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Petrographic, geochemical, and isotope–geochemical studies of the Mesoproterozoic volcanic association (lava flows and the Valaam sill) in the Ladoga graben in the suture zone of the Karelian craton and Svekofennian orogenic complex are conducted. Ferrobasites of close composition and geological position in lava flows and the sill have different Nd isotopic compositions, differing by 5 units of εNd(t): = –4...–5 in ferrobasalts of lava flows and a very low-radiogenic Nd isotopic composition in mafic rocks of the sill (εNd(t) to –11 for ferrogabbros), and identical to the isotopic composition of its acidic rocks (εNd(t) to –11 for granophyres). It is shown that in the crust, which is heterogeneous in age, the process of fractional crystallization along the tholeiitic trend combined with assimilation of melts from wall rocks of magma chambers could have been an effective mechanism for forming such "paradoxical" features of the Nd isotopic composition. The isotopic composition of sill rocks indicates the likely occurrence of Mesoarchean blocks in the lower crust of the Ladoga graben region. [ABSTRACT FROM AUTHOR]

Major-element, lithophile trace element, and Sm–Nd and U–Pb zircon isotopic data are presented for Palaeoproterozoic mid-ocean ridge basalt (MORB)-type tholeiitic dikes ranging in age from 2140 ± 3 to 2126 ± 5 Ma studied at six localities within three terranes in the Karelian Craton, eastern Fennoscandian Shield. All the studied dikes have remarkably uniform geochemical and isotope characteristics. They are tholeiitic basalts with low contents of large ion lithophile elements, high field strength elements, and rare earth elements (REE), nearly flat chondrite-normalized REE patterns [(La/Sm)n = 0·9–1·2, (Gd/Yb)n = 1·0–1·2], and positive Ti, Nb, and Zr anomalies in the primitive mantle-normalized diagrams. The dikes also show relatively uniform initial Nd isotope compositions, with εNd values ranging from +1·4 to +3·0, despite the occurrence of these dikes within Archaean terranes with different crustal history. According to the results of U–Pb (zircon) and Sm–Nd internal isochron dating the crystallization age of the dikes is constrained to be c. 2·14 Ga. The studied MORB-type tholeiitic dikes are probably comagmatic with Palaeoproterozoic MORB-type basalts that have previously been recognized in the Karelian Craton, and might represent relicts of their magma feeder system. The uniformity of ages and geochemical and isotope characteristics of the MORB-type dikes and volcanic rocks suggest that they are probably related to a common magmatic event. This event was near-contemporaneous with the eruption of high-Ti plume-related basalts and intrusion of dikes in the c. 2·1 Ga Jatulian continental flood basalt province. Geochemical modelling indicates that the chemical and isotopic compositions of the dikes are best explained by derivation of their parental magmas by partial melting of a uniformly depleted mantle source in the spinel peridotite stability field, followed by fractional crystallization and minor (<6%) assimilation of continental crustal material. This suggests that magma-storage processes in upper crustal chambers were very short-lived; this could be the result of rapid extension and fast transport of melts through the relatively thin lithosphere of the Karelian Craton. Indirect evidence for the formation of the studied dikes in an extensional tectonic setting is provided by the established presence of extensional tectonics in the eastern part of the Fennoscandian Shield at c. 2·1 Ga associated with the opening of the Lapland–Kola and Svecofennian oceans. The studied continental MORB-type tholeiites, therefore, may play an important role as indicators of the timing of continental breakup. Palaeoproterozoic MORB-type tholeiitic dikes and basalts show significant geochemical similarities to Phanerozoic syn-breakup continental flood basalts of the North Atlantic and Afar provinces; this adds further support to the indicative role of continental MORB-type tholeiites in the reconstruction of continental breakup processes in the Precambrian. [ABSTRACT FROM AUTHOR]

The article discusses a study that focuses on the intraplate magmatism during the period of Paleoproterozoic Huron on the Karelian Craton, a part of the Fennoscandian Shield in Southeast Finland and Karelia, Russia. Topics discussed include decrease in the volume of igneous formation, types of various mafic dykes swarms of different ages including the early composed of gabbro-norite dykes and use of baddeleyite crystals for isotope analysis at the Russian Academy of Sciences.

Mineralogical and petrological-geochemical features of the Mesoproterozoic (1.23-1.20 Ga) alkaline ultrabasic rocks from the Kostomuksha-Taloveis (Russia) and Lentiira-Kuhmo (Finland) areas, West Karelia, have been studied. In terms of mineralogy and geochemistry, these rocks more resemble group II kimberlites of South Africa (orangeites) than olivine lamproites or ultramafic lamprophyres. On the basis of phenocryst composition, the studied orangeites are divided into three types: Cpx-Phl-Ol, Phl-Ol, and Phl- Carb orangeites. The Cpx-Phl-Ol orangeites from the Kostomuksha cluster clearly differ from analogous rocks from the Lentiira cluster. The composition of Phl-Ol orangeites is indicative of derivation by intense fractional crystallization; Cpx-Phl-Ol orangeites from the Kostomuksha area display evidence of intense lithosphere assimilation. The Phl-Carb orangeites from the Taloveis cluster and Cpx-Ol orangeites from the Lentiira cluster most closely approximate primary melts. The Kostomuksha orangeites are characterized by lowto moderate-radiogenic (Sr/Sr)1220 ratio varying from 0.7038 to 0.7067. The Phl-Carb orangeites of Taloveis have less radiogenic Nd isotope composition (ɛ from −11 to −12) as compared to the Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (ɛ from −6.9 to −9.4). The Cpx-Phl-Ol orangeites from Lentiira contain fresh olivine. By morphology and composition, there are three olivine generations: (1) large rounded, usually zoned crystals with Fo core, 0.33-0.37 wt % NiO, and 0.03-0.04 wt% CaO, which are interpreted as xenocrysts from depleted peridotites; (2) anhedral rounded zoned olivines of intermediate size with Fo cores, 0.03-0.05 wt % CaO, 0.12-0.17 wt % NiO, and up to 0.40 wt % MnO. These olivines were entrapped by orangeite melt and presumably represent a cumulate of basaltic melts or were derived from metasomatized peridotites; (3) fine euhedral olivines and xenocryst rims corresponding to Fo with 0.10-0.42 wt % CaO, 0.14-0.35 wt % NiO, and up to 0.07-0.21 wt % MnO; their origin was presumably related to the crystallization from kimberlite melt. The calculation of $$f_{O_2 }$$ of kimberlite melt during crystallization of perovskites using Nb-Fe perovskite oxyba-rometer showed that Cpx-Phl-Ol orangeites of Kostomuksha and orangeites of Lentiira crystallized at similar oxygen fugacities corresponding to ΔNNO from −3.3 to −1.1 and from −3.3 to −0.9, respectively. The Sm-Nd and Rb-Sr isotope study provided evidence for the contribution from ancient enriched source in the genesis of the orangeites. It was proposed that their mantle source was formed in two stages: (1) metasomatic reworking of previously depleted lithospheric source at the Karelian Craton base during Paleoproterozoic orogenic events 2.1-2.0 Ga ago; (2) extension-related generation of orangeite melts 1.27-1.20 Ga ago. [ABSTRACT FROM AUTHOR]