A Pseudobasement in the pre-alpine structure of the Peredovoi Range, north Caucasus

Original Russian Text © M.L. Somin, L.M. Natapov, E.A. Belousova, A. Kroener, A.N. Konilov, V.A. Kamzolkin, 2013, published in Doklady Akademii Nauk, 2013, Vol. 450, No. 4, pp. 445–449.5 page(s

Chemical and isotopic compositions of zircons from amphibolites of the Kamchatksky Cape Peninsula

The paper reports the first data on geochemistry and U-Pb SHRIMP geochronology of zircons from garnet amphibolites whose fragments are hosted by the sole of the ophiolite complex of the Kamchatsky Cape, eastern Kamchatka. The zircons compose homogeneous sampling, have relatively small sizes, are anhedral, have no oscillatory zoning, and possess practically no inclusions. Chemical and photoluminescent characteristics of the zircons testify to their metamorphic genesis. U-Pb SHRIMP dates of the zircons (81.4+/-9.6 Ma) indicate that metamorphism of the amphibolite complex took place in Campanian, Late Cretaceous. These dates seem to correspond to the peak of high-pressure metamorphism, which is thought to be related to origin of an ophiolite complex of the suprasubduction type and its uplift within the Kronotsky Island arc

Mesoarchean mafic dykes of the Belomorian eclogite province (Gridino Village Area, Russia)

Archean processes of eclogitization in the Gridino metamorphic association (the Belomorian eclogite province) developed in mafic dykes, boudins, and acidic rocks of the Archean continental crusts. To determine the U-Pb age of the intrusion of the latest dykes, the geochronological samples were taken from the dyke of ferriferious metagabbro that cross-cuts the dyke of eclogitzed and granulitized olivine gabbronorite. The igneous zircons were dated by the SHRIMP II technique. The zircons showed a concordia age of 2846 ± 7 Ma, which is considered as the time of intrusion of a mafic melt. The younger low-thorium zircon rims of 2.78-2.81 Ga age around the igneous cores are typical formations that appeared under metamorphic conditions in equilibrium with a migmatite melt, and may characterize the time of formation of the granite leucosome under metamorphism, probably of eclogite facies.7 page(s

Mesoarchean Kola-Karelia continent

Mesoarchean Kola-Karelia continent in the eastern Fennoscandian Shield includes three tectonic provinces, Kola, Karelia and Belomoria, that were formed by the Paleoarchean and Mesoarchean microcontinents. Traces of Mesoarchean tonalite-trondhjemite-granodiorite (TTG)-type early crust were documented in all of the most ancient units of the Kola-Karelia continent. Ancient crust was revealed and dated in the Ranua and Iisalmi microcontinents, 3.5-3.4 Ga; Vodlozero and Khetolambina microcontinents, 3.25-3.15 Ga; Kuhmo-Segozero microcontinent, ~3.0 Ga; Murmansk and Inari-Kola microcontinents, 2.93 Ga; and Kianta microcontinent, 2.83-2.81 Ga. In the older (>3.0 Ga) tectonic units and microcontinents, the ancient crust was possibly formed in brief bursts of endogenic activity. In younger microcontinents (3.0–2.93 Ga), these processes could continue until 2.8 and even 2.72 Ga. The tectonic settings in which early TTG crust has been produced are largely uncertain. The primary melt glassy inclusions with a glass phase in cores of prismatic zircon crystals from TTG gneisses provide evidence for the volcanic origin of gneiss protolith. Suggested genetic modeling of TTG-type complexes assumes that felsic K-Na melts with positive Eu anomaly are a product of dry high-temperature partial melting of the previously formed mafic-to-felsic crustal rocks and/or thick older TTG crust. Positive Eu anomaly in the eutectic is directly related to the predominance of plagioclase and K-feldspar in the melt. TTG-type crust melted to produce granite-granodiorite (GG) rocks. Earliest microcontinents are separated by Mesoarchean greenstone belts (mainly 3.05-2.85 Ga, in some cases up to 2.75 Ga), which are fragments of paleo-island-arc systems accreted to their margins: the Kolmozero-Voronya, Central Belomorian, Vedlozero-Segozero, Sumozero-Kenozero, and Tipasjärvi-Kuhmo-Suomussalmi belts; and the mature island arcs (microcontinents): Khetolambina and Kovdozero. These structural units are characterized by significant extent, close to rectilinear trend, localization along the boundaries between Archean microcontinents, and a specific set of petrotectonic assemblages (basalt-andesite-rhyolite, komatiite-tholeiite, and andesite-dacite associations). The recently discovered Meso-Neoarchean Belomorian eclogite province that is structurally linked with the Central Belomorian greenstone belt contains two eclogite associations distributed within TTG gneisses: the subduction-type Salma association and the Gridino eclogitized mafic dikes. The protolith of the Salma eclogites is thought to have been a sequence of gabbro, Fe-Ti gabbro, and troctolite, formed at ca. 2.9 Ga in a slow-spreading ridge (similar to the Southwest Indian Ridge). The main subduction and eclogite-facies events occurred between ca. 2.87 and ca. 2.82 Ga. Mafic magma injections into the crust of the active margin that led to formation of the Grigino dike swarm were associated with emplacement of a mid-ocean ridge in a subduction zone, beginning at ca. 2.87 Ga. Crustal delamination of the active margin and subsequent involvement of the lower crust in subduction 2.87–2.82 Ga ago led to high-pressure metamorphism of the Gridino dikes that reached eclogite-facies conditions during a collision event between 2.82 and 2.78 Ga. This collision resulted in consolidation of the Karelia, Kola, and Khetolamba blocks and formation of the Mesoarchean Belomorian accretionary-collisional orogen. To date, the subduction-related Salma eclogites provide the most complete and meaningful information on the nature of plate tectonics in the Archean, from ocean-floor spreading to subduction and collision. The Kovdozero granite-greenstone terrain that separates the Khetolambina nd Kuhmo-Segozero microcontinents is formed by TTG granitoids and gneisses hosting metasediments and metavolcanics of several greenstone belts, which belonged to the Parandovo-Tiksheozero island arc that existed from ca. 2.81 to 2.77 Ga. The Iringora greenstone belt includes the ophiolite complex of the same name with an age of 2.78 Ga. The collision of microcontinents resulted in the upward squeezing of the island arc and the obduction of its marginal portions onto surrounding structures.74 page(s

The Belomorian eclogite province : unique evidence of Meso-Neoarchaean subduction and collision

Original Russian Text © M.V. Mints, A.N. Konilov, K.A. Dokukina, T.V. Kaulina, E.A. Belousova, L.M. Natapov, W.L. Griffin, S.Y. O’Reilly, 2010, published in Doklady Akademii Nauk, 2010, Vol. 434, No. 6, pp. 776–781.6 page(s

Mesoarchean subduction processes : 2.87 Ga eclogites from the Kola Peninsula, Russia

The nature of tectonic processes on the early Earth is still controversial. The scarcity of high-pressure metamorphic rocks such as eclogite (the high-pressure equivalent of basalt) in Archean cratons has been used to argue that plate tectonics did not operate until Earth had cooled to a critical point, perhaps around the 2.5 Ga Archean-Proterozoic transition. However, eclogites occur as meter- to kilometer-sized lenses enclosed in Archean gneisses of the Belomorian Province of the Fennoscandian shield. Geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros, troctolites, and Fe-Ti oxide gabbros. Greenschist facies mineral parageneses are enclosed in prograde-zoned eclogite garnets, and peak metamorphic conditions define an apparent thermal gradient (12–15 °C/km), consistent with metamorphism in a warm Archean subduction zone. We show here that these eclogites represent the oldest known high-pressure metamorphic rocks. U-Pb dating and Hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2.87 Ga for the Uzkaya Salma eclogite; a 2.70 Ga age for the Shirokaya Salma eclogite is interpreted as the age of a granulite facies overprint. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9–1.8 Ga) orogeny. These new data imply that plate tectonic processes operated at least locally in late Mesoarchean time. The adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce Archean tonalite-trondhjemite-granodiorite–type magmas.4 page(s