Interaction between mafic and felsic magmas in subvolcanic environment (Tastau igneous complex,eastern Kazakhstan)

The paper reports the results of integrated geological, petrological, geochemical, and geochronological studies of the Tastau igneous ring complex in the Zaisan orogen of eastern Kazakhstan. Interaction between felsic and mafic magmas has been studied. Hybrid rocks are represented by gabbros and diorites injected into a granitic magma chamber. They occur as dikes and pillow-like and globular mafic bodies variously disintegrated and mixed with host granitoids. The age of synplutonic rocks is 242 ± 20 Ma (U/Pb zircon dating), which is, with regard to analytical error, substantially younger than it was presumed.Mechanisms of interaction between felsic and mafic magmas have been studied. They include mechanical (mingling) and chemical (mixing) interaction, which produce composite mixtures and hybrid rocks. The ratios of mafic to felsic components involved in the formation of intermediate rocks were calculated from major elements by regression analysis and tested with regard to rare and trace elements. The model for mingling includes rapid quenching of the mafic melt when it is injected into the granitic magma chamber, decomposition of crystalline fragments, dispersion of fragments and crystals in the magma chamber under conditions of rapid turbulent flow, and enrichment of felsic magma with femic components to produce monzonitic magmas.     

Eclogites from the Belomorian Mobile Belt (Kola Peninsula): geology and petrology

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 of their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence of 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 ～14–14.5 kbar for the southern ones. The used geothermometers yielded ～700°C 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 suggest that the Kuru-Vaara eclogites might have reached the field of 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.     

Olivine-bearing rocks of the Lapland granulite belt (Baltic Shield)

Olivine-bearing varieties of garnet–clinopyroxene crystalline schists of the Lapland granulite belt have been studied in detail for the first time. Two types of olivine (iron mole fractions of 27 and 38%) are distinguished. Olivine with lower Fe content occurs as inclusions in clino- and orthopyroxene and in terms of СаО and Cr contents is close to magmatic minerals. Olivine with high Fe content presumably suffered highand moderate-temperature metamorphism. The olivine-bearing rocks contain several grains of omphacite with 30–37 mol % jadeite and garnet with 44–50 mol % pyrope, which can be regarded as relict assemblages of the early stage of eclogitization of a magmatic protolith. The presence of symplectites indicates their retrograde transformation during decompression. The protoliths of the studied rocks could be olivine gabbronorites and pyroxenites. It was found that the rocks contain high-alumina minerals: corundum, spinel, and sapphirine. In addition, Al₂O₃ content in some amphibole grains is as high as 19 wt %. This indicates that the ascent of the deep-seated rocks was accompanied by interaction with Al-rich fluid. The positive Eu anomaly in the olivine-bearing rocks and some of their minerals is indicative of the reducing character of fluid. Activation of fluid reworking leading to the formation and transformation of the olivine-bearing rocks, transfer of alumina and its precipitation at different depths are related to the processes at the base of the Paleoproterozoic rift system of Karelides.     

Finding of corundum-bearing rocks in the Lapland granulite belt

Corundum-bearing rocks are described for the first time in the Kandalaksha structure of the Lapland granulite belt. Corundum is confined to rocks of two types: metagabbro?anorthosites constituting lenses among metaanarthosites of the Kandalaksha massif and basic granulites. Corundum crystals (up to 200 μm long) occur in plagioclase and garnet and differ from each other depending on the host mineral, which serves as evidence against their xenogenic nature. Some corundum crystals exhibit an axial zone, which may indicate their crystallization from the gaseous phase. Corundum-bearing rocks are accompanied by piclogites (pyroxene?garnet varieties with olivine). Piclogites and their minerals (clinopyroxene, garnet) are characterized by a positive Eu anomaly, which implies rock reworking by fluids during corundum formation, when deep-seated complexes were subjected to exhumation.     

New geochronological data on metamorphic and igneous rocks from the Gridino Village area (Belomorian eclogitic province)

Doklady Earth Sciences -     

The degree of oxidation of iron in synthetic iron-magnesian biotites

Summary Chemical analytical and pyrolytical methods have been used to study the Fe⁺²/Fe⁺³ ratios and dehydroxylation reactions in synthetic biotites. It has been found for the biotites with Fe/(Fe + Mg) of 20 to 70 mole % that the oxidation degree decreases from 26 to 16% with increasing iron. Based on the measured amounts of water and hydrogen released during pyrolysis it is inferred that the deprotonization is a dominant reaction at low temperatures (T 600°C), accompanied by dehydration as the temperature increases. Depending on the composition, a complete dehydroxylation takes place at T 900 °C, and the measured amount of water corresponds to the iron oxidation degree in the starting samples. The results of this study have important implications with respect to determination of the formation conditions of biotite-bearing rocks, and also for improvement of the techniques for determination of different valence of iron and water.

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Le degré d'oxidation du fer en biotites synthétiques contenants le fer et le magnésium

Résumé Des méthodes chimiques et pyrolitiques ont été utilisées pour l'étude des rapports Fe⁺²/Fe⁺³ et de la réaction de la déhydroxilation en biotites synthétiques. On a trouvé pour les biotites avec Fe/(Fe + Mg) de 20-70 mole % que le degré d'oxidation décroît à partir de 26 jusqu'à 16% pendent que le contenu du fer s'accroît. Sur la base de la quantité d'eau et hydrogène liberée pendant la pyrolyse, on infère que la déprotonisation est une réaction dominante à températures basses (T = 600°C), mais quand la température s'accroît, la déprotonisation est accopagnée de la déhydratation. Dépendant de la composition il y a une déhydroxilation complète à T = 900°C, et la quantité de l'eau mesurée correspond au degré d'oxidation du fer dans les specimens initials. Les résultats de cette étude infuencent la détermination des conditions de formation des roches contenantes biotite et aussi l'amélioration des techniques de la détermination du fer de valences différentes et de l'eau.