Sm-Nd data as a key to the origin of the Archean sanukitoids of Karelia, Baltic shield

Abstract

New Sm-Nd isotopic data were obtained for the Late Archean sanukitoids of the Karelian granite-greenstone terrain of the Baltic shield. Regional variations in their Nd isotopic composition were detected. The Nd isotopic characteristics of sanukitoids from the youngest Central Karelian domain are similar to those of the depleted mantle, whereas the intrusions of the older western Karelian and Vodlozero domains show lower ɛNd(t) values. This isotopic heterogeneity is explained by different time intervals between the enrichment and partial melting of the mantle sources of sanukitoids from particular domains. A two-stage model was proposed for the formation of sanukitoid magmas. The first stage included mantle metasomatism by slab-derived fluids and/or melts. During the second stage (2.74–2.70 Ga), a tectonothermal anomaly caused partial melting of the metasomatized mantle and generation of sanukitoid melts. Most of the sanukitoid intrusions are cut by calc-alkaline lamprophyre dikes, which are geochemically similar to the sanukitoids. The new Sm-Nd isotopic data suggest a genetic link between these rocks. A comparison of the geochemical features of the sanukitoids and Phanerozoic subduction-related magmas showed that the Archean sanukitoids have no modern analogues.