GEOCHEMICAL AND PETROPHYSICAL CHARACTERISTICS OF PLUTONIC ROCKS FROM THE ARCHAEAN KARELIA PROVINCE IN FINLAND

This study considers the geochemical and petrophysical characteristics of plutonic rocks from four terrains in the Archaean Karelia Province in central Finland: Pudasjarvi, Iisalmi, Kuhmo and Ilomantsi terrains. The samples investigated are mainly dioritic to granodioritic in composition, although some mafic intrusions have been sampled from each of the areas with the exception of the Ilomantsi terrain. The densities and magnetic susceptibilities of the samples are generally low; this reflects their relatively felsic compositions and general paucity of magnetite. The majority of the samples show enrichment in LREE and compatible elements relative to HREE when normalized against averages calculated from nearly 3000 plutonic rock samples representing the Proterozoic and Archaean bedrock in Finland. These features are especially striking in plagioclase-rich adakitic rocks (TTGs, sanukitoids), whose chemical spectra are consistent with fractionation in the lower crust or upper mantle, at depths greater than 40–60 km. Using normalized Nd/Zr, La/Ce and Er/Lu ratios, the restitic minerals of adakitic melts can be estimated to mainly consist of clinopyroxenes, amphiboles and garnet, with lesser amounts of orthopyroxene. An important observation is the similarity of the adakitic samples from each of the terrains, suggesting similarities in their respective evolutionary history. The evolution of adakitic rocks can be explained with respect to subduction-related magmatism and fractionation at great depth. However, when considering the wide distribution and large volume of Archaean adakitic rocks compared to those that are unequivocally interpreted as resulting from Archaean subduction processes, an alternative explanation is favoured here, involving crustal thickening by tectonic stacking, combined with underplating by anomalously hot upper mantle material. The underplating processes generated plagioclase-rich adakitic melts, leaving restites rich in compatible elements and HREE, which are locally evident as high velocity layers in the lower crust (assuming that they have not been removed by delamination). Adakitic plutonic rocks appear to represent a continuous compositional series, from TTGs with variable Na 2 O/K 2 O and low Ba+Sr to sanukitoids with low Na 2 O/K 2 O and increasing Ba+Sr. These compositional variations can be explained by varying fractionation depths and degrees of subduction-related mantle metasomatism.