Growth of Archaean crust in the Kuhmo district, eastern Finland: U–Pb and Sm–Nd isotope constraints on plutonic rocks

Neoarchaean plutonism in the Kuhmo district in eastern Finland, Karelia, includes tonalites, sanukitoids, leucocratic granodiorites, and leucogranites. A U–Pb (SIMS and TIMS) study of zircons and titanites places age constraints on this plutonic activity, and whole-rock Sm–Nd isotopes are used to characterise source variation. U–Pb results indicate that the tonalites were emplaced during at least three separate periods; >2.81, ∼2.78, and 2.76–2.74 Ga. The initial ɛNd values vary from +0.3 to +2.2, and suggest no major involvement of significantly older crust in the genesis of tonalites. The tonalites were followed by 2.74–2.70 Ga (possibly also 2.68 Ga) sanukitoids with initial ɛNd values of −0.7 to 1.2. Close temporal relations between the ∼2.74 Ga tonalites and sanukitoids indicate that their genesis may be linked. Subsequent leucocratic granodiorites and leucogranites were emplaced at 2.70–2.68 Ga and commonly contain inherited zircons. The initial ɛNd values of these granodiorites and granites vary from +1.1 to −3.4, indicative of multiple magma sources. The general decrease in initial ɛNd values with decreasing age demonstrate progressive recycling of pre-existing crust, thus stressing the importance of crustal recycling as a process in forming the leucocratic granitoid rocks. Overall, the Archaean plutonic rocks of the Kuhmo district are temporally similar to those in the adjacent Russian Karelia, suggesting that 2.83–2.68 Ga was one of the most extensive periods of crustal growth throughout the Karelia craton.