ISOTOPE EVIDENCE FOR IJOLITE FORMATION BY FENITIZATION - SR-ND DATA OF IJOLITES FROM THE TYPE LOCALITY IIVAARA, FINLAND

Ijolites from the type locality at Iivaara, Finland, form a continuous series of magmatic rocks ranging from urtites to melteigites. Both Ni and Cr, but also the large ion lithophile light-rare-earth elements, Zr, Hf, Nb, Rb, Sr and Ba are low in concentration. The Nd contents equal those of the neighboring fenites, Sr is distinctly less abundant, and there is no significant Eu anomaly. The Nd-143/Nd-144 and Sr-87/Sr-86 of the ijolites demonstrate a systematic covariation between the data of carbonatites from the Kola Alkaline Province (epsilon(Sr) - 13.8, epsilon(Nd) + 5.6) and those of the fenites at Iivaara (epsilon(Sr) + 132.9, epsilon(Nd)24. 7), with epsilon(Sr) varying from + 0.3 to + 23.9 and epsilon(Nd) varying from - 9.2 to - 19.3. The trace element abundances and the isotopic data give evidence for a crystallization of the rocks from a liquid generated by melting (rheomorphism) of high-grade fenitized country rocks rather than from a primary mantle-derived magma which was contaminated at crustal levels. The fenitization of wall rocks preceding the ijolite magma formation was element selective. Mixing of elements during the fenitization process between the designated components carbonatite (or derivative fenitizing fluid) and wall rock should have been dynamical depending on the stability of the wall rock mineral assemblages in contact with the fenitizing fluids, the migration velocity of these fluids, and their capacity of the respective elements. Such dynamical mixing explains best the variation of the isotope ratios without systematic covariation of the respective element concentrations.