae in Na-clinopyroxenes in both types of eclogites and their bulk compositions corresponding to high-Mg basalts indicate that the Kuru-Vaara eclogites might have reached ultrahigh-pressure metamorphism. Analysis of the tentative P-T paths of metamorphic evolution for both types of eclogites showed that their burial-exhumation cycle might have taken as short as a few million years.
The set of presented data suggests that the formation of the Kuru-Vaara eclogites was related to the subduction of the Archean oceanic crust, which should have differed in composition and structure from the modern oceanic crust.