ISOTOPIC HETEROGENEITY OF SOURCE REGIONS FOR ALKALINE MAGMATISM OF ARCTIC SIBERIA - ND ISOTOPE EVIDENCE

New data are reported on Sm and Nd abundances and Nd isotope compositi on for the rocks of the Gulinskii alkali-ultramafic massif. The alkali -ultramafic rocks (meymechite, dunite, picrite, khatangite, and koswit e) originated at about 230 Ma from a depleted mantle source (epsilon N d-T = +5.1), yet a possibly coeval eudialyte syenite originated from a source significantly enriched in lithophile elements (epsilon Nd-T ap proximate to -8). A model is proposed in which the mantle in this area separated at about 1150 Ma into two contrasting regions: depleted (so urce of the ultramafic rocks) and enriched (source of the syenite).