The recently discovered Niva agpaitic syenite intrusion is located 50 km southwest of the Khibina complex, central Kola Peninsula. It forms a lense-like body 1.5-2 km in size that outcrops over an area of 300 X 200 m on the southern bank of the Imandra lake. Rb-Sr (379 ± 21 Ma) and Sm-Nd (383 ± 58 Ma) isochron ages of the intrusion coincide with the age of the Khibina and Lovozero complexes. A characteristic feature of the Niva rocks is their extremely high agpaicity and enrichment in Sr, Ba, Y, Zr, Nb, Rb, LREE, and volatiles. The mineral association in the agpaitic syenite is K-feldspar, aegirine-diopside, biotite, lamprophyllite, aenigmatite and shcherbakovite. Isotopic compositions indicate the absence of contamination by continental crust during formation of the Niva alkaline magmas. A depleted asthenospheric mantle reservoir mixed with a slightly enriched mantle component is suggested to be the main source of the Niva magma. A metasomatically altered garnet granulite xenolith from the western contact of the Niva intrusion contains a primary paragenesis of diopside, garnet and plagioclase with chemical characteristics similar to those in the granulite suite of the adjacent Lapland zone and lower crustal granulite xenoliths from Devonian pipes in the Kandalaksha area. Additionally, the granulite contains vinogradovite, neptunite, pectolite and natrolite which partially replace both primary minerals of the granulite assemblage and secondary minerals which were formed during a later regional metamorphism. PT estimates support the lower crustal origin of the xenolith. The U-Pb zircon age of the Niva garnet granulite is 2711 ± 7 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.