Agpaitic magmatism in the northeastern Baltic Shield: a study of the Niva intrusion, Kola Peninsula, Russia

The recently discovered Niva agpaitic syenite intrusion is located 50 km so uthwest of the Khibina complex, central Kola Peninsula. It forms a lense-li ke body 1.5-2 km in size that outcrops over an area of 300 x 200 m on the s outhern 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, an d 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 continenta l crust during formation of the Niva alkaline magmas. A depleted asthenosph eric mantle reservoir mixed with a slightly enriched mantle component is su ggested to be the main source of the Niva magma. A metasomatically altered garnet granulite xenolith from the western contact of the Niva intrusion co ntains a primary paragenesis of diopside, garnet and plagioclase with chemi cal 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, n eptunite, pectolite and natrolite which partially replace both primary mine rals of the granulite assemblage and secondary minerals which were formed d uring a later regional metamorphism. PT estimates support the lower crustal origin of the xenolith. The U-Pb zircon age of the Niva garnet granulite i s 2711 +/- 7 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.