Geochemical features of carbonatites of the Fennoscandian shield

摘要

The petrochemistry of carbonatites of three formation types were studied: (1) ultrahigh-pressure garnet-containing carbonatites (UHPC) of the Caledonian sheet (Tromso, Norway); (2) rocks of the carbonatite-alkaline-ultrabasic Kovdor massif (the Kola Peninsula); and (3) rocks of the carbonatite-alkaline-gabbroid Tikshozero massif (north of Karelia). The samples of carbonatites were examined and tested with a microprobe; the microelements were determined using the ICP-MS technique at the Institute of Microelectronics Technology and High Purity Materials (Chernogolovka). The carbonatites of the Kovdor and Tikshozero massifs are characterized by similar negative REE trends, with a degree of REE enrichment of the Tikshozero carbonatites. The UHPC from Tromso are different from those of the Kovdor and Tikshozero massifs in the negative trend along with lower concentrations of light REEs. The Tromso UHPC are similar to the carbonatites of the Kovdor and Tikshozero massifs in the trend and concentrations of heavy REEs. The carbonatites of the Fennoscandian shield of various formation times and types are characterized by the geochemical similarity to those in different regions of the world with the sources associated to mantle plumes. This similarity might be caused by the formation of the mantle carbonated magmas of carbonatite-containing igneous complexes from a mantle source enriched under either mantle metasomatism or plume-lithosphere interaction, with similar mechanisms of formation. The appearance of the formations as such within a wide time interval points to the long-term occurrence of a superplume at the Fennoscandian shield and to permanent activation of the related processes of magma formation.