The 2.31 Ga mafic dykes in the Karelian Craton, eastern Fennoscandian shield: U–Pb age, source characteristics and implications for continental break-up processes

• The 2.31 Ga mafic dykes recognized in the Karelian Craton.
• Trace-element and isotopic characteristics of the dykes indicate long-term crustal storage of their parental magmas.
• The 2.31 Ga possible formed in the pre-breakup or failed breakup tectonic setting.

Major- and trace-element geochemistry, Sm–Nd isotopic and U–Pb geochronological (ID TIMS, baddeleyite) data are presented for Paleoproterozoic mafic dykes in the Karelian Craton, eastern Fennoscandian shield. Mafic dyke swarms recognized in the Lake Upper Kuito area in the Western Karelian terrane of the Karelian Craton include (i) NE-trending ca. 2450 Ma gabbronorite dykes, (ii) NW-trending ca. 2310 Ma dolerite dykes, and (iii) NNW-trending ca. 2130 Ma continental MORB-type tholeiitic dykes. Each swarm belongs to a distinct intraplate igneous event and indicates specific mantle melting and extensional processes.The ca. 2.31 Ga doleritic dykes have high-Fe, high-Ti tholeiitic series affinity and characterized by high concentrations of lithophile trace elements. The enrichment of LREE and unfractionated HREE patterns together with depletion of Nb on multi-element diagrams and initial ɛNd values of +0.5 to +0.8 are characteristic for ca. 2.31 Ga dolerites. The geochemical modelling results suggest that these features could be originated via combination of fractional crystallization and crustal contamination processes, and do not indicative of a mantle source enrichment. It also indicates long-term storage of melts in an intracrustal magma chamber and a relatively slow ascent to the surface. The primary melts of ca. 2.31 Ga dolerites possibly originated by partial melting of a DM-type mantle source in the spinel peridotite stability field.In contrast to previous ca. 2.45 Ga event studied ca. 2.31 Ga dolerites do not indicate high-temperature mantle-plume induced mantle melting. On the other hand, the ca. 2.31 Ga dykes have some similarities with syn-breakup ca. 2.13 Ga continental MORB-type tholeiites, although there is no evidence for the break-up of the continental lithosphere of the Karelian Craton at ca. 2.31 Ga. Thus ca. 2.31 Ga dykes could indicate the existence of relatively stable extensional tectonic environment and an unsuccessful attempt of break-up of the continent.The ca. 2.31 Ga dykes together with previously studied Runkaus basalts and Taivalkoski dykes could be considered as a component of distinct relatively wide-spread Kuito-Taivalkoski igneous event within the ca. 2.2–2.4 Ga “crustal age gap” period in the eastern Fennoscandian shield. This probably indicate that the number and intensity of endogenous events of age 2.4–2.2 Ga are greatly undervalued. This also argues against a decrease of endogenic activity and magmatic “slowdown” during ca. 2.4–2.2 Ga period.