ALBERT

Description: 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 Nd 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.