ND AND SR ISOTOPE SIGNATURES OF THE KHIBINA AND LOVOZERO AGPAITIC CENTERS, KOLA-ALKALINE-PROVINCE, RUSSIA

Nd and Sr compositions of the highly evolved agpaitic nepheline syenit es and associated ijolites and carbonatites from the Khibina and the L ovozero alkaline centres define three magma sources. Isotopes of the v oluminous nepheline syenites and ijolites of Khibina intrusions III, I V, V, VI and VII as well as of nepheline syenites of Lovozero lie on t he Kola Carbonatite Mixing Line which is close to the ''mantle array'' defined by the components ''bulk earth'' and ''prema'' on a Sr-Nd plo t. The Khibina carbonatites and associated silicate rocks of intrusion VIII, which have more radiogenic Sr, did not evolve from the same par ent magma as the nepheline syenites. Isotopic constraints exclude a pr e-enrichment of Rb, Sr, Sm and Nd in the lithospheric mantle below Kol a over more than 10 Ma prior to the crystallization of the magmas. A f ormation of the melts involving major participation of the Precambrian crust of the Baltic Shield is also excluded. The lack of significant Eu anomalies in the Lovozero nepheline syenites gives evidence that th e agpaitic magmas in the Kola region did not form from basaltic liquid s by fractional crystallization of plagioclase or anorthoclase at crus tal levels. A formation from nephelinite or nepheline benmoreite magma s at mantle pressures is more likely, possibly by dynamic flow crystal lization. Enrichment factors suggest that large-ion lithophile and hig h field-strength elements as Ta, La, Nb and Zr, which are highly conce ntrated in the agpaites, were scavenged from mantle volumes of some 10 0,000 km3. An enrichment of these elements prior to magma formation ma y have been performed by volatile transfer. The well-defined whole-roc k isochrons of the Khibina III-VII and the Lovozero agpaites of c. 370 Ma date the magma separation for the different intrusions, if these m elts are cogenetic and formed by fractional crystallization in a Khibi na and a Lovozero magma chamber. If, however, Rb and Sr were collected by a process of volatile transfer, and the initial Sr isotopic compos itions of the two distinguished agpaite suites are, therefore, average s of the sampled mantle volumes, the Rb-Sr whole-rock isochron ages of c. 370 Ma would date this process of element collection. The concorda nce of the whole-rock ages with the mineral ages of Khibina and Lovoze ro samples is then further evidence for the short period between magma genesis, intrusion and crystallization.