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. Dobretsov et al. [1985] first described the rock complexes in Eastern Sayan as ophiolites. Ophiolites formed in Dunzhugur island arc and were obducted onto Gargan block, a Neoarchean crystalline basement of the Tuva-Mongolian Massif (TMM), as a single nappe [Khain et al., 2002;Kuzmichev, 2004]. Zircons from plagiogranite were dated at 1021±5 Ma by multigrain TIMS and 1020±1 Ma by Pb-Pb single-grains evaporation method [Khain et al., 2002]. Later [Kuzmichev, Larionov, 2013] analysed 12 grains of detrital zircons from gravelstone of the Dunzhugur formation and obtained 206 Pb/ 238 U ages from 844±8 to 1048±12 Ma. Careful examination of these data shows that 206 Pb/ 238 U ages for concordant zircons only vary from 962±11 to 1048±12 Ma. Two groups of data give Concordia ages of 974±11 and 1028±10 Ma. Rocks of the Dunzhugur complex are characterized by slightly negative to slightly positive εNd(t) values from -1.0 to +1.5 and Late Palaeoproterozoic depleted mantle Nd model ages of 1.8-1.6 Ga [Sklyarov et al., 2016]. The Dunzhugur complex was intruded by tonalite plutons of the Sumsunur complex dated at 785±11 Ma [Kuzmichev et al., 2001] and 811±7 Ma [Kovach et al., 2012]. These tonalites have strongly negative ɛNd(t) values from -13.2 to -12.3 and Nd model ages of 2.5-2.4 Ga, suggesting formation of these melts from a mixture of N.L.

We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–reprecipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ∼2.86–2.87, ∼2.82–2.80, and ∼2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.

In the central Vetreny Belt, southeastern Baltic Shield, an areally extensive 110 m deep lava lake is exposed consisting of remarkably fresh differentiated komatiitic basalt. During eruption, the liquid had a temperature of 1380-1400 °C and contained &15% MgO. The lava ponded in a large topographic depression soon after eruption. The differentiation of the lava lake was controlled by settling of transported olivine and chromite phenocrysts and caused the origin of prominent internal layering. The last portions of the trapped liquid crystallized at temperatures of 1250-1070 °C. A Sm-Nd isochron of 2410$34 Ma for whole rock samples, olivine, augite and pigeonite separates from the lava lake provides a reliable estimate for the time of formation of the uppermost sequences in the Vetreny Belt. This age is in good agreement with the Sm-Nd and Pb-Pb isochron ages of 2449$35 and 2424$178 Ma for the volcanic rocks from the same stratigraphic level in the northwestern Vetreny Belt. Modeling of Nd-isotopes and major and trace elements shows that the komatiitic basalts at Lion Hills may have had a komatiite parent depleted in highly incompatible elements. It can be shown that this initial liquid I.S. Puchtel ( ) • A.