Origin of continental crust of 1.9-1.7 Ga age: Nd isotopes and U-Pb zircon ages in the Svecokarelian terrain of South Finland

Abstract

Initial Nd isotopic ratios are reported for 23 samples representing magmatic crustal components in the Svecokarelian terrain of South Finland. U-Pb zircon ages are determined for all geologic units, involving 21 separate upper concordia intercept ages based on more than 100 UP-b analyses. The ages range for all the rocks from 1.90 Ga for primitve plutonic rocks to 1.79 Ga for post-tectonic intrusions. The well-known gabbro-diorite-tonalite-trondhjemite association of the Kalanti district appears to consist of components with different ages: trondhjemites are probably 1.90 Ga or older, diorites/tonalites belong to the main Svecokarelian plutonic eposide at 1.89-1.87 Ga, and at least some gabbro has a post-tectonic age of 1.80 Ga. ɛ Nd (T) values range between +2 to +3 for meta-andesites, large gabbros and primitive granitoids to −0.5 for more evolved granitoids. A magma source with ɛ Nd of at least +2 to +3 was available during 1.90 to 1.87 Ga, but evolved granitoids have ɛ Nd close to zero. The preferred interpretation is that depleted mantle with ɛ Nd=+4 to +5 was present beneath the Svecokarelian crust forming during 1.9-1.8 Ga, and that all rocks have been affected more or less by addition of an Archean crustal component with ɛ Nd=−9 to −13. The primitive rocks with ɛ Nd=+2 to + 3 were only slightly affected, while granitoids with ɛ Nd close to zero include a ∼10% Archean component. The widespread nature of the Archean addition and the distance of up to 500 km to actual exposed Archean crust make it most realistic that the Archean component was added to the form of sediments delivered by subducting Proterozoic ocean crust. The plutonic rocks of the Finnish Svecokarelian crust in areas away from Archean cratons consist of ∼90% newly mantle-derived material.