MINERALOGY AND GEOCHEMISTRY OF DEVONIAN ULTRAMAFIC MINOR INTRUSIONS OF THE SOUTHERN KOLA-PENINSULA, RUSSIA - IMPLICATIONS FOR THE PETROGENESIS OF KIMBERLITES AND MELILITITES

Minor magmatic intrusions of kimberlite, melilitite and cpx-melilitite occur in the southern part of the Kola Peninsula, Russia, on the Ters kii Coast and near the town of Kandalaksha. They yield K-Ar ages of 38 2 +/- 14 Ma and 365 +/- 16 Ma, similar to the magmatic rocks from the Kola Alkaline Province. The Terskii Coast kimberlites have mineralogic al and geochemical affinities with group 1 kimberlites, whereas the Ka ndalaksha monticellite kimberlite more closely resembles calcite kimbe rlites. The lower Al2O3 content in the Kola kimberlites indicates a st rongly depleted harzburgitic source, while higher Al2O3 in the melilit ites suggests a lherzolitic source. The Terskii Coast kimberlites are anomalously potassic and significantly enriched in P and Ba compared t o other group 1 kimberlites. In contrast, the melilitites are sodic an d are anomalously depleted in P compared to worldwide melilitites. Tra ce element patterns of the Kola kimberlites and melilitites indicate t he presence of K- and P-rich phases in the mantle source. To account f or the K-troughs shown by both magma types, a K-rich phase such as phl ogopite is thought to be residual in their sources; however, the anoma lous K-enrichment in the Terskii Coast kimberlites may indicate that a n additional metasomatic K-rich phase (e.g. K-richterite and/or a comp lex K-Ba-phosphate) existed in the kimberlite source. The P-depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual in the melilititic source. However, anomalous P-enri chment in the kimberlites cannot be explained by complete melting of t he same phase because the kimberlites are a smaller degree melt; thus, it is most likely that another metasomatic phosphate mineral existed in the source of the kimberlites. The Kola kimberlites and melilitites are all strongly LREE-enriched but the kimberlites have a steeper REE pattern and are significantly more depleted in HREE, indicating a hig her proportion of garnet in their source. Higher Nb/Y ratios and lower SiO2 values in the kimberlites indicate that they were a smaller degr ee partial melt than the melilitites. The presence of diamonds in the Terskii Coast kimberlites indicates a relatively deep origin, while th e melilitites originated from shallower depth. The non-diamondiferous Kandalaksha monticellite kimberlite has lower abundances of all incomp atible trace elements, suggesting a higher degree of partial melting a nd/or a less enriched and shallower source than the Terskii Coast kimb erlites. The Sr-87/(S6)Sri, Nd-143/Nd-144(i) and Pb isotope compositio ns confirm that the Terskii Coast kimberlites have close affinities wi th group 1 kimberlites and were derived from an asthenospheric mantle source, while the Kandalaksha monticellite kimberlite and Terskii Coas t melilitites were derived from lithospheric mantle. Impact of a Devon ian asthenospheric mantle plume on the base of the Archaean-Proterozoi c lithosphere of the Kola Peninsula caused widespread emplacement of k imberlites, melilitites, ultramafic lamprophyres and other more fracti onated alkaline magmas. The nature of the mantle affected by metasomat ism associated with the plume and, in particular, the depth of melting and the stability of the metasomatic phases, gave rise to the observe d differences between kimberlites and the related melilitites and othe r magmas.